The region of the mouth is not separated from that which follows it (the pharynx) by an epiglottis, which in ourselves protects the entrance into the air-passages, nor is there any uvula to guard the posterior orifice of the nose by which the air reaches the throat. The succeeding portion of the gullet (the œsophagus) is very long, as might be supposed from the length of the neck in most birds, and it is very frequently either dilated at one side, or produced into a cæcal pouch (crop, ingluvies), which may, or may not, be separated by a narrow connection, from the rest of the gullet, and which may be divided into two compartments. This crop serves for the detention of the food, which cannot have undergone any complete process of mastication, and it is here treated to a process of maceration by the fluid secreted from the walls of this organ. Passing from this receptacle, the food becomes subject to the action of the stomach proper, which differs, however, from our ordinary conception of a stomach, as seen in man, by being divided into two distinct portions. The anterior one is known as the proventriculus, and it is in this that the gastric juice is brought to bear upon the food, and its walls are consequently thickened by a glandular layer; the hinder division, which is known as the gizzard, forms an elongated sac, with two orifices—one from the proventriculus, the other leading to the small intestine—in its upper portion. The characters of its walls are very different in those birds which live on animal, as compared with those that live on vegetable (grains) food; in the former they are membranous and thin, but in the latter they are enormously thick and very muscular. On examination, it is seen that the dark colour of the muscles is on each side of the gizzard relieved by a shining spot of tendinous material, and the walls of the gizzard have consequently been compared to a double-bellied (digastric) muscle. The internal cavity of the gizzard is lined with a dense and rough coat, and is ordinarily found to contain small stones, and occasionally other hard materials. These obviously take the place of the absent teeth, when the muscles of the gizzard set up that (grinding or compressing) action by which the ingested seeds are broken down. The wall of the gizzard may itself also act as a rasping organ, being, as it often is, provided with a firm glandular layer, the secretion of which is converted into a hard lining, the structure of which has been observed in some cases to be due to interlaced filaments secreted from and continuous with the glands in the wall of the gizzard.
Notwithstanding the differences in the character of the gizzard in carnivorous and graminivorous birds, it has been shown by the ever-famous John Hunter that carnivorous birds can be brought to live on grains, and grain-eating birds on meat.
It is interesting, further, to note, with regard to the opening into the small intestine, that in a number of grain- or fruit-eating birds there is no valvular arrangement to detain the food in the gizzard till it is completely triturated, for it is thus that many plants have their area of distribution increased, the escaped seeds passing uninjured from the intestine to find, perhaps, a suitable soil in a new district. In those that swallow large stones a valve is often to be observed. The difference which we have already had so frequently to notice, as obtaining between the carnivorous and “vegetarian” birds, is seen to be continued into their small intestine; just as in mammals, this portion of the tract is longer in the latter than in the former birds. The anterior, or duodenal portion, is always characterised by forming a loop, within which lies the gland known as the pancreas, and the succeeding portion is, as compared with most mammals, short. A slight elevation, hardly ever of any great size, may at times be observed on the course of the short intestine. This represents all that remains of the duct by which the hatching bird was connected with the yolk. The short and straight large intestine is ordinarily separated from the preceding by a cæcum; this is generally paired (in the Herons and some others it is single), and varies in length; in many cases these cæcal tubes are hardly more than papillæ. In the Parrot, as in the Woodpecker and some others, these cæca are absent. In the desert-dwelling Ostrich (Struthio) they are said to be as much as two feet long; but in the Emu they do not exceed six inches in length.
DIGESTIVE ORGANS OF THE KINGFISHER.
(After Macgillivray.)
(a) Tongue; (b, c, d) Œsophogus; (d, e, f) Stomach; (f, g, h, i, j, k) Intestine; (j, k) Cloaca.
The intestine ends in a cavity, which is common to it, and to the other organs that open to the exterior in this region. This cloaca (sewer) is found in reptiles also, and in one division of the Mammalia, the Monotremata. In birds it is provided with a special glandular appendage on its upper (or dorsal) aspect, which goes by the name of the Bursa Fabricii. Neither the history nor the functions of this peculiar organ can be said to be thoroughly understood.
Of the organs which are appended to the intestine, the lungs will be described elsewhere; of the rest we have to consider the liver, the pancreas, and the spleen. The first-named organ is large, and covers over the pancreas, the proventriculus, the spleen, part of the gizzard, and part of the small intestine. It is ordinarily divided into two “lobes,” between which, on the upper edge, is placed the tip of the heart. In the common fowl the left lobe is often divided into two; but this organ is never broken up into so large a number of parts as it is in many mammals, from which animals birds also differ in always having more than one duct to carry off the secretion of the liver (bile) to the small intestine, except in the Ostrich; in this, as in some other birds, there is no gall-bladder in which the bile may be collected, so that in such this secretion passes directly into the intestine.
As has been already pointed out, the commencement of the small intestine forms a loop, in which is set the organ known as pancreas, which may for simplicity be described as the salivary gland of this region, although in truth the fluid secreted from it is a much more powerful aid to the digestion of food than that of any known salivary gland. It has always two, and in a number of cases three ducts, which do not unite with the bile ducts, but open separately from, though near them, into the end of the “duodenal loop.” The spleen, which is a small oval body, and is placed to the right of the proventriculus, has no ducts; in birds of prey it is more cylindrical in shape.