Essay on Frogs

Match this with Real Hypothalamus

Nuclei of Hypothalamus – Mnemonic
William Wordsworth 's poetry described the wonder of the natural world, which had formerly been viewed as a threatening place. During the Neoproterozoic era, freezing temperatures covered much of the Earth in glaciers and ice sheets. The sigmoid colon is the part of the large intestine after the descending colon and before the rectum. Ahmad Shah Notes revised, edited and perfected. However, when there is mechanistic information that raises doubt about the relevance of the effect for humans, classification in Category 2 may be more appropriate. Generally there is a requirement for more complete information to decrease than to increase the level of concern.

12,000 BIOMEDICAL ABBREVIATIONS

Urine: Composition, Constituents and Deposits

The carbohydrates and fats remain unaffected by the proteolytic enzymes. In the stomach the food is stored for quite a long time. Due to the massive peristalsis of the stomach wall, the food is broken into smaller pieces and thoroughly mixed with the gastric secretions including the mucus. The food is now in a semi-liquid state known as chyme. The latter passes into the duodenum through pyloric sphincter at regular intervals.

Digestion that takes place in the intestine is called intestinal digestion. In the duodenum, the acidity of the chyme is neutralized by the salts of bile and now the chyme becomes alkaline in nature. With the entry of bile, pancreatic juice, the intestinal juice and more of mucus, the food is now in a more liquid form and is called chyle. A hormone, the cholecystokinin, produced by the duodenum reaches the gall bladder through blood circulation where it stimulates the gall bladder to release the bile.

The bill contains no enzymes, but its salts provide the alkaline medium necessary for the action of the pancreatic and intestinal enzymes. The duodenum also secretes secretin hormone which reaches the pancreas through blood circulation, where it stimulates the pancreas to release the pancreatic juice.

The pancreatic juice contains pro-enzymes trypsinogen, chymotrypsinogen and enzymes peptidase, amylase, maltase and lipase. The inactive trypsinogen is converted into trypsin by the action of an activator enzyme, the enterokinase of the intestinal juice. The trypsin converts inactive chymotrypsinogen into chymotrypsin. All these three enzymes— trypsin, chymotrypsin and peptidase change proteoses and peptones into peptides.

Amylase converts starch into maltose. Maltase converts some maltose into glucose. Lipase changes fat into fatty acid and glycerol. Intestinal juice contains peptidases, maltase and lipase enzymes. The enzyme enterokinase present in this juice converts inactive trypsinogen of pancreatic juice into trypsin. Many peptidases tri and dipeptidases , change peptides into amino acids. Maltase changes the rest of the maltose into glucose and the lipase converts the remaining fat in fatty acids and glycerol.

It is a process by which the digested food diffuses into the circulatory system blood and lymph through the wall of the alimentary canal. The absorption starts from the stomach. Most of the absorption is done by small intestine. The wall of the alimentary canal is richly traversed by blood and lymph capillaries. The lymph capillaries of the intestine are known as lacteals. Amino acids, glucose, vitamins and water are absorbed into blood capillaries, while fatty acids and glycerol enter the lymph-capillaries lacteals.

The indigestible food comes to rectum by peristaltic action, where water is mainly absorbed. After that, the faecal matters are transferred to the cloaca and from there they are egested via cloacal aperture. The skin of frog provides an extensive surface for the exchange of gases. It is thin, richly supplied with blood and kept moist by the mucus and water. The cutaneous respiration is always carried out.

During hibernation winter sleep and aestivation summer sleep , it is the only method of respiration in frog. It occurs when the animal is on the land or partially immersed in water.

There are present two external nares situated at the tip of the snout. Each external naris nostril leads into a nasal chamber which opens into the buccopharyngeal cavity by an internal naris. During this mode of respiration, the mouth and glottis are tightly closed while the nares are kept open. There are two sets of muscles which operate the mechanism of buccopharyngeal respiration. These muscles are stemohyal muscles and petrohyal muscles.

It is less frequent than the cutaneous and buccopharyngeal respiration. It occurs when need for oxygen is increased. Respiratory path includes external nares, nasal chambers, internal nares, buccopharyngeal cavity and glottis.

This chamber leads posteriorly to the lungs through a pair of apertures. The lungs are two delicate, elastic, pinkish, ovoid, thin walled hollow sacs lying on either side of the oesophagus. Each lung looks like a honey-comb. The inner surface of the lungs is divided by a series of partitions, the septa into many small cavities known as alveoli.

Exchange of gases takes place through alveoli. During pulmonary respiration, the mouth is tightly closed. The sternohyal and petrohyal muscles play very important role in the mechanism. A thin walled laryngotracheal chamber is the sound producing organ in the frog. It corresponds to the larynx and trachea of higher animals. There is also present a pair of elastic bands, the vocal cords which extend longitudinally across the laryngotracheal chamber. The vocal cords are the true sound producing organs.

A narrow slit like gap, the rima-glottis is present between the free inner edges of the vocal cords, which helps in the production of sound. The inner edges of the vocal cords, which lie freely, get vibrated when air from the lungs is expelled out forcibly to produce a characteristic sound croak.

