28 MORPHOLOGY OF THE VERTEBRATA. 



The right auricle is larger than the left. The inner surface of 

 the auricles is marked by the columnse carnae and musculi pectinati. 



The right ventricle is a narrow cavity of a triangular shape, 

 placed to the right and anterior side of the left ventricle, but not 

 extending to the apex. The right ventricle is characterised by the 

 smoothness and evenness of its inner surface ; the pulmonary artery 

 takes its origin from it, and is provided at its commencement with 

 three semilunar valves. It divides into two branches — one for 

 each lung. The left ventricle is a cone-shaped elongated cavity, 

 the walls of which are about three times as thick as those of the 

 right ventricle, and have strong fleshy columns developed on their 

 inner surface, which extend from base to apex. 



The aorta springs from the left ventricle — its orifice being 

 guarded by three semilunar valves. 



The openings of the auricles into the ventricles are guarded 

 by the auriculo- ventricular valves, which prevent the regurgita- 

 tion of the blood backwards from the ventricles into the auricles. 



The aorta is peculiar in crossing over the right bronchus. It 

 immediately after its commencement gives origin to the two 

 innominate arteries, which send off the common trunk of the 

 •carotid and vertebral arteries. 



The basilar artery is formed by the internal carotids, and not 

 by the vertebrals. The arteries of the hind extremities do not 

 arise from a single branch or external iliac, but from two arteries 

 which come off successively from the aorta at a considerable 

 distance from each other, and leave the pelvis by two separate 

 ■apertures. 



The inferior vena cava receives the hepatic veins before it opens 

 into the right auricle. There is no renal portal system. 



BRAIN. 



The cerebrum does not cover the cerebellum, and its chief 

 mass consists of the corpora striati. The lateral lobes of the 

 •cerebellum and pons varolii are rudimentary. There are no con- 

 volutions on the cerebral hemispheres, and the corpus callosum is 

 not developed. 



The optic thalami are of small size, and are not united by a soft 

 •commissure : between them is the cavity of the third ventricle. 



SKELETON 



Exhibits a far higher type than that of fishes. 



As the beak in birds is prehensile, the cervical vertebrae are 

 numerous, and freely moveable. In Columbus they are SKive"? 

 in number. 



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