ZOOLOGY FOR MKDIfAL STUDENTS 



CHAP. 



Doth atrium and ventricle are in the mammal completely divided 



Jit and left chambers. The conus and the cardiac end of the 



ventral aorta become split completely into two vessels (Fig. 186) the 



pulmonary artery (/>) and the systemic aorta (S) which course round one 



another in the customary spiral fashion. The modifications undergone 



d.c. 



v.c. 



d ; c. 



T.SC: 



FIG. 186. 



11 illn-UMiing the arterial system of a mammal as seen from the ventral side. Portions 

 df the embryonic, arterial system which are no longer present in the adult are drawn in outline. 

 A, I> il.c, dorsal (internal) carotid; i, innominate; l.c, left (common) carotid; 



l.p, left pulmonary ; l.sc, left subclavian ; />, pulmonary ; r.c, right (common) carotid ; ./>, right 

 jiiilmo lit Mibclavian ; S, systemic, ; v.c, ventral (internal) carotid. 



of the neck between arches III and IV brings about a wide separation of 

 in the .idult, with a corresponding lengthening of the common carotid arteries.] 



by the main arterial trunks of the embryo are on similar lines to those 



.ilrrady described for the Reptilia but there is one striking peculiarity 



ol the mammal, namely that the aortic root of the right side completely 



Dpears from the point at which it gives off the subclavian artery 



. 1 86, r.sc) backwards to the point at which it joins its fellow to 



form the unpaired dorsal aorta. It follows that in the mammal as in the 