The male frogs produce louder and prolonged sound than the females because males have a pair of vocal sacs. These sacs are the pouches of the skin on the throat, opening into the buccopharyngeal cavity and acting as resonators.

When the air is expelled out from the lungs the vocal sacs are inflated with air to raise the pitch of the sound in the male frog.

The circulatory system of frog consists of blood vascular system and lymphatic system. It is of closed type as the blood flows in the blood vessels. It represents single circulation. It means both the oxygenated and the deoxygenated blood enters the heart and get mixed in the ventricle.

Blood vascular system comprises blood, heart and blood vessels. The blood is a mobile connective tissue, composed of a fluid, the plasma and the cells, the blood corpuscles. These are large, oval and biconvex nucleated cells. The cytoplasm contains a respiratory red pigment, known as haemoglobin, which is made up of a blood protein, the globin and haematin.

They are small, numerous and spindle shaped nucleated cells. They are produced by the bone marrow. They help in the clotting of blood, by producing the thrombin enzyme. The heart is situated mid-ventrally in the thorax in between the two lungs, just anterior to the liver. The heart is protected by the pectoral girdle ventrolaterally and by vertebral column dorsally.

The outer layer of the sac is called parietal pericardium and inner layer is known as visceral pericardium. In between the two layers, a space, the pericardial cavity is present, which is filled with a pericardial fluid.

The pericardium protects the heart from the shocks, and mechanical injuries and also allows its free movements. After removing the pericardium, the external characters of the heart are clearly seen. The heart of frog is three chambered, viz. The two thin walled auricles are separated from each other by the inter auricular septum.

The sinus venosus opens into right larger auricle through sinuauricular aperture which is guarded by a pair of sinuauricular valves. It only allows the blood to flow towards the right auricle.

The common opening of pulmonary veins is present in the smaller left auricle. The opening is not guarded by any valve. Both the auricles open into a single ventricle by a common wide auriculo-ventricular aperture, which is guarded by a valve, the auriculoventricular valve bearing four flaps— one dorsal, one ventral and two lateral in position.

The free edges of these flaps project into the ventricle and are connected to the wall of the ventricle with fine contractile but tough thread-like structures known as chordae tendineae.

The chordae tendineae regulate the movement of the auriculo-ventricular valve so as to allow the blood to pass from the auricles into the ventricle and further prevent the backward flow of the blood into the auricles. The ventricle is a muscular and thick walled chamber, the inner surface of which is projected into ridges, the columnae carneae.

The latter divide the cavity of the ventricle into smaller spaces, known as fissures. The right side of the ventricle opens into truncus arteriosus on the ventral side. At the place from where the truncus arteriosus arises from the ventricle, a row of three pocket shaped semilunar valves is present.

These valves allow blood flow from the ventricle to the truncus arteriosus. The latter consists of a proximal muscular part, the pylangium and the distal part, the synangium. A row of the three semilunar valves separates the pylangium from the synangium. The pylangium part is further divided vertically into two incomplete chambers by a spiral twisted valve.

The right chamber is called cavum aorticum and the left cavum pulmocutaneum. The spiral valve is a flap of tissue running vertically in the pylangium and is attached with the middle valve of the anterior row of the three semilunar valves. The spiral valve is free ventrally and posteriorly. The cavity of the synangium is further divided into a dorsal and ventral chamber by a horizontal partition called primary septum or septum principle.

The synangium bifurcates into two branches as aortic trunks. A small opening of the pulmocutaneous arch is present in the cavum pulmocutaneum just behind the anterior row of semilunar valves. This opening is guarded by a small valve. Thus, the cavum pulmocutaneum leads into the dorsal chamber of synangium and then dorsal chamber opens into pulmocutaneous arch. While the cavum aorticum leads into the ventral chamber of synangium and then the ventral chamber opens into both systemic and carotid arches.

In addition to the above mentioned structures, there is present one sinus venosus. Histologically, the wall of the heart consists of three layers: Heart of frog is myogenic heart beat originates from a muscle, however, it is regulated by the nerves. Wave of contraction originates from the sinus venosus and spreads over the wall of the sinus venosus and both the auricles. It compels the heart to beat.

The contraction of the heart is termed as systole while its expansion is called diastole and both constitute one heart beat. The sinus venosus contracts, whose contraction drives the mixed blood from the sinus venosus to the right auricle. At the same time, the left auricle receives oxygenated blood through the opening of the pulmonary veins from the lungs. When the two auricles are full of blood, they contract simultaneously. The backward flow of the blood into the sinus venosus is prevented by the sinuauricular valves.

Thus, the blood from both the auricles is forced into the single ventricle through auriculoventricular aperture. The auriculoventricular node is stimulated by the auricular contraction and passes a fresh wave of the contraction over the wall of the ventricle. Thus both oxygenated and deoxygenated bloods are mixed in the ventricle.

