REPTILIA. 



39 



is no complete ventricular septum, but a strong muscular ridge projects 

 from the ventral wall of the ventricle, and is attached anteriorly between 

 the roots of the pulmonary artery and left aorta. When the heart con- 

 tracts this ridge or septum pulmonale cuts off a cavum pulmonale which 

 contains purely venous blood from the rest of the ventricle which contains 

 mixed blood or on its left side arterial blood. The pulmonary artery 

 rising from the cavum pulmonale is thus filled entirely by venous blood, 

 whilst the roots of the two aortae are so disposed that the left aorta rising 

 rather on the right contains more venous than arterial blood, and the right 

 aorta, vice versa, more arterial. The apex of the ventricle is attached 

 by a fibrous band to the pericardium. The whole heart is broad and 

 flattened dorso-ventrally in Chelonia. The roots of the two aortae and 

 pulmonary artery are closely united in Lacertilia, Chelonia and Crocodilia. 

 In many Lacertilia, e. g. Lacerta, the third or carotid aortic arch is com- 

 plete and falls into the fourth or aortic arch : in other Lacertilia 

 and Reptilia there is no connection between the two. The right aortic 

 arch gives off the carotids, except in the Lacertilia first mentioned, and 

 the subclavian arteries which are absent in Ophidia, and then unites with 

 the left aorta to form the subvertebral aorta. The left aorta gives off the 

 caeliac artery before it unites with the right, and in Chelonia and Crocodilia 

 the artery in question is of so large a size that it appears to be the continu- 

 ation of the arch. In the Crocodilia the left aorta arising from the right 

 ventricle carries only venous blood to the viscera, whilst in other Reptilia it 

 carries a mixture of arterial and venous. There are always two venae cavae 

 superiores. The cava inferior is formed by the union of the two efferent 

 renal veins. There is a renal-portal circulation except in Chelonia. The 

 caudal vein divides into two branches, each of which goes to the kidney of 

 its own side in Ophidia ; so too in the Lacertilia, but here each branch 

 receives on the way the veins of the corresponding hinder extremity. In 

 the Crocodilia the afferent kidney veins are derived from a transverse vessel 

 uniting the two branches of the caudal. These are continued on into the 

 two epigastric veins, remnants of the foetal umbilical veins, which receive the 

 veins of the hinder extremity and conduct the blood to the liver, anasto- 

 mising with the hepatic portal system. The two branches of the caudal 

 vein are connected to the epigastrics in Chelonia. There is a single epi- 

 gastric in Lacertilia which receives blood from the abdominal walls and the 

 bladder and is distributed to the liver. The correspoding vein in Ophidia 

 is resolved into a plexus, ramifying in the fat body (see p. 69) and re- 

 ceiving veins also from the abdominal walls. Anastomoses exist, however, 

 between the renal-portal veins and the rootlets of the hepatic portal 

 system. The lymphatic vessels take the shape of loose sheaths or plexuses 

 surrounding the large arteries, and communicate, as in Amphibia, with the 

 superior cavae anteriorly and the ischiadic veins posteriorly. At their 



