DIGESTIVE APPARATUS OF BIRDS.

185

siderable modification of form, resulting from the fact that, as these animals do not masticate their food, they require some Esophagus

Ventriculus
Succenturiatus)

Gizzard

Liver

Gall-bladder
Bile ducts

Pancreas

Duodenum

Cœca

Large Intestine

Ureter

Oviduct

Cloaca
Anus

Fig. 111.-DIGESTIVE APPARATUS OF FOWL.

other means of reducing it. This means is provided for them in their stomach. In the tribes whose food is of such a nature as to require being moistened before it is rubbed down, and especially in those which feed upon grains, the oesopha gus has a pouch-like dilatation, termed the crop or craw (fig. 111); in this it is retained, and exposed to the action

186

TRITURATING ACTION OF GIZZARD.

of fluid secreted by its walls, just as it is in the paunch of ruminant quadrupeds. This crop is of enormous size in some of the granivorous (grain-eating) birds, such as the Turkey. The second stomach (or ventriculus succenturiatus) is the one in which the gastric juice is secreted; but this is seldom large enough to retain the food, which passes-on through it to the gizzard, a hollow muscle, furnished with a hard tendinous lining. In the granivorous birds this is extremely strong and thick; and pieces of gravel are swallowed by them, which, being worked-up with the food by the action of the gizzard, assist in its reduction. In the rapacious flesh- or fish-eating birds, however, no such assistance is required, the food being easy of solution; the walls of their gizzard are thin, possessing but few tendinous fibres; and the three cavities of the stomach are almost united into one.

201. Various experiments have been made to test the mechanical powers of the gizzard of Birds. Balls of glass which they were made to swallow with their food, were soon ground to powder; and the points of needles and of lancets, fixed in a ball of lead, were blunted and broken-off by the power of the gizzard, whilst its own internal coat did not appear to be in the least injured. On the other hand it has been ascertained, that grain enclosed in metal balls which protected it from the mechanical action of the gizzard, but which were perforated so as to afford the gastric fluid free access to their contents, was not in the least digested; so that the utility, and even the necessity of this operation, become evident.

202. As there are few animals, save the Mammalia, that perform any proper masticaton in their mouths, the grinding down of their food (where it is of such a nature as to require it) must be performed in the stomach; and accordingly we find many tribes, belonging to different divisions of the animal kingdom, in which a gizzard, or something analogous to it, exists. It is possessed by almost all Cephalopods, and by many of the Gasteropods. In the walls of the stomach of some of these last, there is a considerable amount of mineral matter deposited, intermixed with the hard tendinous fibres of which they chiefly consist. A powerful gizzard is also found in many Insects, but here it is placed above the digestive stomach (fig. 112, c). The accompanying figure exhibits

REDUCING APPARATUS OF INSECTS, ETC.

187

the alimentary canal of a Beetle, from its commencement to its termination. At a is seen the head, bearing the jaws, &c. ; from this the gullet passes straight backwards, and is dilated

Fig. 112.-DIGESTIVE APPARATUS OF

BEETLE.

a

into a crop at b, below which is the gizzard, c. This opens at its lower end into the true digestive stomach, d; which is surrounded by an immense number of little follicles or bags, by which the secretion of the gastric juice is effected (§ 204). Into the lower end of this, b the long vessels, e, open, C which constitute in Insects ths only rudiment of a liver (§ 358). In many of the Crustacea, the walls of the d stomach are beset with regular rows of teeth, which are moved by the action of powerful muscles. These teeth are cast or shed at the same time with the shell. e In the Wheel-Animalcules, the place of the gizzard is occupied by a curious pair of jaws, armed with teeth; by the working of which, the food is effectually crushed. In the Bryozoa, a gizzard exists between the œsophagus and the true digestive stomach; and the stomach itself is surrounded by the little follicles which secrete the bile, and pour it into that cavity (§ 115).

203. In animals which subsist exclusively on flesh, however, no such complicated apparatus exists. Thus in Serpents (fig. 34), the stomach is but a slight dilatation of the alimen

188

DIGESTIVE APPARATUS-GASTRIC DIGESTION.

tary tube; and it is not easy to say where it commences and terminates. In Spiders and Scorpions, too, which live upon the juices they suck from other animals, the alimentary tube is very simple; and it is scarcely dilated into a proper stomach. And in most of the Radiated classes, we find the stomach to possess only one orifice, through which the undigested residue of the food is cast out, as well as fresh supplies taken in. But this stomach is not always a simple bag; thus in the Star-fish it sends prolongations into the rays, the use of which is at present undetermined. There are certain animals in which no digestive cavity exists: their sustenance being derived either from the juices prepared by other animals, in whose tissues or cavities they are imbedded, and being introduced by absorption through the whole surface, as is the case in the lower Entozoa (fig. 53); or from particles which are drawn into the midst of the soft gelatinous substance of their bodies, and undergo a sort of diges tion there, as is the case with the Rhizopoda (§ 129).

A

Gastric Digestion :-Chymification.

204. The food which has been reduced in the mouth by the action of the teeth, or in the stomach itself by the movement of its own tendinous walls, is prepared for the real process of digestion; by which it is converted into a fluid, and thus made fit to be truly received into the system, by being absorbed into its vessels. The chief agent in the digestive process is a fluid termed the gastric juice, which is secreted or sepaas seen in a vertical section rated from the blood by a vast number of the Stomach, magnified of little bags or follicles (fig. 113), imtwenty diameters at B. bedded in the walls of the stomach. When the cavity is empty, this fluid is secreted in very small quantities; but, like the salivary secretion, it is poured out

B

Fig. 113.

GASTRIC FOLLICLES,

SENSE OF HUNGER-SECRETION OF GASTRIC JUICE. 189

in abundance when the lining membrane is stimulated by the contact of food, especially solid food. Only a limited quantity is secreted at any one time; and this quantity is just that which is sufficient to dissolve food enough for the supply of the natural wants of the system. The contact of any solid substances with the interior of the stomach, is sufficient to produce a flow of this fluid into its cavity; but the secretion soon ceases if the substance be not of an alimentary nature.

205. The sense of hunger appears due to the distension of the blood-vessels of the stomach, which takes place in preparation for the secretion of the gastric fluid. This determination of blood towards the stomach seems to occur whenever the body needs a fresh supply of nourishment; and it ceases as soon as a sufficient amount of gastric fluid has been drawn off. Hence it is, that hunger is relieved by eating; and hence it is, also, that hunger is for a time relieved by taking solid substances into the stomach, even though they contain no nourishing matter. It is from having experienced this, that savage nations are in the habit of mixing indigestible solid matter with the fluids that sometimes constitute their principal articles of food. Thus the Kamschatdales mix earth or saw-dust with the train-oil on which alone they are frequently reduced to live; and the Veddahs, or wild hunters of Ceylon, mix the pounded fibres of soft or decayed wood with the honey on which they feed when meat is not to be had. One of them being asked the reason of the practice, replied, "I cannot tell you, but I know that the belly must be filled." It has been found by experiment, that soups and other forms of liquid aliment are not alone fit for the support of the system, even though they may contain a large amount of nutritious matter; and the medical man well knows, that many persons have stomachs too weak and irritable to retain "slops" (as they are commonly termed), who can yet digest solid food of a simple kind. All these instances show, that the contact of a solid substance with the walls of the stomach, is the proper stimulus or excitement to the secretion of the gastric fluid.

206. This fluid, when poured upon the food, is thoroughly mixed-up with it by a peculiar movement of the walls of the stomach, which is continually bringing fresh portions of the