MOUTH, (ESOPHAGUS, AND STOMACH.
We have already shown the most im portant circumstances relating to the mouth. Many species of the genus si mia, as well as the hamster, (marmota cricetus) and some similar species of the marmot, are provided with cheet pouch es, in which the former, who live on trees, place small quantities of food as a re serve : the latter employ these bags to convey their winter provision to their burrows.
The peculiar glandular and moveable bag, (bursa faucium,) which is placed behind the palate, has hitherto been only observed in the camels of the old world and it probably serves to lubricate the throats of these animals, in their abode in the dry sandy deserts which they inha bit.
The oesophagus of quadrupeds ia dis. tinguished from that of the human sub ject by possessing two rows of muscu lar fibres, which pursue a spiral course, and decussate each other. In those car nivorous animals which swallow vora ciously, as the wolf, it is very large ; on the contrary, in many of the larger her bivora, and particularly in such as ru minate, its coats are proportionably stronger.
No mammalia possess an uvula, except man and the simia.
In some herbivora the stomach has an uniform appearance externally ; but it is divided into two portions internally, ei ther by a remarkable difference in the two halves of its internal coat, as in the horse, or by a valvular elongation of this membrane, as in several animals of the mouse-kind. This is also the case in the hare and rabbit, where also the food in the two halves of the stomach differs very much in appearance, particularly if the animal has been fed about two hours before death.
In these animals the left hdlf of the sto mach is covered with cuticle, while the other portion has the usual villous and se creting surface. The left portion of the cavity may be regarded as a reservoir, from which the food is transmitted to the true digestive organ ; and the different states in which the food is found in the two parts of the cavity justify this sup position. Hence these stomachs form a connecting link between those of rumi nating animals on one side, and those which have the whole surface villous on the other.
On the whole internal surface of the horse's stomach there are found, in vast abundance, particularly in spring, the larva' of two species of cestrus ; viz. the cestrus equi (which Linnxus called cestrus bovis,) and the ce. lizmorrhoidalis, the true history of which has been eluci dated, for the first time in modern days, by that excellent veterinary surgeon Mr.. Bracy Clark, in the "Transactions of the Linnean Society," vol. 3.
These animals, which are called botts, attach themselves to every part of the stomach, but are in general most nume rous about the pylorus ; and are some times, but much less frequently, found in the intestines. They hang most com monly in clusters, being fixed by the small end to the inner membrane of the stomach, where they adhere by means of two small hooks or tentacula. When re moved from the stomach, they will at tach themselves to any loose membrane, and even to the skin of the hand : for this purpose they draw back their hooks al most entirely within the skin, till the two points of these hooks close to each other; they then present them to the mem brane, and keeping them parallel till it is pierced through, they expand them in a lateral direction ; and afterwards, by bringing the points downwards, or towards themselves, they include a suffi cient piece of the membrane with each hook, and thus remain firmly fixed, for any length of time, without any further exertion of the animal. They attain their full growth about the latter end of May, and are coming from the horse from this time to the latter end ofJune. On drop ping to the ground, they soon change to the crysalis, and in six or seven weeks the fly appears. This bott is larger and whit er than that of the oestrus hxmorrhoidalis,. which has a reddish cast ; but in its struc ture, and situation in the animal, resem bles the former. It is found, however, to hang about the rectum, previously to quitting it,1which the large horse-bott ne ver does.