204 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



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 



</ n farther, for 



in them the 

 aortic trunk 

 is single 

 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 (Fijj. 

 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 cireulus 

 cephaliciBS, by means of which the supply of blood 

 to the head is the better regulated ; they supply the 

 head and neck. 



-As 



A(7 



Fig. 88. Heart of Laoerta inuralis. 



A, Auricles ; v, ventricle; tr, arterial cone (or triincus 

 anonym us): 1, 2, first and second arterial arches; 

 RA, root of aorta; AC, aorta; AS AS', subclavian 

 arteries; Ap, pulmonary artery ; rp, pulmonary vein; 

 j, jugular ; vs. subrlavian veins; ci, vena cava in- 

 ferior : these last three pass into the sinus venosus 

 (8) which lies beneath the right auricle. (At ter Wie- 

 dersbciiuj 



