476 MANUAL OF ZOOLOGY SECT. 



ventricular valve consists of two membranous flaps connected 

 with the wall of the ventricle by tendons, but the correspond- 

 ing valve of the right side (R. V.) is a large muscular fold, 

 very characteristic of the class. 



The right auricle receives the right and left precaval 

 veins (A, r. prc, I. prc] and the postcaval (ptc.\ the left, four 

 large pulmonary veins (p. v). The left ventricle (/. vn\ 

 as in the Crocodile, gives origin to the right aortic arch 

 (a. ao\ but the right ventricle (r. vri) gives off only one 

 trunk, the pulmonary artery, which soon divides into two 

 (r. p. a., 1. p. a). The left aortic arch is absent in the adult, 

 and it is the right alone which is continued into the dorsal 

 aorta. The result of this is that the systemic arteries 

 receive pure arterial blood from the left side of the heart, 

 and the only mingling of aerated and non-aerated blood is 

 in the capillaries. This is perhaps the most important 

 physiological advance made by Birds over Reptiles. 



The aortic arch curves over the right bronchus to reach 

 the dorsal body-wall, and then passes directly backwards as 

 the dorsal aorta. The renal-portal system is only imper- 

 fectly developed. 



The brain completely fills the cranial cavity, and is 

 remarkable for its short, broad, rounded form. The 

 cerebellum (c. b) is of great size, and has a large median 

 portion and two small lateral lobes or flocculi (/) ; the 

 surface of the middle lobe is marked by grooves passing 

 inwards in a radiating manner and carrying with them the 

 grey matter, the extent of which is thus greatly increased. 

 The hemispheres (c. h) extend backwards to meet the 

 cerebellum, and the optic lobes (o. /) are thereby pressed 

 outwards so as to take up a lateral instead of the usual 

 dorsal position. 



The eye is not even approximately globular, but has the 