The mixed blood of the ventricle enters the truncus arteriosus. From the truncus arteriosus mixed blood goes to three arches carotid, systemic and pulmocutaneous and ultimately reaches all the parts of the body. The contraction of heart systole and the relaxation of heart diastole constitute the heart beat. The rate of heart beat is controlled by the medulla oblongata of the brain and by certain hormones such as adrenalin and thyroxine.

Increase in carbon dioxide and temperature also affects the sinus venosus and thereby it increases the rate of heart beat. In frog cardiac cycle is completed in about 0.

Arteries, arterioles, veins, venules and blood capillaries are called blood vessels. Arteries carry blood from the heart to different body parts. Veins bring blood from different body parts to the heart. Arteries divide to form arterioles. The arterioles divide to form blood capillaries which unite to form venules. The venules join to form veins. The veins have valves to prevent backward flow of blood. Exchange of nutrients, hormones, gases, etc.

The renal portal system collects the blood from the hind parts on the body from which the urea and uric acid present in the blood are first got filtered in the kidneys before the blood goes into postcaval and then to the heart. Thus, the blood going to the heart contains comparatively less impurities after passing through the renal portal system. The excess of glucose is converted into glycogen which is stored in the liver for later use. When an individual feels deficiency of food, the glycogen is converted into glucose and is transferred to the blood stream via hepatic veins,.

Thus the blood is detoxified purified of harmful nitrogenous waste;. It is the second part of the circulatory system, through which a mobile connective tissue, the lymph, is circulated. Lymphatic system comprises a Lymph, b Lymph capillaries, c Lymph sinuses and d Lymph hearts. It is similar to blood plasma but has got less number of proteins. Mainly globulin proteins are present. Other components of the lymph plasma are very much like that of blood plasma, i.

Red blood corpuscles Erythrocytes are completely absent. Thrombocytes spindle shaped cells are few in number due to which the clotting of the lymph is very slow. Lymph capillaries lie very close to the blood capillaries but differ from them, as they end blindly in contact with the body cells or the tissue spaces. The lymph always flows from the tissues towards the lymph hearts. Their walls are extremely thin and permeable to colloids, crystalloids and water. They do not have a constant shape.

The lymph capillaries of the intestine transport the digested fat, which does not pass through blood stream directly. In the frogs, lymph vessels are not found. Instead of these vessels, there are present thin walled spaces or lymph sinuses around the tissues and between the organs. The important lymph sinuses are subcutaneous sinuses and sub-vertebral sinus. There are present two pairs of thin walled, muscular lymph hearts, one pair is located behind the transverse processes of third vertebra and the other is situated in the region of urostyle.

It is a dark, red oval structure lying near the anterior end of the rectum. It includes the nerves and ganglia that control and coordinate such organs which are not under voluntary control.

It comprises sympathetic nervous system and parasympathetic nervous system. It is lodged in the cranial cavity of the skull. The brain is covered by two membranes or meanings sing meninx. The outer tough, thick membrane is duramater, and the inner thin, more delicate and vascular membrane is pia arachnoid membrane.

The brain is divisible into three parts: Fore brain, mid brain and Hind brain. It comprises two olfactory lobes, two cerebral hemispheres and a diencephalon. Dorsal surface of the diencephalon has anterior choroid plexus to supply nourishment to the anterior parts of the brain. From the dorsal side of the diencephalon also arises a short cylindrical pineal stalk, which carries knob like pineal body.

The pineal body is an endocrine gland. The position of pineal body is indicated externally by the brow spot. A pair of thick optic nerves crosses and forms optic chiasma on the ventral side of the diencephalon. Further, behind the infundibulum and attached to it a flattened ovoid sac, the hypophysis pituitary gland is present. It produces very important hormones. It consists of optic lobes and crura cerebri. Behind the diencephalon two ovals, large slightly pressed to the outside, optic lobes are present.

These are well seen in dorsal view. On the ventral side, the brain is thickened to form two fibrous strands of nervous tissue, the crura cerebri sing, crus cerebrum , which carry the stimuli between the cerebral hemispheres and medulla oblongata.

It comprises cerebellum and medulla oblongata. Immediately behind the optic lobes, there is present a sort of transverse ridge on the dorsal surface known as cerebellum, which is poorly developed in frog. It maintains balance of the body. The last part of the brain is medulla oblongata, which is stout and somewhat triangular in shape and continues as spinal cord posteriorly.

The dorsal side of the medulla oblongata is non-nervous but highly vascular and forms posterior choroid plexus. The cranial nerves from 5th to 10th are connected with the medulla oblongata. The brain is hollow containing cavities, which are filled with the cerebro spinal fluid secreted by anterior and posterior-choroid plexuses. The cavities of the brain are known as ventricles. Each olfactory lobe encloses a ventricle termed as rhinocoel which leads into the ventricles of the cerebral hemisphere, the lateral ventricles or paracoels or first and second ventricles.

Two lateral ventricles open into the ventricles of the diencephalon, the diocoel or third ventricle through an aperture, the foramen of Monro. The ventricle of the cerebellum is called the metacoel. The myelocoel communicates with the central canal of spinal cord. They also control the voluntary activities of the animals. It also regulates the autonomic nervous system. Diencephalon also controls the metabolism of fat, water and carbohydrates.

