THE appearance which an animal or plant presents in the first stage of its existence is exactly the same, that of a cell, or space surrounded by walls. In the plant, CO, and HO are absorbed into these cells, and are converted into solid matter; but in the animal, there is no such power existing. The animal increases in size, and prevents its substance from wasting by appropriating to itself its ready-formed food in plants, or in other living beings of its own nature. The cellulin which constitutes the sides of the cells in plants, consists of carbon, hydrogen, and oxygen, and yields by distillation acetic acid; while the cellular matter of animals contains, in addition to these three gases, a fourth, nitrogen, and yields to heat ammonia. A plant may therefore be defined to be an organized being, which derives its food from gases and from the soil; while an animal subsists by incorporating into itself, from foreign sources, matter similar to that of which its own substance is composed. The food therefore of animals being stored up in plants for their use, it is obvious that animals are originally derived from plants, and hence the dependence of the one class of beings on the other. The main constituents of the food have already been described in treating of the composition of flour. It remains now to trace the changes which such substances undergo in their passage through the animal system. These may be divided into various stages:- 1. Mastication or chewing, and Deglutition or swallowing; 2. Primary digestion in the stomach and intestines, or solution and preparation for absorption by the lacteals to be conveyed into the blood; 3. Secondary digestion, or sanguification, or conversion into blood; 4. Deposition of solid matter from the blood, or nutrition; 5. Removal of substances from the blood, or secretion and excretion; and, 6. Exhalation, including the action of the skin and lungs, or respiration.

1. Mastication. - The organs of mastication are situated in the mouth, and consist of the teeth, tongue, and cheeks. The teeth

consist of 3 parts: the enamel (a), which covers the crown of the tooth, is formed of fibres or tubes, laid parallel to each other, fig. 99.; the ivory or dentine (b) constitutes the body of the tooth, while the crusta petrosa, or cement (d), is laid exteriorly over the fangs. The specific gravity of human enamel is 2.711; of ivory, 2.105; of elephant's ditto, 1.728; of decayed teeth, 1.533; of a human humerus, 175; of a whale rib, 2.102; of Walrus ivory, 1.909. The composition of these structures in man is represented in the following table, along with that of decayed teeth, and of bone dried at 2120.

The structure of the teeth affords some insight into the mode in which they decay, for when the enamel is worn off, the ivory is exposed in the form of tubes filled with calcareous matter. The acid of the unhealthy mucous membrane and saliva dissolves out the carbonate of lime, and renders the tubes empty, and allows foreign matter to enter. The teeth thus lose their earthy matter, and become gradually dissolved down to the pulp and nerve (c), when toothache occurs. The enamel contains very little animal matter, and is usually removed principally by friction, when the ivory disappears by decay or solution, as described. The teeth are not supplied with nerves or blood vessels, and are therefore incapable of restoring lost portions. In an adult they are 32 in number, consisting in each jaw of 4 incisors, cutting or front teeth; 2 canine, dog or eye teeth; 4 bicuspid, smaller grinders or molars, and 6 grinding teeth, larger grinders or molars. In order that the food may be reduced to a properly formed pulp by the teeth, it is necessary that it should be mixed with a proper proportion of fluid. This is supplied by the parotid gland before the ear (when swelling causing the mumps), and by the glands under the tongue and jaws, which pour out the saliva or spittle to the extent of 13 lbs. per day. This fluid contains in 100 parts 98.88 water, animal matter 0.63, chloride of calcium 0.18, potash and soda, with some animal matter, 292, phosphate of lime 017, silica '015. When the pulp has been

properly formed, it is carried to the back part of the tongue, by which organ it is projected into the throat; when the morsel is grasped by the muscles and conveyed involuntarily by the gullet, and without sensation, into the stomach. This process is usually termed deglutition or swallowing.

