Page:The American Cyclopædia (1879) Volume V.djvu/185

 COMPARATIVE ANATOMY 181 feeders, the intestine is the longest. In testu- dinata, instead of teeth the jaws are armed with sharp edges of horn ; the tongue and gullet are provided with long papillae, sharp in the marine species ; the salivary glands are tolerably de- veloped ; the gullet is long, wide, and muscular, and the stomach wide and fleshy ; the intestine is about six times the length of the body, and the colon has a short, wide caecum ; the canal opens into a general cloaca. The food of birds is so various that their digestive apparatus would be expected to present considerable dif- ferences, and in no part is there greater variety than in the bill, which in most species is the principal organ of prehension, whether the food be seeds, insects, fish, or the flesh of animals. The bill furnishes to the zoologist as good char- acters for the classification of birds as do the teeth for that of mammals ; its exterior and the sharp edges are covered with solid horn, but it never has any true teeth, so that there is no proper mastication in this class ; the birds of prey have the upper mandible short, strong, curved, and terminating in a sharp point, and hi the falcons with a tooth -like process on each side, indicating by these characters the more or less carnivorous propensities of the genus ; the tooth-billed hawks are the boldest, while the vultures, with their more elongated beaks, rarely attack living animals ; in the parrots and granivorous birds it is broad and powerful to break hard shells and seeds ; in aquatic genera it is broad for obtaining worms and insects from water and mud ; in insect-eaters it is long and slender, or short and broad, according as the prey is taken on the wing or not, as in the bee- eater in the first case, and the whippoorwill in the second ; it is long and straight in the king- fisher and heron for seizing small fish or rep- tiles ; in the pelican the under mandible is pro- vided with a large pouch for holding fish, and in the hornbill the upper is surmounted by a large and hollow casque. As the food does not undergo mastication in the mouth, the salivary glands are small ; the gullet is wide and mus- cular, and capable of great distention in birds of prey ; at the lower part of the neck it com- municates with a membranous pouch, called the crop, in which the food undergoes a soft- ening preparatory to stomachal digestion ; the crop is largest in the granivorous birds, but it is found in the rapacious orders, though absent in the ostrich and the fish-eaters. Below this the gullet becomes smaller, but shortly dilates again into a second digestive cavity, or proven- triculus, whose internal surface is studded with numerous follicles, generally of small size, some- times hardly perceptible, but large in birds which have no crop ; it secretes a fluid analo- gous to the gastric juice. This second stomach opens into a third, the gizzard, where chy- mification is completed, of variable size and structure ; in carnivorous birds the gizzard is thin and membranous, while in the granivorous it is thick and muscular for compressing and crushing their hard food, performing the office of teeth ; the lining membrane assumes a hard cartilaginous character, just as the skin of the palm and heel of man does; when circum- stances favor or require it, mucous membrane may thus change into skin, as far as its dense cuticular layer is concerned ; the power of the gizzard in the ostrich is enormous, wearing down the hardest substances; in gallinaceous birds its grinding action is assisted by the swallowing of pebbles, which serve the purpose of the gas- tric teeth of crustaceans and other inverte- brates ; the food of a bird may be known by the simple inspection of its gizzard, so close is the relation between its muscular power and the substances to be reduced by it. The intestine is much shorter than in most mammals, and consists of small and large, the latter having two tubular appendages, or cceca, at its junction with the former ; these caeca are very small in the birds of prey, and largest in the gallinaceous order; the rectum is dilated near the anus, forming a cloaca, in which the ureters, ovi- ducts, and male genitals end. The liver is large, filling a considerable part of the thorax and abdomen, usually two-lobed, with a gall blad- der and hepatic ducts ; the pancreas is long and narrow, lodged in the first convolution of the small intestine ; the spleen is small, variously shaped, and situated beneath the liver. The teeth are most perfectly developed in mammals, in which they serve not only for mastication, but for defence, attack, and locomotion. The structure of teeth presents three different sub- stances : a central portion, forming the princi- pal part of the bulk, characterized by minute canals radiating from the pulp cavity, the ivory or dentine ; the enamel, investing the exterior and crown with a thin layer of extreme hard- ness ; and the cementum, or crusta petrosa, covering the roots and sometimes around the crown with a thin lamina like bone. These three substances are well seen in the grinder of an elephant, in which the central part is made up of ivory, in a series of ridges covered with enamel, and this last, except on the surface, concealed by crusta petrosa; the hard enamel leaves a projecting grinding surface, while the other two substances are worn away. The teeth in vertebrates are shed at least once, and in some many times ; those of fishes and reptiles are continually undergoing this process ; in most fishes and snakes the teeth shed have between them others which remain attached to the jaw or the soft parts, until new ones are formed, to be shed in their turn ; in some fishes, the crocodile, and most mammals, the new teeth are developed below the old, which they push out, after the roots are absorbed and the crown drops off; in the elephant and mastodon the new teeth are formed behind the old ones, gradually sliding forward as the latter are worn away. In man and in most mammals there are three kinds of teeth: the incisors, in front, with thin and cutting edges ; the canines, coni- cal, next to the incisors, four in number, in all animals except man longer than the other teeth,