The secretion of pituitary body influences a number of activities of the animal. It is a reduced structure in frog. It lies in the neural canal of vertebral column and extends from the medulla oblongata of the brain to almost the end of back-bone. The spinal cord is surrounded by the same two protective membranes; the meanings, as in the brain viz.

In frog, it is short, somewhat flattened structure which widens anteriorly and tapers towards the posterior end into the urostyle, where it is called filum terminate. The spinal cord shows two swollen parts, one near the arms and the other near the hind limbs known as brachial and sciatic enlargements respectively.

These enlargements supply large nerves to the arms and legs. All along its length, spinal cord has two median longitudinal grooves, the dorsal fissure on the dorsal side and ventral fissure on the ventral side while the latter is more grooved. The nerves which connect the brain and leave the brain box cranium are known as cranial nerves. The number of cranial nerves is definite in a particular group of animals. In frog, the number is ten pairs. The serial number of a nerve is also definite. It carries nerve impulses smell from the nasal chamber to the brain, hence, the nature of the nerve is sensory.

It carries impulse of sight from the eye to the brain, and its nature is sensory. It innervates four eye muscles viz. This nerve caries the impulse from brain to these muscles for controlling the movements of the eye ball and is motor in nature. It is distributed to one eye muscle, the superior oblique. This nerve helps in controlling the movement of eye ball and its nature is motor. It innervates the skin receptors of the snout. Thus, it conveys impulses from the skin of this area to the brain and, hence sensory in nature,.

It supplies the receptors of the skin, upper jaw, upper lip, lower eye-lid and carries the impulses from these areas to brain and hence is sensory in nature,. It innervates the skin and muscles of the lower jaw and tongue. It is mixed branch. It carries sense of touch from lower jaw to the brain. It also controls the movement of lower jaw. It supplies the roof of the buccal cavity palate and carries the sense of taste to the brain. So, it is sensory in nature,.

It innervates the muscles of hyoid apparatus, muscles and skin of lower jaw. It controls the activities of these organs and also conveys the sense of touch from skin of lower jaw. Its nature is mixed. Thus, total nature of the facial nerve is mixed. It supplies the ear and carries impulses from the ear to the brain, thus helps in hearing and balancing the organs. The nature of this nerve is sensory. It supplies the taste buds of tongue and muscles of the pharynx. Some fibres carry impulses from tongue, while the other fibres are responsible for pharynx movement as they are concerned with swallowing reflex.

Thus, both sensory and motor nerve fibres are present in this nerve. The nature of this nerve is mixed. It is to be noted that it is the only cranial nerve, which enters the body cavity after emerging from the brain box cranium.

It innervates the lungs. Vagus is mixed in nature because it carries both motor and sensory fibres. These nerves arise from the spinal cord and are normally nine pairs, sometimes 10th unpaired nerve is also seen. Rarely 10 pairs of spinal nerves are found.

As soon as two roots come out of the vertebral column, they unite to form a very small branch, which immediately divides into three branches. It is thicker, larger and supplies the skin and muscles of the ventral and lateral side of the trunk mostly, but in some cases it is also connected with the limbs. It is a short fine branch, which arises near the origin of each ventral branch and joins sympathetic cord of its side. Except the first pair of spinal nerve, all are distributed to the skin and muscles of the respective parts of the body.

They carry impulses from the skin receptors to the spinal cord and from the spinal cord to the muscles. Thus, their nature is mixed. The first spinal nerve is also known as hypoglossal Nerve. It supplies muscles of the tongue, floor of the buccal cavity and some muscles of the shoulder and back.

It also takes part in the formation of brachial plexus. It is a motor branch. The hypoglossal nerve, the second and the third spinal nerves form the brachial plexus. From the brachial plexus, the second nerve continues as a brachial nerve to the skin and muscles of fore-limb.

The 4th, 5th and 6th spinal nerves supply the skin and muscles of the body wall of the belly region. The 7th, 8th and 9th spinal nerves form the sciatic plexus, which gives off branches to the skin and muscles of the abdomen and hind-limbs. This plexus also supplies some branches to the large intestine, genital ducts and urinary bladder. The tenth nerve is also known as coccygeal nerve, which is not commonly found in Rana.

Whenever this nerve is present, it emerges from the urostyle through an aperture and joins the sciatic plexus. The fine branches of 10th nerve supply the urinary bladder and cloaca. There is a noteworthy point that the roots of seventh to tenth nerves first run inside the neural canal of the vertebral column for some distance to form a horse-tail shaped structure, the cauda equina.

Later on these nerves come out of the neural canal. It is a system of nerve fibres and ganglia which control and coordinate the involuntary activities of the visceral organs, such as secretion of digestive fluid, action of heart, etc. This system is autonomic in the sense that it regulates such activities of the body in which the will power of the animal is not involved, for example, the secretion of the digestive fluid is always under the control of autonomic nervous system, but the animal is not aware of it.

It consists of two longitudinal sympathetic cords, one on either side of the vertebral column ventral to the dorsal aorta, which run forward along the outer sides of the systemic arches.

