Page:The American Cyclopædia (1879) Volume V.djvu/184

 180 COMPARATIVE ANATOMY eyes, like the ears, are proportionally larger than in the day feeders, and their pupil, when contra'cted, assumes the form of a vertical slit instead of a circular aperture ; in the moles and subterranean species the eyes are extremely small, and sometimes quite rudimentary ; in the aquatic genera the lens is more spherical, as in fishes, and in cetacea the lids are imperfectly developed and the whole organ comparatively small ; in carnivora especially there is at the bottom of the eye a brilliant tapetum, which shines at night with metallic reflections ; the lids are generally formed as in man, the upper being the largest and the most movable ; ex- cept in man and monkeys, there is usually a third nictitating membrane. The direction of the eye in man and monkeys is forward ; but as we descend in the scale it becomes lateral, so that the animal cannot see directly before him, and the sphere of vision becomes different for each eye. Organs of Alimentation and Digestion. There is no organ so characteristic of the animal, as distinguished from the vege- table, as an internal digestive cavity for the conversion of organic substances into nutritive material. In the sac-like polyps the food is introduced into the simple stomach, and dis- solved without any mechanical division ; in the echinoderms there is a complicated apparatus of teeth, and the digestive cavity is arranged in a radiating manner ; in the higher invertebrates and all the vertebrates there is a distinct mouth, an apparatus for mastication, a stomach for di- Jgestion, and an intestine from which the nutri- ent matters are absorbed and the useless mate- rials are expelled. Accessory salivary, biliary, and pancreatic organs are found from the higher radiates up to man ; in vertebrates the teeth are confined to the cavity of the mouth, and generally to the jaws, none being found in the stomach. Fish are very voracious, and most of them live upon animal food, swallow- ing whatever small prey they can obtain; a few have no teeth, but they are usually well provided with them, the sharks having several rows; the teeth are found not only in the jaws, but on the palate and vomer, the tongue, the branchial arches, and pharyngeal bones; they are numerous, without roots, united to the bone which supports them, deciduous and replaced by others growing under or by the side of the old ones, and thinly covered with enamel ; the form is generally conical, as they serve only to retain or tear their food, rarely to crush and grind it; they are numerous and sharp in the pike and salmon, serrated in some sharks, flat and pavement-like in the rays, strong fangs in the wolf-fish, in others soft and velvety, tuberculated, or sharp-edged, and ab- sent in the sturgeon and the sucking genera : to fit them better for their prehensile office, they are placed alternately, and not opposed to each other as in masticating animals. The salivary glands are absent, or very rudimentary; the gullet is short and wide, with its membrane folded longitudinally; the stomach varies in shape from globular to long and tapering, with both orifices near together and guarded by con- strictor muscles ; the intestine is short as in all carnivorous feeders, and not divided distinctly into large and small ; to compensate for its shortness, the intestine in the sharks is provided with folds arranged in a spiral or longitudinal direction, which delay the passage of the food and greatly increase the absorbing surface ; the anus varies in position from under the throat to the base of the tail. The liver is soft, light- colored, of large size, and many-lobed, discharg- ing its bile into the commencement of the intes- tine, while the pancreas pours in its secretion on the other side ; the latter organ is a large gland in the shark, but it is more commonly a series of tubes or crecal appendages, the simplest form of a gland, placed around the pylorus; digestion is rapidly performed, and the chyle is taken up by numerous lacteals which end in the venous system near the heart. The spleen is small, of various forms, attached to the stomach, generally simple, but lobulated in the sharks. The amphibia resemble fishes in their digestive apparatus, in their prehensile teeth in the palate and jaws, in the absence or rudi- mentary condition of the salivary glands, in the short and wide gullet, narrow stomach, and short and simple intestine ; in some of the higher forms they approach the reptiles, in the less numerous teeth, elongated tongue, and distinct small and large intestine. In snakes and saurians, mostly carnivorous, the intestirial canal is shorter than in the herbivorous testu- dinata. In serpents, which feed on living prey, the sharp conical teeth are directed back- ward, and the bones to which they are attached are freely movable, enabling them to swallow animals considerably larger than themselves; the venomous genera have in front of the teeth of the upper jaw two long curved fangs, com- municating by a canal or a groove with the poison gland behind and below the orbit ; the muscles which close the jaws press the venom into the wound made by the teeth ; in the rat- tlesnake these fangs are movable, and may be bent backward in a fold of the gum when not in use; behind the ones actually employed, there are the rudiments of others which soon complete the terrible armature if one fang hap- pens to get broken. The tongue is long, sheath- ed, and bifurcated ; salivary glands are present ; the gullet is long and very extensible; the stomach capacious, simple, and capable of great distention, separated from the intestine by a distinct valve ; the duodenum receives the bil- iary and pancreatic secretions, and begins to present a villous surface ; the large intestine is distinguishable from the small by its size, and ends in the cloaca with the ureters and genital openings ; the liver, spleen, and pancreas are elongated to conform to the shape of the body. In the carnivorous saurians the arrangement is equally simple, though the teeth are fewer, chiefly in the jaws, the stomach short and rounder; in the iguana and other vegetable