THE LAKYNX IN SAUKOPSIDA. 267 



The septum of the cavum pulmonale remains as a small 

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

 ventricular aperture has become a distinct membranous valve. 



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

 municate only in the pulmonary and systemic capillaries. The 

 auricular and ventricular septa are complete, as in the Groco- 

 dilia / 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 semilunar 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 trunk of the dorsal aorta. 

 The left commonly gives off visceral arteries, and becomes a 

 good deal diminished in size before joining the common trunk. 



In many Iiacertilia, 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 Heptilia, 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 carried to it mainly by the anterior abdominal veins, 

 which are represented by two trunks in most Heptilia, by one 

 in the Ophidia. 



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. 



All the Sauropsida possess a larynx, a trachea, and one 

 or two lungs. The bronchi do not divide dichotomously, as 

 tliey do in Mammalia. 



In Ghelonia and Grocodilia the larynx consists of a cir- 

 cular cartilage apparently corresponding with both the thyroid 

 and the cricoid of the higher Vertebrata ; and of the arytenoid 

 cartilages, articulated with its anterior and dorsal edge. 



The Laeertilia have, for the most part, a similar larynx, 

 but the circular cartilage is often interrupted by round, or 