Each cord bears at intervals small black nine to ten sympathetic ganglia connected with the corresponding spinal nerves by the ramus communicans a branch of spinal nerve. Anteriorly, each sympathetic cord enters the skull along the vagus cranial nerve 10th and joins with the vagus ganglion and ends in the gassenan ganglion of the trigeminal cranial nerve 5th. Posteriorly each sympathetic cord joins the 9th spinal nerve and later on ends there. The nerves arising from the sympathetic ganglia supply the respective visceral organs, such as heart, liver, stomach, intestine, kidneys, gonads, blood vessels, urinary bladder, etc.

On stimulation the sympathetic nerve fibres secrete a chemical sympathin, which stimulates the organs to function. It increases heart beat. There is no such cord or ganglia as found in the sympathetic nervous system. It consists of very small parasympathetic ganglia situated in the walls of visceral organs viscera and nerve fibres.

These are connected with the central nervous system after travelling in some cranial and spinal nerves. On stimulation, the parasympathetic nerve fibres secrete a chemical called acetylecholine, whose function is just opposite to that of sympathin. It slows heart beat. The function of sympathetic and parasympathetic nervous system is antagonistic to each other. These are nerve endings and touch corpuscles which are found in the skin.

Nerve endings are also present in the viscera soft internal organs ,. These are found as taste buds which are present in the epithelium of the tongue and the buccopharyngeal cavity,. Each eye is provided with upper and lower eye lids which are actually simple folds of skin. The upper eye lid is thick and very slightly movable. The lower eye lid is vestigial and immovable. This membrane is retracted when the frog is on land.

A harderian gland is present below the lower eye lid whose secretion lubricates the eye ball and nictitating membrane. Six other muscles are attached to the eyes which bring about the movements of the eye ball. The four muscles are recti, namely, superior rectus, inferior rectus, internal rectus, external rectus and two oblique muscles: The superior oblique is supplied by fourth cranial nerve, inferior oblique, internal rectus, superior rectus and inferior rectus are innervated by 3rd cranial nerve while external rectus gets sixth cranial nerve.

The wall of the eye ball mainly consists of three layers: Outer fibrous coat, middle vascular coat and inner nervous coat. It is the outermost layer of eye ball which is fibrous in nature. Two third part of this layer is opaque and lies inside the eye orbit and is known as sclerotic. The muscles controlling the movement of eye ball are attached on it. The remaining one-third of the outer layer is transparent and bulges out to form the cornea. The cornea is lined externally by a thin transparent membrane, the conjunctiva, which is continuous with the lining of the eye lids.

Choroid lies next to outer layer which is richly supplied with blood capillaries and black pigmented cells. It forms a circular pigmented yellow membrane, the iris towards the anterior side to enclose an oval aperture, the pupil. The iris contains both circular and radial muscles. Due to the contraction of radial muscles the diameter of the pupil is increased and by the contraction of circular muscles the diameter of the pupil is decreased.

A vascular fold of the choroid, the ciliary body lies behind the iris. The ciliary processes arise from the ciliary body. The ciliary body is poorly developed in frog.

Just behind the iris, a transparent, crystalline and almost spherical lens is situated, which is enclosed in the delicate transparent lens capsule. It is held and kept in position by the fibres of the suspensory ligament, which extend from the lens capsule to the ciliary body.

The lens divides the cavity of the eye ball into aqueous chamber and vitreous chamber. The aqueous chamber contains a transparent watery fluid, the aqueous humour, while the vitreous chamber contains a jelly like substance, the vitreous humour.

Both these humours help in refracting the light and in maintaining the shape of the eye ball. There are two protractor lentis muscles, one dorsal and one ventral, which extend between the cornea and the inner part of the ciliary body. They help in the slight adjustment of the object. It is the innermost layer of the eye on which the image is formed, that is why it is nervous and perceptive layer.

It is also called retina. It is made up of four layers. The outer layer is non-sensory pigmented cuboidal epithelium, which lies very close with the choroid. Next to the epithelial layer, a layer of cones and rods is present. Rods and cones contain visual pigments. The rods mainly enable the animal to see in the darkness, that is why these are present in large numbers, in nocturnal animals.

The cones are chiefly concerned with distinguishing colour and day light vision. The rods and cones are followed by a layer of bipolar neurons. This means that all available information bearing on the classification of hazard shall be considered together, including the results of valid in vitro tests, relevant animal data, and human experience such as epidemiological and clinical studies and well-documented case reports and observations.

Information on chemicals related to the material being classified shall be considered as appropriate, as well as site of action and mechanism or mode of action study results. Both positive and negative results shall be considered together in a single weight-of-evidence determination. Where evidence is available from both humans and animals and there is a conflict between the findings, the quality and reliability of the evidence from both sources shall be evaluated in order to resolve the question of classification.

Reliable, good quality human data shall generally have precedence over other data. However, even well-designed and conducted epidemiological studies may lack a sufficient number of subjects to detect relatively rare but still significant effects, or to assess potentially confounding factors.

Therefore, positive results from well-conducted animal studies are not necessarily negated by the lack of positive human experience but require an assessment of the robustness, quality and statistical power of both the human and animal data.

