Sauropsida: Class Aves 



529 



which hardens to form a thick protective lining resembling in appear- 

 ance and texture the substance of horny structures produced by the 

 skin. Integumentary horn, however, is not a secreted substance (see 

 p. 18). 



The degree of development of the gizzard is closely correlated with 

 the character of the food. It attains maximum development in the 

 grain-eaters. In these birds the food, softened during its stay in the 



if— Digestive epithelium^ 

 --Peritoneum^ \ 

 • Muscle-^ " x 

 -Esophagus.^ "* 



-Proventriculus- 



-Gastric glands 



Gizzard 



Pylorus' 

 N Gizzard muscle 

 Keratoid layer 



Fig. 412. (Left) Stomach of turkey, a grain-eating 

 bird. (Right) Stomach of buzzard, a flesh-eating bird. 

 Diagrammatic sections. (After Kingsley: "Compara- 

 tive Anatomy of Vertebrates," Philadelphia, The 

 Blakiston Company.) 



crop and mingled with digestive fluids derived from the proventriculus, 

 is subjected to the vigorous crushing and grinding action of the muscu- 

 lar gizzard. It is the common habit of these birds to swallow small 

 pebbles or other hard indigestible objects which assist in the grinding 

 process. The gizzard is more or less well developed in insectivorous 

 birds. Hard-shelled beetles must be crushed before they can be di- 

 gested. Obviously the gizzard serves in place of teeth. It is least differ- 

 entiated in birds of prey (eagle, hawk, owl, gull), in which its muscular 

 layer is relatively thin and no hard lining is produced. 



The length of the intestine varies from twice to several times the 

 total length of the bird, in general being longer in vegetarians than in 

 carnivores. Two main regions are recognized, the anterior small in- 

 testine and the posterior large intestine, the "small" and "large" 



