Birds are structurally very different than mammals, and nowhere is this difference so striking as it is in the arrangements of their digestive systems. The evolutionary lineage of modern birds is in dispute, but one widely-held theory, for which there is some considerable support, holds that they're more or less direct descendants of the dinosaurs. As you might expect, therefore, there's not much similarity to the typical mammalian pattern; furthermore, the avian digestive tract is suited to the peculiarities of bird life.
The central fact of being a bird is that everyone wants to eat you. With a few notable exceptions, birds are on nearly every predator's menu. Consequently their anatomy reflects adaptations for evasion and escape. To be able to "eat and run" (or "eat and fly") has real selective value, so birds have evolved a digestive system that permits them to "eat now, digest later," after they get away from whatever is chasing them.
The most obvious adaptation to life as a bird is the presence of the hard bird's beak, which is made of keratin. Birds aren't the only group of vertebrates with beaks, but the variety and form of the structure reaches its zenith in them. Birds haven't any teeth (hence the expression "scarce as hen's teeth") but the beak makes up for it in many ways. Other than the beak and the absence of teeth, however, much of the oral cavity is similar to that in mammals. Slide 642 is a longitudinal section of the tongue of a chicken. (This isn't the whole tongue—the distal end has been cut off in handling.) The surface of the tongue is covered with stratified squamous epithelium, quite heavily keratinized. Note that there are apparently no tongue papillae except for perhaps a few filiforms at the proximal end.
Birds are reputed to have little or no sense of taste or smell, and considering that some of them are carrion eaters (such as vultures) and others eat things like skunks (owls are known to do this) it seems likely that, at least for some of them, it's true. A distinct oddity of the avian tongue is that it has a bone in it, running the full length. The proximal end of this bone is replaced with hyaline cartilage. Since birds don't use the tongue in articulation (as do mammals) a rigid tongue doesn't affect vocalization. Avian sound is produced in a special organ of the respiratory system, and the tongue is used just to move food around. It's widely believed that "splitting the tongue" of a parrot or a crow will enable it to talk better, but this is not the case, and the bird probably won't appreciate it.
To see the articulated joint in the tongue of a bird, click here.
At the proximal end you will find some glands of the buccal cavity. These are simple branched tubular, mucus type salivary glands.
To see these glands, click here.
Slide 637 shows a section of the crop. This structure is common to all birds. It's a diverticulum of the esophagus, used for temporary food storage, and a nice example of the specialization of the avian digestive system. It enables a bird to snatch what food it can, fly away at the approach of danger, and digest the meal at leisure. It's simply amazing how much food the crop of even a small bird can hold. I've often opened up the crops of pigeons (Columba livia) and found nearly half a pound of corn in them, roughly half the live weight of the pigeon! I once found in the crop of a small wood duck (Aix sponsa) 10 acorns, all better than 3/4" in diameter. This trick was accomplished by a bird whose gullet couldn't have been much more than 3/4" when fully opened. To put it into perspective, this would be the equivalent of a human swallowing an equivalent number of baseballs.
The crop is lined with simple columnar epithelium (although you will see stratified squamous described in some texts). The deep regions of the mucosa at first glance appear to have glands in them, but actually these are crypts which have been cut in cross section. There is, however, some increased basophilia in the deep crypts. Mitotic figures are common in the deep regions of the crop as well. The tunica muscularis is scanty and has two layers.
Slide 644 is a section of the proventriculus. This is the true glandular stomach of the bird, but it's nothing like what you see in mammals.
The epithelium of the tunica mucosa is simple columnar. There are no goblet cells. Looking at this slide, at first glance you might also think there are villi present; but what appear to be villi are actually the mucosal ridges cut in cross section. If you examine a gross specimen of the proventriculus, you will see that the mucosa is arranged into discrete areas of concentric circular folds, like rings on a bull's eye. Another useful analogy is that of a stone dropped into a pool of water; if you can imagine the ripples from the stone's fall "frozen" in place, they would present an appearance very much like that of the mucosal ridges. The muscularis mucosae is scanty and discontinuous, and consists of small bundles of smooth muscle fibers cut in cross section as you view them.
To see the mucosa of the proventriculus and the relationships to the underlying glands, click here.
In the center of each "bulls eye" area is a small orifice, opening into the lumen of a gland below. The section on slide 644 shows these concentric ridges cut from top to bottom. These are the most striking feature of this organ.
The secretory part of the proventriculus is composed of a number of large units in the submucosa. These (adenomeres) are encapsulated by connective tissue. Each adenomere opens onto the luminal surface; their secretions are discharged through the opening. The composition of avian gastric juice is similar to that of mammals, and the cells of the glands produce enzymes and hydrochloric acid, just as the chief cells and zymogenic cells of mammals do. But birds seem to do this with only one cell type, rather than two as in the mammalian system.
The secretory parts of each adenomere resemble plates of cells with a blood vessel running between the cell layers. The cells are ranged along these plates and give a peculiar "saw toothed" look to them in sections like this. The serrated appearance results from the lack of junctional complexes at the apical ends of the cells. All of the cells are sitting on a basal lamina overlying the blood vessel of each plate, but they aren't connected at the top.
For a closer look at the secretory cells of the proventriculus, click here.
The ventriculus, on slide 643, is colloquially called the "gizzard." As noted above, birds don't have teeth, but they still have to masticate food before it can be passed into the intestine. The gizzard is the avian solution to the problem.
Birds in the wild tend to pick up gravel and small stones in their food. These pass through to the ventriculus, where they're retained for a while. The ventriculus is extremely muscular, and the contractions of the walls grind the food to a fine pulp by mashing it with these swallowed rocks. It's said that if you hold a chicken to your ear, you can hear the stones in her gizzard clicking; I have never done this myself, but if you have access to a chicken, it might be worth trying. By the way, you can easily examine a fresh gross specimen of the ventriculus the next time you buy a chicken; there should be one in the giblets bag.
The gizzard is lined with a very thick cornified layer produced by the cells of the mucosa. Note, however, that that this is NOT a keratinized epithelium. The epithelium produces this cornified material is a simple cuboidal type. The amorphous substance is called a "pellicle" or "cuticle" and in life it looks like a thick sheet of cellophane or Mylar.
To view the mucosal layer of the ventriculus, click here.
The mucosa is arranged into deep, straight tubular crypts, and the material is secreted at the bottom of these crypts. As the ventriculus grinds away, the free edge of the cornified material wears away and is constantly renewed from below.
For a little more detailed view of the secretory region of the ventriculus, click here.
The tunica muscularis of the ventriculus is immensely thickened, with at least two layers, and an overlying tough coat of dense fibrous connective tissue on the outside. It's far and away the most prominent portion of the wall of this part of the digestive tract.
Slide 638 shows the anus. As with mammals, this area is derived from embryonic surface ectoderm, and it's lined by stratified squamous epithelium.
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