When such information raises doubt about relevance in humans, a lower classification may be warranted. When there is scientific evidence demonstrating that the mechanism or mode of action is not relevant to humans, the chemical should not be classified. However, a single positive study performed according to good scientific principles and with statistically and biologically significant positive results may justify classification.

For these three hazard classes, mixtures shall be classified based upon information on the ingredient substances, unless on a case-by-case basis, justification can be provided for classifying based upon the mixture as a whole. In these cases the mixture may be classified according to those data. The data must exclude the possibility that the ingredient will behave in the mixture in a manner that would increase the hazard over that of the pure substance.

Furthermore, the mixture must not contain ingredients that would affect that determination. When performing an assessment in accordance with these requirements, the evaluator must take into account all available information about the potential occurrence of synergistic effects among the ingredients of the mixture.

Lowering classification of a mixture to a less hazardous category on the basis of antagonistic effects may be done only if the determination is supported by sufficient data.

These principles ensure that the classification process uses the available data to the greatest extent possible in characterizing the hazards of the mixture. For mixtures classified in accordance with A. If the latter occurs, a new classification is necessary.

If mixture i or ii is already classified based on test data, the other mixture can be assigned the same hazard category. Acute toxicity refers to those adverse effects occurring following oral or dermal administration of a single dose of a substance, or multiple doses given within 24 hours, or an inhalation exposure of 4 hours. See the footnotes following Table A. Acute toxicity hazard categories and acute toxicity estimate ATE values defining the respective categories.

Otherwise, ii the appropriate conversion value from Table 1. Test data already generated for the classification of chemicals under existing systems should be accepted when reclassifying these chemicals under the harmonized system. When experimental data for acute toxicity are available in several animal species, scientific judgment should be used in selecting the most appropriate LD50 value from among scientifically validated tests.

The flow chart of Figure A. Tiered approach to classification of mixtures for acute toxicity. When there is relevant evidence of acute toxicity by multiple routes of exposure, classification is to be conducted for all appropriate routes of exposure.

All available information shall be considered. The pictogram and signal word used shall reflect the most severe hazard category; and all relevant hazard statements shall be used. Where the mixture itself has been tested to determine its acute toxicity, it is classified according to the same criteria as those used for substances, presented in Table A.

If test data for the mixture are not available, the procedures presented below must be followed. Dilution, Batching, Concentration of mixtures, Interpolation within one toxicity category, Substantially similar mixtures, and Aerosols. The acute toxicity estimate ATE of ingredients is considered as follows: Ingredients that fall within the scope of this paragraph are considered to be ingredients with a known acute toxicity estimate ATE.

See note b to Table A. The ATE of the mixture is determined by calculation from the ATE values for all relevant ingredients according to the following formula below for oral, dermal or inhalation toxicity:. This information may include evaluation of:. If sufficient information is not available to reliably estimate acute toxicity, proceed to the provisions of A. In this situation the mixture is classified based on the known ingredients only.

A statement that x percent of the mixture consists of ingredient s of unknown toxicity is required on the label and safety data sheet in such cases; see Appendix C, Allocation of Label Elements and Appendix D, Safety Data Sheets. Conversion from experimentally obtained acute toxicity range values or acute toxicity hazard categories to acute toxicity point estimates for use in the formulas for the classification of mixtures. Corrosive reactions are typified by ulcers, bleeding, bloody scabs, and, by the end of observation at 14 days, by discoloration due to blanching of the skin, complete areas of alopecia, and scars.

Histopathology should be considered to evaluate questionable lesions. Skin irritation is the production of reversible damage to the skin following the application of a test substance for up to 4 hours. Emphasis shall be placed upon existing human data See A. Classification results directly when the data satisfy the criteria in this section. In case the criteria cannot be directly applied, classification of a substance or a mixture is made on the basis of the total weight of evidence See A.

Corrosive reactions are typified by ulcers, bleeding, bloody scabs and, by the end of observation at 14 days, by discoloration due to blanching of the skin, complete areas of alopecia and scars. Histopathology should be considered to discern questionable lesions.

A separate irritant criterion accommodates cases when there is a significant irritant response but less than the mean score criterion for a positive test. For example, a substance might be designated as an irritant if at least 1 of 3 tested animals shows a very elevated mean score throughout the study, including lesions persisting at the end of an observation period of normally 14 days.

Other responses could also fulfil this criterion. However, it should be ascertained that the responses are the result of chemical exposure. Addition of this criterion increases the sensitivity of the classification system. Reversibility of skin lesions is another consideration in evaluating irritant responses. When inflammation persists to the end of the observation period in 2 or more test animals, taking into consideration alopecia limited area , hyperkeratosis, hyperplasia and scaling, then a chemical should be considered to be an irritant.

In vitro alternatives that have been scientifically validated shall be used to make classification decisions. Solid substances powders may become corrosive or irritant when moistened or in contact with moist skin or mucous membranes. Generally, such substances are expected to produce significant effects on the skin. In some cases enough information may be available from structurally related compounds to make classification decisions.

