338 



CLASSIFICATION OF VERTEBRATES. 



TR 



in a few birds between the gizzard and the intestine. The 

 length and coiling of the intestine varies greatly, the large 

 intestine usually being short and straight. There are usually 

 two caeca at the junction of ileum and colon. The cloaca re- 

 ceives on its dorsal wall the urinary and genital ducts, and 

 farther back, in the young of all, there is developed a sac, the 

 bursa Fabricii, of unknown function. It usually degenerates in 

 the adult (Fig. 131). 



The liver is usually two lobed, and there are two or three 

 bile ducts. A gall bladder usually occurs. The pancreas is 

 large and compact, and lies in the duodenal loop. The spleen 

 is small. 



The trachea is usually straight, but it may be folded or con- 

 voluted, the convolutions lying beneath the skin of the abdomen 



or inside the keel of the breast 

 bone. The tracheal rings are 

 ossified. The syrinx (p. 29) is 

 usually formed by trachea and 

 bronchi, but it may be purely 

 bronchal or entirely tracheal in 

 character. The lungs do not 

 lie free in the coelom, but are 

 bound to its dorsal wall by 

 cellular tissue. The air sacs 

 are usually large, and it is by 

 changes in the size of these 

 more than by alterations in the 

 volume of the lungs themselves 

 that inspiration and expiration 

 are effected. 



The brain is characterized by its compact form and the 

 large size of the non-convoluted cerebrum, which reaches back 

 to meet the cerebellum, thus forcing apart the optic lobes. The 

 cerebrum is largely made up of the corpora striata, and a small 

 corpus callosum is present. The olfactory lobes are small 

 (much larger in extinct birds). The cerebellum has a median 

 vermis, showing an arbor-vitae in section, and a pair of small 

 lateral floccular lobes ; a pons is lacking. The twelve cranial 



TR 



FIG. 333. Diagrammatic section of 

 syrinx, after Boas. f, walls of drum; 

 S, bridge; BR, bronchus; TR, trachea. 