2. Primary digestion takes place throughout the whole of the intestinal canal, which extends from the mouth to the lower

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extremity of that tube-including, therefore, the stomach and bowels. When the masticated food arrives in the stomach, in the course of an hour or two it becomes acid, from the extrication of lactic and acetic acids. If starch has formed a part of the food, we find on filtration that the liquid does not render iodine solution blue, but purple, a proof that the starch has become dextrin or soluble starch; and in the solution there exists much sugar, although none may have been swallowed. The fibrin, casein, and albumen are all dissolved apparently by the assistance of matter derived from the secreting coats of the stomach; while fat is not dissolved, but passes directly into the blood. The lactic acid (CHO) is formed at the expense of the sugar (produced again from the starch), by the division of the atom of that proximate principle. The cause of the solution of the nitrogenous principles appears to depend upon the action of a substance, probably modified albumen, which is contained in the secretion from the coats of the stomach. The true explanation of this solution is, however, yet to be sought for. In the stomach (fig. 101.), the food assumes a more pulpy appearance than when recently chewed. A portion of it passes by the lower extremity of the stomach into the small intestines, but much of it appears to be carried away through the coats of that organ, probably into the veins. That portion which passes into the duodenum and small intestines (6) is white, and called chyle. It is taken up by the lacteals, or small vessels, which convey it into glands, and

Human stomach and intestines (Grant). 1. Gullet; 2. Stomach; 3. Small intestines; 4. Termination of small in large intestines; 5. Colon or curved gut; 6. Rectum or straight gut.

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ultimately into the thoracic duct, a tube passing upwards along the spine towards the heart. The Bile.-A little below the infe

STOMACHS OF Cow (Carus and Jones.)-(1.) Gullet by which the food passes into (2) the paunch or first stomach, whence the food passes into (3) the second stomach, or honey-comb. From this stomach it passes into the mouth, and is again masticated (chewing the cud). It is then swallowed, and goes to (4) the third stomach, or manyplies, and then into (5) the fourth stomach, or red, the true digesting stomach. Lastly, it enters (6) the small intestines.

rior or pyloric orifice of the stomach, the pancreatic or sweetbread fluid, a substance analogous to the saliva, and the bile from the gall bladder of the liver, are poured into the intestine. The bile

Lacteals which absorb the digested food from the intestinal canal, and carry it to the thoracic duct.

is a yellow fluid, becoming greenish by exposure to the air, with a sweetish bitter taste. Test.- Bile may be detected in solu

tions, by adding to the liquid containing it its bulk of SO„, and then a few drops of a weak solution of sugar. A fine pink colour is produced. When evaporated at 2120, a greenish yellow residue remains, from which alcohol takes up the greater portion, leaving a green colouring matter and some mucus undissolved. The true bile is dissolved by the alcohol, and may be deprived of colour by animal charcoal. It is then a compound of bilic or choleic acid and soda, and may be crystallized by dissolving it in 20 parts of absolute alcohol, and then adding ether till the solution becomes slightly milky; on standing, needle-shaped crystals are formed, consisting of NaO C44 H40 NSO. Bilic acid may be separated by precipitating by acetate of lead, and throwing down the lead by SH. The following table gives the composition of the substances obtained from bilic or choleic acid.

Bilic acid is colourless or yellow, like gum, of a resinous consistence, and is very bitter, dissolving in water and alcohol, but not in ether. When boiled with HCl, a resin falls down, which is choloidic acid (C60 H50 011) a yellow resinous body, insoluble in water, soluble in alcohol, uniting with bases. In the solution we find common salt, sal ammoniac, and taurin (C4 H, NS2 06), fine crystalline needles, soluble in water, insoluble in alcohol, by the addition of a large quantity of which fluid it crystallizes out of the liquid. When choloidic acid is boiled with NO, in a retort, a violent action occurs, and much foam is produced. There distil over a heavy and light oil. The upper layer consists of capric, 'caprylic, valerianic, butyric, and acetic acids, principally the acids of butter, while the heavy oil contains nitrocholic acid (corresponding with 2NO, and 1 urea), forming a salt with potash and cholacrol (C, H, N, O13), both of which substances have a powerful odour. In the retort remain choloidanic acid in hairy tufts, (C16 H12 O,) and cholesteric acid, a gummy substance (C ̧ H404), forming a crystalline salt with silver: this acid is isomeric with pyrogallic acid. Cholic acid in fine needles is obtained by boiling bilic acid or bile with caustic soda, and precipitating the acid by acetic acid. By NO, it is converted into cholesteric acid. In the bile we find a pearly substance, melting at 279°, soluble in