Although information might be gained from the evaluation of single parameters within a tier, there is merit in considering the totality of existing information and making an overall weight of evidence determination. This is especially true when there is information available on some but not all parameters.

Emphasis shall be placed upon existing human experience and data, followed by animal experience and testing data, followed by other sources of information, but case-by-case determinations are necessary. Tiered evaluation of skin corrosion and irritation potential. A weighting factor of 10 is used for corrosive ingredients when they are present at a concentration below the concentration limit for classification with Category 1, but are at a concentration that will contribute to the classification of the mixture as an irritant.

The approach explained in A. For mixtures containing strong acids or bases the pH should be used as classification criteria since pH will be a better indicator of corrosion than the concentration limits of Table A. A mixture containing corrosive or irritant ingredients that cannot be classified based on the additivity approach shown in Table A.

Classification of mixtures with ingredients for which the approach in Table A. Concentration of ingredients of a mixture classified as skin Category 1 or 2 that would trigger classification of the mixture as hazardous to skin Category 1 or 2.

Concentration of ingredients of a mixture for which the additivity approach does not apply, that would trigger classification of the mixture as hazardous to skin. Eye irritation is the production of changes in the eye following the application of test substance to the anterior surface of the eye, which are fully reversible within 21 days of application. A single hazard category is provided in Table A.

Category 1, irreversible effects on the eye, includes the criteria listed below. These observations include animals with grade 4 cornea lesions and other severe reactions e. In this context, persistent lesions are considered those which are not fully reversible within an observation period of normally 21 days. A single category is provided in Table A. In vitro alternatives that have been scientifically validated and accepted shall be used to make classification decisions.

Generally, such substances are expected to produce significant effects on the eyes. Although information might be gained from the evaluation of single parameters within a tier, consideration should be given to the totality of existing information and making an overall weight of evidence determination. This is especially true when there is conflict in information available on some parameters. Notes to Figure A. At present, there are no internationally accepted test methods for human skin or eye irritation testing.

Examples of, scientifically validated test methods for identifying eye corrosives and severe irritants i. Positive test results from a scientifically validated in vitro test for skin corrosion would likely also lead to a conclusion to classify as causing Serious Eye Damage. The weight of evidence including information on skin irritation could lead to classification of eye irritation. It is recognized that not all skin irritants are eye irritants as well. Professional judgment should be exercised in making such a determination.

When considering testing of the mixture, chemical manufacturers are encouraged to use a tiered weight of evidence strategy as included in the criteria for classification of substances for skin corrosion and serious eye damage and eye irritation to help ensure an accurate classification, as well as avoid unnecessary animal testing.

For mixtures containing strong acids or bases, the pH should be used as classification criteria See A. A mixture containing corrosive or irritant ingredients that cannot be classified based on the additivity approach applied in Table A. In these cases the mixture could be classified according to those data See also A. In those cases, the tiered weight of evidence strategy should be applied as referred to in section A. A mixture may be classified as Eye Category 2B in cases when all relevant ingredients are classified as Eye Category 2B.

Concentration of ingredients of a mixture for which the additivity approach does not apply, that would trigger classification of the mixture as hazardous to the eye. Skin sensitizer means a chemical that will lead to an allergic response following skin contact. The second phase is elicitation, i. For skin sensitization, an induction phase is required in which the immune system learns to react; clinical symptoms can then arise when subsequent exposure is sufficient to elicit a visible skin reaction elicitation phase.

As a consequence, predictive tests usually follow this pattern in which there is an induction phase, the response to which is measured by a standardized elicitation phase, typically involving a patch test. The local lymph node assay is the exception, directly measuring the induction response.

Evidence of skin sensitization in humans normally is assessed by a diagnostic patch test. Substances may be allocated to one of the two sub-categories 1A or 1B using a weight of evidence approach in accordance with the criteria given in Table A. Hazard category and sub-categories for respiratory sensitizers. The condition will have the clinical character of an allergic reaction.

However, immunological mechanisms do not have to be demonstrated. Relevant information includes aggravating factors both in the home and workplace, the onset and progress of the disease, family history and medical history of the patient in question. The medical history should also include a note of other allergic or airway disorders from childhood and smoking history.

It is, however, recognized that in practice many of the examinations listed above will already have been carried out. Hazard category and sub-categories for skin sensitizers. For Category 1, a stimulation index of three or more is considered a positive response in the local lymph node assay.

EC3 refers to the estimated concentration of test chemical required to induce a stimulation index of 3 in the local lymph node assay. Situations in which a high proportion of those exposed exhibit characteristic symptoms are to be looked at with special concern, even if the number of cases is small; c Positive data from appropriate animal studies; d Positive data from experimental studies in man See paragraph A.

However, in cases where evidence is available from both sources, and there is conflict between the results, the quality and reliability of the evidence from both sources must be assessed in order to resolve the question of classification on a case-by-case basis. Normally, human data are not generated in controlled experiments with volunteers for the purpose of hazard classification but rather as part of risk assessment to confirm lack of effects seen in animal tests. Consequently, positive human data on skin sensitization are usually derived from case-control or other, less defined studies.

