312 THE ANATOMY OP VEETEBEATED ANIMALS. 



The septmn of tlie cavum pulmonale remains as a small 

 muscular band, and the fold of the outer lip of each 

 auriculo-ventricular apertiire, has become a distinct mem- 

 branous valve. 



3. In Aves, the venous and arterial blood cun-ents, com- 

 municate only in the pulmonary and systemic capillaries. 

 The am-icular and ventricular septa are complete, as in 

 the Crocodilia ; but the right ventricle gives off only the 

 pulmonary artery, the left aortic arch having disappeared. 

 The septum of the cavum pulmonale becomes a great 

 muscular fold, and takes on the function of an auriculo- 

 ventricular valve. At the origin of the pulmonary artery, 

 and at that of the aortic arch, three semikinar valves are 

 developed. 



In Reptiles there are usually only two aortic arches, one 

 on each side, answering to the fourth pair of arches of the 

 embryo. The right gives off the carotid and subclavian 

 arteries, and passes directly into the tmnk of the dorsal 

 aoi'ta. The left commonly gives off visceral arteries, and 

 becomes a good deal diminished in size before joining the 

 common trunk. 



In many Lacertilia, four aortic arches, (answering to the 

 third and fourth pairs of the embryo,) persist, two anterior 

 arches, from which the carotids are given off, springing, by 

 a common trunk, from the right ordinary aortic arch. 



In the Rcptilia, most of the blood of the hind limbs and 

 tail passes through one or other of two " portal systems " 

 before reaching the heart, the one portal system lying in 

 the kidney, the other in the liver. The portion which goes 

 to the liver is can-ied to it mainly by the anterior abdominal 

 veins, which are represented by two trunks in most Reptilia, 

 by one in the Opliidia. 



In Aves there is no renal portal system, and the anterior 

 abdominal vein opens into the inferior vena cava close 

 to the heart. Nevertheless a median trunk, which is given 

 off from the caudal vein, carries a considerable proportion 

 of its blood directly into the hepatic portal system. 



