Page:The American Cyclopædia (1879) Volume V.djvu/186

 182 COMPAKATIVE ANATOMY destined to tear and to cut ; and the molars or grinders, with a wide and irregular surface, for crushing and bruising the food. These differ- ent kinds of teeth are arranged in mammals according to the nature of their intended nour- ishment ; and a simple inspection of the denti- tion indicates the kind of food, the habits, and even the structure of most animals. In the carnivora, or flesh-eaters, the canines are long, and the molars are compressed and sharp- edged, shutting like the blades of scissors ; in the insect-eaters the molars are beset with conical points, meeting accurately in each other's interstices ; in the fruit-eaters the sur- face of the molars is provided with rounded tubercles ; in those which feed on hard grains, the grinding surface is flat and rough like a millstone ; in the gnawing animals the in- cisors are greatly developed, and the enamel is so arranged that the wearing of the tooth keeps its edge sharp and chisel -shaped. Of these different forms of teeth the molars are the most constant and important, bearing the strictest relation to the habits of the animal. Between the carnivora and the herbivora, the flesh-eaters and the vegetable-feeders, stands man: he has all the kinds of teeth equally de- veloped, of the same length, and in an uninter- rupted series, and of course his natural food is a mixture of animal and vegetable substances; placed in unnatural circumstances and in ex- tremes of heat and cold, the animal or the vege- table element must predominate for the preser- vation of health, according to latitude. Teeth are sometimes used wholly as weapons of at- tack or defence, as the long incisors or tusks of the mastodon and elephant ; or as the single incisor of the narwhal, developed in the male, generally on the left side, into a long spirally twisted horn, that of the right side being ru- dimentary and concealed in the jaw, and both being rudimentary in the female. The tusks of the walrus serve also in locomotion, for pull- ing its unwieldy body up steep banks or blocks of ice. The teeth are wanting in the ant- eaters and pangolins ; in the Greenland whale they are replaced by large, flexible, horny plates, called whalebone, in the upper jaw, the lower one having neither teeth nor plates ; the upper incisors are wanting in ruminants, the lower in the walrus ; the canines are absent in rodents, some ruminants, and most female soli- peds ; in the ornithorhynchus of New Holland the muzzle is prolonged into a wide, horny beak, flattened like that of a duck, and like it furnished with transverse lamellae on the edges. The salivary glands are largest in herbivora. The gullet is wide and dilatable in carnivora, narrow in herbivora, and its fibres are ar- ranged generally very much as in man. The shape of the stomach varies much according to the food ; it is usually simple, but multiple in the ruminants. The animal food of the car- nivora requires a simple stomach and a short alimentary canal ; in the quadrumana and less carnivorous families, where the food is more mixed, the organ becomes larger transversely ; in the lion the intestine is not more than three times the length of the body, and in some others of the family it is difficult to distinguish the large from the small. In the ruminants the stomach consists of four distinct cavities : the first, or paunch, receives the crude, unmas- ticated food while the animal is grazing ; this occupies a large part of the abdomen, espe- cially on the left side ; the second, or honey- comb, is small, on the right of the gullet and in front of the paunch, of which it seems a mere appendage; its mucous membrane pre- sents the polygonal cellular structure noticed in tripe; the third, from its numerous folds, is called the manyplies ; and the fourth, at the right of the last, the abomasum or ren- net bag ; this secretes the gastric juice, and is the proper digestive cavity. The first three stomachs communicate directly with the gullet, which opens almost equally into the first and second, and by a narrower canal into the third. It may appear strange that while the coarse food enters the first cavity, after it has been brought back to the mouth and masti- cated it should enter the third ; the experi- ments of Flourens show that this is the neces- sary result of the anatomical disposition of the parts, as follows : the unchewed food, when it arrives at the part of the gullet which is con- tinued in the form of a tube, mechanically separates its borders and falls into the paunch ; but when drinks or fine semi-fluid substances present themselves, the canal is not dilated, and they naturally pass chiefly into the third cavity ; it is consequently the occlusion or dila- tation of this canal which determines the en- trance of the food into the first or third stom- ach, and it is the volume of the substances swal- lowed which opens or not this canal. The act of regurgitation from the paunch to the mouth is supposed by some to commence in the second cavity, which seizes a portion of the alimentary mass, compresses it into a rounded form, and forces it into the gullet, by whose vermicular contractions it reaches the mouth ; others be- lieve, with Flourens, that the two stomachs force the mass into the oesophageal canal, which detaches a portion and forces it upward. Of these cavities the first is by far the largest, and the third the smallest ; in early life the fourth is the only one developed for the recep- tion of milk. In the camel and dromedary the paunch is fitted to receive large amounts of water in one of its compartments, the first answering to a paunch, and the second, the water cavity, to the honeycomb. As a gen- eral rule we find the stomach most com- plicated in herbivora, and the simplest in car- nivora ; yet there are many exceptions to this ; for instance, in the horse, whose food differs but little from that of the ox, the stomach is simple ; in some carnivorous cetaceans it is very complicated. The intestines are generally long, large, and sacculated in proportion to the vegetable nature of the food, but to this also