Evaluation of human data must, therefore, be carried out with caution as the frequency of cases reflect, in addition to the inherent properties of the substances, factors such as the exposure situation, bioavailability, individual predisposition and preventive measures taken. Negative human data should not normally be used to negate positive results from animal studies.

For both animal and human data, consideration should be given to the impact of vehicle. However, a combination of two or more indicators of skin sensitization, as listed below, may alter the decision.

This shall be considered on a case-by-case basis. Consideration shall be given to classifying these substances as skin sensitizers. Therefore, classification will normally be based on human evidence, similar to that for skin sensitization. When reliable and good quality evidence, as described in the criteria for substances, from human experience or appropriate studies in experimental animals, is available for the mixture, then the mixture shall be classified by weight of evidence evaluation of these data.

Care must be exercised in evaluating data on mixtures that the dose used does not render the results inconclusive. Dilution, Batching, Concentration of mixtures, Interpolation, Substantially similar mixtures, and Aerosols.

The term mutation applies both to heritable genetic changes that may be manifested at the phenotypic level and to the underlying DNA modifications when known including, for example, specific base pair changes and chromosomal translocations. Genotoxicity test results are usually taken as indicators for mutagenic effects. The two-category system is described in the Figure A.

The scheme is, therefore, not meant for the quantitative risk assessment of chemical substances. Evaluation of the test results shall be done using expert judgment and all the available evidence shall be weighed for classification.

In those instances where a single well-conducted test is used for classification, it shall provide clear and unambiguously positive results. The relevance of the route of exposure used in the study of the substance compared to the route of human exposure should also be taken into account. The classification may be modified on a case-by-case basis based on the available test data for the mixture as a whole.

In such cases, the test results for the mixture as a whole must be shown to be conclusive taking into account dose and other factors such as duration, observations and analysis e. Dilution, Batching, and Substantially similar mixtures. Carcinogen means a substance or a mixture of substances which induce cancer or increase its incidence. Substances and mixtures which have induced benign and malignant tumors in well-performed experimental studies on animals are considered also to be presumed or suspected human carcinogens unless there is strong evidence that the mechanism of tumor formation is not relevant for humans.

Classification of a substance or mixture as posing a carcinogenic hazard is based on its inherent properties and does not provide information on the level of the human cancer risk which the use of the substance or mixture may represent.

In certain instances, route-specific classification may be warranted. The evaluations are to be based on all existing data, peer-reviewed published studies and additional data accepted by regulatory agencies.

Sufficient human evidence demonstrates causality between human exposure and the development of cancer, whereas sufficient evidence in animals shows a causal relationship between the agent and an increased incidence of tumors.

Limited evidence in humans is demonstrated by a positive association between exposure and cancer, but a causal relationship cannot be stated. Limited evidence in animals is provided when data suggest a carcinogenic effect, but are less than sufficient. Guidance on consideration of important factors in the classification of carcinogenicity and a more detailed description of the terms "limited" and "sufficient" have been developed by the International Agency for Research on Cancer IARC and are provided in non-mandatory Appendix F.

Beyond the determination of the strength of evidence for carcinogenicity, a number of other factors should be considered that influence the overall likelihood that an agent may pose a carcinogenic hazard in humans. The full list of factors that influence this determination is very lengthy, but some of the important ones are considered here.

The relative emphasis accorded to each factor depends upon the amount and coherence of evidence bearing on each. Generally there is a requirement for more complete information to decrease than to increase the level of concern. Additional considerations should be used in evaluating the tumor findings and the other factors in a case-by-case manner. It is recognized that genetic events are central in the overall process of cancer development.

Therefore evidence of mutagenic activity in vivo may indicate that a substance has a potential for carcinogenic effects. If a Category 2 carcinogen ingredient is present in the mixture at a concentration between 0.

However, a label warning is optional. A mixture may be classified based on the available test data for the mixture as a whole.

Where the mixture itself has not been tested to determine its carcinogenic hazard, but there are sufficient data on both the individual ingredients and similar tested mixtures to adequately characterize the hazards of the mixture, these data will be used in accordance with the following bridging principles as found in paragraph A. Dilution; Batching; and Substantially similar mixtures.

Some reproductive toxic effects cannot be clearly assigned to either impairment of sexual function and fertility or to developmental toxicity. Nonetheless, chemicals with these effects shall be classified as reproductive toxicants. For classification purposes, the known induction of genetically based inheritable effects in the offspring is addressed in Germ cell mutagenicity See A.

This includes, but is not limited to, alterations to the female and male reproductive system, adverse effects on onset of puberty, gamete production and transport, reproductive cycle normality, sexual behaviour, fertility, parturition, pregnancy outcomes, premature reproductive senescence, or modifications in other functions that are dependent on the integrity of the reproductive systems.

These effects can be manifested at any point in the life span of the organism. The major manifestations of developmental toxicity include death of the developing organism, structural abnormality, altered growth and functional deficiency. Effects on sexual function and fertility, and on development, shall be considered.

Systems Biology in Toxicology and Environmental Health