AVES 361 



of the body. The ileum is not ordinarily long, but in the owls it is 

 nearly as long as the duodenum. 



The rectal ceca are diverticula at the point where the ileum 

 enters the rectum. In the grebe, a single cecum in present. In 

 most birds there are two ceca, variable in length according to their 

 habits. In the pigeon the ceca are not more than 2 inches long 

 (shorter than in any other vegetable feeder) while in the common 

 domestic fowl, they may reach a length of 3 feet. 



Rectum. — The large intestine (called the rectum rather than the 

 colon) is wider than the small intestine, and has coarse, short villi. 

 It terminates by a valvular opening in a dilated cavity, the remains 

 of the allantois (see page 302), now a rudimentary urinary bladder. 

 The ureters and generative ducts open into a transverse groove at 

 the lower part of the urinary dilation. The anal follicles are in a 

 conical glandular cavity communicating with the posterior part of 

 the cloaca and named the " Bursa Fabricii." 



Liver and Gall Bladder. — There is usually a gall bladder, with 

 two bile ducts leading from the large liver to the duodenum. The 

 pigeon has no gall bladder but the fowl has an extremely large one. 



Pancreas. — The pancreas is a compact elongated reddish gland 

 lying in the loop of the duodenum, into which it discharges its 

 ferments through three ducts. The three enzymes digest proteins, 

 carbohydrates and fats as in mammals (p. 432). 



Circulatory System. — The heart is comparatively large. (Figure 

 203.) Two auricles are present and the two ventricles are com- 

 pletely separated. The right auricles receive impure blood from 

 the right and left precavals, and the postcaval veins. The blood 

 passes from the right auricle through the auriculo-ventricular valve 

 to the right ventricle. From the right ventricle it goes past the 

 semilunar valves to the pulmonary artery and thence to the lungs. 

 Four pulmonary veins from the lungs bring the blood to the left 

 auricle. From the left auricle through the mitral valve it passes to 

 the left ventricle. From the left ventricle it passes the semilunar 

 valves to the right aortic arch ^ which gives off the innominate 

 arteries, then continues as a dorsal aorta. Venous and arterial 

 blood do not mix in the bird. The jugular veins are united by a 



8 In the reptiles, the right aorta transmits pure blood, while the left aortic arch 

 contains mixed blood. 



The pigeon (Columba) has about 2,000,000 red corpuscles in a cu. mm. of blood. 

 Erythrocytes vary in size from 12.1 microns in the fowl to 14.7 in the pigeon. 



