204 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



.-As 



t 



pulmonary. While three arches arise from the 

 heart in many reptiles, only two are directly given 

 off in Ophidia, one of which is aortic and one pul- 

 monary. In the bird and mammal this reduction is 



carried still 

 farther, for 

 in them the 

 aortic trunk 

 single 



is 



throughout 

 its whole ex- 

 tent, the left 

 half being re- 

 duced in the 

 former, and 

 the right in 

 the latter. 

 These reduc- 

 tions are best 

 explained by 

 a study of the 

 figures of 

 Rathke (Fi<*. 

 89). 



The fol- 

 lowing are 

 the names 

 and areas of 

 distribution of the more important arteries : 



1. Carotids. These may be double, when they 

 are known as external and internal, or one or other 

 may be reduced or disappear ; in some Fishes the 

 carotids are not direct continuations of branchial 

 vessels, but the latter first unite to form a circiilus 

 cephaJicios, by means of which the supply of blood 

 to the head is the better regulated ; they supply the 

 head and neck. 



Iff 



Fig. 88. Heart of Lacerta muralis. 



Auricles ; v, ventricle ; tr, arterial cone (or truncus 

 anonymus) : l, 2, first and second arterial arches ; 

 RA, root of aorta; AO, aorta; As As', subclavian 

 arteries; Ap, pulmonary artery ; vp, pulmonary vein; 

 J, jugular ; vs. subclavian veins; ci, vena cava in- 

 ferior ; these last three pass into the sinus venosus 

 (s) which lies beneath the right auricle. (After Wie- 

 aersheim.j 



