The end and use of swallowing these stones have been very differently ex plained. Cxsalpinus considered it rather as a medicine than as a common assist ance to digestion ; Boerhaave, as an ab sorbent for the acid of the stomach ; Redi, as a substitute for teeth ; accord ing to Whytt, it is a mechanical irritation, adapted to the callous and insensible na • ture of the coats of the stomach ; Spallanr. zani rejected all supposition of design or object, and thought that the stones were swallowed from mere stupidity. There seems not much sagacity to be discover ed in this opinion, when we consider that these stones are so essential to the doe digestion of the corn, that birds grow lean without them, although they may be most copiously supplied with food. This paradoxical opinion has, however, been already refuted by J. Hunter and 0 Fordyce. Blumenbach thinks that the stones kill the grain, and deprive jt of its vitality, which otherwise resists the action of the diges tive powers. Thus it has been found that if the oats and barley given to horses be previously heated, the animal only re quires half the quantity, and thrives equally well.
Reptiles and birds have their nostrils terminated by two longitudinal slits on the palate ; they have no velum palati, nor epiglottis.
The esophagus of the serpent kind is of immense magnitude ; for these reptiles swallow animals larger than themselves, which are retained for a considerable time in the tube, and descend into the stomach by degrees, where they are slowly subjected to the action of the gas tric juice. The whole process sometimes occupies many days, or even weeks. There is hardly any distinction between esophagus and stomach.
From the peculiar formation of the nose of fishes, and from their respir ing by means of gills, their fauces have no connection with any nasal cavity, or glottis.
The esophagus is of great width in fishes ; and is distinguished with difficul ty in many cases from the stomach. These animals swallow their food whole, without subjecting it to any mastication ; and if the stomach will not hold the Whole, a part remains in the oesophagus, until that which has descended lower is digested. The alimentary canal is gene rally very short ; sometimes extending straight from the mouth to the antis with very little dilatation, as in the lamprey (petromyzon marinus.) The crustacea, and some insects, are furnished with organs of mastication of similar structure. Their mouth is form ed of two or more pairs of jaws placed laterally. These move from without in wards, and vice versa ; whereas those of red-blooded animals move from above downwards, and back again. The parts which are termed the lips of insects are two bodies, of which one is placed above or in front of the jaws, and the other be low or behind them. The palpi or feel
ers are articulated to the jaws. All in sects which have jaws, possess the power of masticating hard animal and vegetable substances ; for these parts are of a firm horny texture, and in many cases are very large, when compared with the size of the animal.
The locust (grylli,) the dragon-fly (libellula,) the beetles, and particularly the lucanus cervus, or stag-beetle, and the staphylinus maxillosus, are examples in which the jaws are very large and manifest, and often possess denticulated edges. All the genera of the following order have jaws ; viz, the coleoptera, orthoptera, neuroptera, and hymenopte ra. The insects of the remaining orders derive their nourishment chiefly from li quids; which they get either from ani mal or vegetable substances by means of a spiral and tubular tongue, or soft pro boscis, as in the lepidoptera ; with a broad opening, admitting of extension and retraction (the hemiptera ;) or a horny pointed tube, containing sharp bristly bodies internally (the diptera and ap tera.) The stomach of the bee is a transparent membranous bag, in which the nectar of the flowers is elaborated and converted into honey. The animal vomits it up from this reservoir, and deposits it in the hive. The stomach of the crab and lobster is a very singular organ. It is formed on a bony apparatus, in short, a species of skeleton, and does not therefore collapse when empty. To certain parts of this bony structure, round the pylorus, the teeth are affixed. Their substance is ex tremely hard, and their margin is serra ted or denticulated ; as they surround the tube, near the pylorus, nothing can pass that opening without being perfectly comminuted. These bones and teeth are moved by peculiar muscles.
In those molluscs, which possess jaws, these parts are fixed in the flesh of the animal, as there is no head to which they can be articulated. They are two in num ber in the cuttlefish, are composed of a horny substance, and resemble exactly the bill of a parrot. They are placed in the centre of the lower part of the body, and are surrounded by the tentacula, which enable the animal to attach itself to any object. By means of these parts, the shell-fish, which are taken for food, are completely triturated. The common snail and slug have a single jaw, semilunar in its form, and denticulated. The tritonia has two jaws, which act like the blades or a pair of scissars. The other molluscs possess no organs of this kind, but have, in some instances, a sort of proboscis ; as the buccinum, murex, voluta, doris, scylkea, &c.