But although it may be supposed, that the division of the tube into the great and small in testines refers to their difference of size alone, it is to be observed that they perform very differ ent functions, and are subservient to very differ ent purposes in the animal ceconomy. It is in the small intestines, and more especially in the first portion of them, termed the duodenum, that what must be considered as the most essen tial or specific part of the function of digestion is effected, the formation of chyle, while it is almost exclusively in the duodenum and the other small intestines, the jejunum and the ileum, that the chyle thus produced is taken up by the lacteals, in order to be conveyed to the thoracic duct, and finally deposited in the bloodvessels.
The use of the large intestines, and more es pecially of the colon, which constitutes a con siderable proportion of the whole, appears to be more of a mechanical nature, serving as a depo sit or reservoir, in which the residuary matter is received and lodged, fora certain period, until it is finally expelled from the system. The division between the parts of the small intestines, to which the names jejunum and ileum have been applied, is entirely arbitrary, as they ap pear to be precisely similar to each other, both in their structure and their functions. But the case is very different with respect to the duode num, which in both these respects possesses a clearly marked and distinctive character. Of this anatomists have long been well aware, and it has accordingly been made the object of par ticular attention, and has even received the ap pellation of the accessory stomach ; but we shall enter more particularly into the consideration of this subject when we come to treat upon the difference between chyme and chyle, and the nature of the process by which it is effected.
The peculiarities of the digestive organs in the different classes of animals arc interesting, not merely as affording remarkable examples of the adaptation of the animal to the situation in which it is placed, but are especially worthy of our notice on this occasion, as serving to illus trate the nature of the operation generally, and the mode in which its various stages are related to each other. The most remarkable examples of this kind are the complicated stomachs of the ruminant quadrupeds, and the muscular sto machs of certain classes of birds.• The ruminant animals belong to the class of the mammalia, and are such as feed principally upon the stalks arid leaves of plants. The quan tity of food which they take is very consider able ; it is swallowed, in the first instance, al most without mastication, and is received into the first stomach, a large cavity, which is termed the venter magnus, pease, or paunch.t The food, after remaining for some time in this sto mach, for the purpose, as it would appear, of being macerated, is next conveyed into the second stomach, a smaller cavity, the internal coat of which is drawn up into folds that lie in both directions, so as to form a number of an gular cells, from which circumstance it has received the appellation of reticulum, bonnet, or honeycomb. The reticulum is provided with a number of strong muscular fibres, by which the food is rounded into the form of a ball, and is propelled along the oesophagus into the mouth. It is now completely masticated,
after having been properly prepared for the pro cess by its previous maceration in the paunch ; this mastication constitutes what has been termed chewing the cud, or rumination.
When the food has been sufficiently com minuted it is again swallowed, but by a pecu liar mechanism of muscular contraction the passage into the venter magnus is closed, while an opening is left for it to pass into the third stomach, termed omasum,,leuillet, or muniplies ; it is smaller than any of the other cavities, and its internal coat is formed into a series of strong ridges and furrows, but without the transverse ridges of the reticulum. From the omasum the food is finally deposited in the fourth stomach, the abomasum, caillt Ile, or reed, a cavity consi derably larger than either the second or third stomach, although less than the first. It is of an irregular conical form, the base being turned to the omasum ; it is lined with a thick mucous or vinous coat, which is contracted into ridges or furrows, somewhat in the manner of the oma sum, and it appears to be that part of the diges tive apparatus which is analogous to the single stomach of the other mammalia, where the ali ment undergoes the process of chymification, the three first stomachs being intended to macerate and grind it down, in order to prepare it for the action of the gastric juice. (See IttimINANTIA.) Although we conceive that the operation of the different parts of this complicated apparatus is pretty well understood, it still remains for us to inquire into the final cause of the arrange ment, or why the maceration and mastication of the food in certain classes of animals should he effected in a manner so different from what it is in others, which, in their general structure and functions, the most nearly resemble them. The opinion which was entertained on this subject by the older anatomists, and which may be still regarded as the popular doctrine, is, that the nature of the food of these animals, and the large quantity ofit necessary for their support, requires a greater length of time for its comminution and a greater quantity of the mucous secretions than it could obtain by the ordinary process. But although there may be some foundation for this opinion, the more extended observations of modem naturalists show, that it does not apply in all cases, and that there are so many excep tions to the general rule as to lead us to doubt the truth of the position.* It is to be ob served, that when animals with ruminant sto machs take in liquids, the fluid passes immedi ately into the second stomach,t where it is mixed with the aliment after it has been macerated in the venter magnus, and probably moulds it into the proper form, for its return along the esophagus into the mouth. While the young animal is nourished by the mother's milk, the fluid is conveyed, in the first instance, through the third stomach into the fourth, and it is not until it begins to take solid food, that the process of rumination is established. It is hence concluded, that the animal possesses the power of conveying the food at pleasure either into the first or the third stomach, and of return ing it from the second into the mouth ;I these, like many other voluntary acts, being of the kind which are termed instinctive.