ORGANS DIGESTION.
THE organs employed in the digestion of aliments are numerous and important. The function of diges tion includes several operations, as mastication, or the chewing of the food ; deglutition, or the conveying of it into that organ in which its solution is to be effected ; chylification, or the reducing of it into a fluid homoge neous mass, proper for the nutrition of the animal. The several organs by which these operations are carried on. will require our particular attention ; and we shall con sid:r them in the order in which we have enumerated the operations.
When the food is received into the mouth, it is there, by the action of the lower jaw pressing the teeth against each other, reduced into small pieces, or so divided that it may be intimately mixed with the saliva, and thus prepared for a more easy and speedy solution in the stomach. The organs by which mastication is per formed are chiefly the teeth, but these are assisted in the process of insalivation by the lips, the tongue, and other parts of the mouth. The mouth in general, as including so many important organs, first deserves no tice ; and after haying considered its general form and varieties, we shall briefly describe each of its compo nent parts, so far as they have not already been antici pated.
There are five principal cavities generally described by anatomists in the human those of the skull, the mouth, the chest, the belly, and the pelvis. We base already examined the contents of the first of these cavities, and we nosy proceed to the second. The ca vity of the mouth is bounded on the forepart, or antinial aspect, by the lips ; on the back part, or intact, by the velum pendulum palati, or hanging palate; on the supe rior, or coronal aspect, by the vault of the palate ; be low, or basilad, by the tongue, and the general mucous membrane ; on the sides, or lateral, by the cheeks. In the ordinary position of the human body, the direction of this cavity is nearly horizontal. Its diameter from before backwards, is determined chiefly by that of the palatine arch, while its diameter from side to side va ries with the motion of the cheeks. The size of this
cavity is of course extremely various in different indi viduals ; but in general, it is said by Ilichat to bear an inverse proportion to the magnitude of the brain. Its general form is that of air oval, being- determined by the circumference of the lower jawbone at the chin, and by that of the hanging palate, both of which are rounded.
The lips generally differ from each other in point of thickness, tire upper being in most individuals thicker and more prominent than the lower ; though one of our most celebrated old poets, Suckling, has considered the opposite proportion as constituting a principal feature inn the portrait of his mistress.
" Iier Ups were small, and one was thin, Compar'd to that was next her elnri, Some bee had stung it newly." The upper lip has a superficial channel in the place of the mesial line, running from the root of the partition of the nostrils, and arising from tire strong adhesion of skin to the muscles in this part. The lower lip has a slight prominence, situated vertically in its middle, an to the channel in the upper Hp, and it is divi ded from the chin by an evident transverse depression. The union of the lips at the corners of the mouth, con stitutes what are called C07717088 IlreR of the lips.
The substance of the lips is composed of three prin cipal layers, a peripheral or outer layer, formed by the skin ; a central or inner layer, formed by the mucous membrane of the mouth by doublings, to be presently described ; and an intermediate layer, formed by the muscular fibres of the orbiciduris enis, and the It vator and depressor muscles of the mouth. The first of these lavers is remarkable for its thinness, especially on the edges of the lips ; the second is thick, of a loose texture, very red, from its numerous blood-vessels, and is fur nished with a great many mucous glands of considerable size, lying between it and the muscular fibres, and com municating with the cavity of the mouth by several ex cretory ducts. The muscular fibres of the third layer are separated from the first layer by cellular substance, in which there is scarcely any appearance of fat.