CHAPTER XI. PLATE XXII.- - PACKAGE OF THE VISCERA, AND MESENTERY. FIG. 1. In this plate the parietes of the chest and abdomen, with the omentum, are removed to show the viscera in situ; a, the heart; b, the aorta; c, the descending vena cava; d, the lungs divided by the mediastinum into two portions; three lobes belong to the right, and two to the left portion of the lungs; e, the diaphragm, or that muscle which separates the thorax from the abdomen; f, the liver; g, the gall-bladder; h, the stomach; i, the spleen; k, the large intestines; l, the small intestines; m, the bladder. The viscera of the thorax and abdomen, i. e. the viscera of organic life, are irregularly disposed. The agents of volition are double, but the instruments of involuntary motion, namely, the in terior life, are single, and at least are irregular in their form. The several viscera are correctly described in the Theology, and sufficient is said for the purposes for which they are introduced. To the supposed use of the spleen only an objection must be taken: various hypotheses have been entertained as to its office, but none are conclusive; the most probable is, that it is a source of supply of blood for furnishing the gastric secretion, or that the blood undergoes some important change in it. FIG. 2. The mesentery. This membrane is formed by a reflection of the peritoneum from each side of the vertebræ ; it connects the intestines loosely to the spine, to allow them a certain degree of motion, yet retains them in their places; and furnishes their exterior covering. Between the laminæ of, a, the mesentery, are received the glands, vessels, and nerves; and its extent admits of a proper distribution of each. CHAPTER XII. PLATE XXIII.· FIG. 1. The upper mandible of the duck, on which are distributed the first and second branches of the fifth pair of nerves; the former passing through the orbit to the extremity of the bill, and, together with the latter, supplying the whole palatine surface. This gustatory sensibility is the more necessary to those races of birds called palmipedes, such as penguins, the wild goose, ducks, &c. and the grallæ, such as water-hens, curlews, woodcocks, &c. their sight being of no assistance to them in finding their prey in the mire. FIG. 2. A small portion of the human intestine cut open in order to show the valvule conniventes. It may be questioned, whether these extremely soft rugæ or folds of the villous coat of the intestine can in the least retard the passage of the food through its canal; nor does the erect attitude of man require them; for, since there are as many of the convolutions of the intestines ascending as there are descending, the weight of the food can have no influence in the action of the intestine: it is certain, however, that this arrangement of the internal coat, affords a more extensive surface for the lacteals and secreting vessels ; and this appears to be the real use of the valvula conniventes. FIG. 3. The air-bladder in the roach. This vessel differs in size and shape, in different species of fish; generally communicating, by one or more ducts, either with the esophagas or stomach; by which means the fish receives or expels the air, thus sinking or rising without effort: but as some are destitute of this organ, it is considered as an accessary instrument of motion. Such fish live almost uniformly at the bottom of the water. FIG. 4. The head of a viper of the natural size. FIG. 5. The fang magnified, at the root of which is the gland which secretes the venom: a hair is represented in the tube through which the poison is ejected. FIG. 6, 7. See note, p. 126 |