Major Differences Between Avian and Mammalian Digestive Systems Include, in Birds, Both the Lack of Teeth and the Separation of Gastric Functions into DistinctAnatomicaI Regions

Important anatomical differences exist between the digestive systems of birds and mammals. These differences affect motility functions more than other aspects of digestion, such as secre­tion, digestion, and absorption.

Figure 28-13 illustrates the general anatomy of the avian digestive system. The pharynx of birds is simpler than that in mammals, with birds having no soft palate. There are no teeth, although in carnivorous species, the beak is modified for tearing food into pieces small enough to swallow. The esopha­gus has a large diameter so as to accommodate unmasticated food. An outpouching of the esophagus is known as the crop. The extent of development of the crop varies widely among avian species. The glandular portion of the stomach is the proventriculus, which is separated from the muscular stomach, known as the ventriculus or gizzard, by a short isthmus. The small intestine varies greatly in length among avian species but is generally rather short compared with that of mammals of similar size. The ceca are usually paired and vary tremend­ously in development among avian species. In some carni­vorous species, such as the hawk, the ceca are rudimentary, whereas in some of the nonflying herbivores, such as the ostrich, cecal development is extensive (see Figure 28-13). The colon and rectum are very simple; the rectum ends in the cloaca, which is a common passageway for digestive, urinary, and reproductive discharges.

The crop performs a storage function. In some species the crop is little more than a pouch in the esophagus, whereas in others, such as the chicken, there is a distinct sphincterlike opening between the esophagus and crop.

The proventriculus is a low-volume organ with a glandular epithelium resembling that of the stomach of mammals (see Chapter 29). Motility functions of the proventriculus are to propel ingesta and digestive secretions into the gizzard for mixing and grinding. The gizzard is a muscular organ that grinds and liquefies ingesta. In addition, particle-size dis­crimination occurs in the gizzard; small particles are passed into the duodenum, whereas large particles are retained for further comminution or ejected back into the proventriculus for further addition of digestive secretions. In carnivorous birds, concretions of bone, hair, feathers, and other indiges­tible material accumulate in the gizzard and are occasionally ejected orally in an action known as Cgestion. In grain-eating birds, small stones or gravel are swallowed and retained in the gizzard to aid in the comminution of ingesta. This inorganic material is referred to as grit, and its presence increases digestive efficiency, although it is not essential. The mucosa of the gizzard is covered by a tough coating known as koilin. This coating is composed of glandular secretions and desquamated cells.

The motility and function of the various stomach areas of birds are easily comparable with gastric motility and function in mammals. The crop and proventriculus function in much the same manner as the fundus and body of the mammalian stomach, with storage and secretory functions. The gizzard functions in much the same way as the antrum of the mam­malian stomach, with grinding and particle-size discrimina­tion functions. The major functional differences between birds and mammals include the physical separation of the stomach compartments in birds and the advanced grinding function of the gizzard.

The motility patterns of the avian small intestine appear to be generally similar to those in mammals. The motility of the hindgut also shares some characteristics of other animals. Reverse peristalsis is a dominant characteristic of the avian colon and rectum, moving ingesta into the ceca. Urinary excretions arriving at the cloaca become incorporated with ingesta and move in a retrograde manner into the ceca, thus facilitating reabsorption of the remaining water and electro­lytes from urine. Cecal motility is characterized primarily by mixing and reverse peristalsis, with occasional mass move­ments resulting in the evacuation of the ceca. These mass movements in avian species are followed by defecation.