HEART AND ARTERIES OF VERTEBRATA. 



581 



just as in Lepidosircn, or the pulmonary artery (p) is continued on 

 directly from the branchial artery. 



The disappearance of the gills 

 causes certain changes in some of the 

 Amphibia, in this vascular apparatus, 

 which is persistent only in the 

 Perennibranchiata. The direct con- 

 nection (cf. Fig. 328), which already 

 exists between the branchial arteries 

 and veins, is developed in such a way 

 that some of the arterial arches are 

 continued directly from the heart into 

 the roots of the aorta. The last arch, 

 which already gives off the pulmonary 

 artery, is developed into the trunk of 

 this artery, and is either connected by 

 small vessels only (ductus arteriosus) 

 with the root of the aorta, or is cut 

 off from it, and forms a separate 

 vessel. In this way several arterial 

 arches are connected with the root 

 of the aorta just as in Lepidosiren, 

 while one of the primitive vascular 

 arches is converted into the pulmonary 



artery. The two kinds of blood are enabled to commingle, owing 

 to the arrangement of the heart, and the characters of the large 

 vascular trunks. 



Fig. 328. Heart aud arterial trunks 

 of a larva of Triton, a Auricle. 

 v Ventricle. ba Arterial bulb. 

 1 — 4 Arterial arches ; the first 

 three are the branchial arteries. 

 c Carotid, p Pulmonary artery. 

 ao Aorta. vb Branchial veins 

 (after M. Rusconi). 



Brccke, E., Denkschr. d. Wien. Acad. I. 



§ 434. 



A great step in the differentiation of the circulatory organs is 

 seen in the Reptilia, where the heart is placed at a greater distance 

 from the head. It is gradually moved backwards from the point at 

 which it is formed, and is embedded in the thoracic cavity ; this is 

 its position in all the Amniota. The ventricular portion is generally 

 elongated, but it is broad in the Chelonii (Fig. 330), and several 

 Saurii. The two auricles are always separated from one another by 

 a septum, as in the Amphibia ; the right one receives the systemic, 

 and the left the pulmonary veins. The former is always the larger. 

 The strong muscular wall of the ventricle is continued into a net- 

 work which diminishes its cavity, just as in Fishes and Amphibia ; 

 this is especially the case in the Ophidii, Chelonii, and Saurii. The 

 ventricular partition is chiefly formed by this network, but some of 

 the muscular bands are more largely developed. The right half 

 of the ventricle contains venous, and the left arterial, blood, so that 

 the two portions can be distinguished. The incompleteness of the 

 separation of the two cavities is partly, at least, made up for by 



