442 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



Both atrium and ventricle are in the mammal completely divided 

 into right and left chambers. The conus and the cardiac end of the 

 ventral aorta become split completely into two vessels (Fig. i86) — the 

 pulmonary artery (^) and the systemic aorta (5) which course round one 

 another in the customary spiral fashion. The modifications undergone 



d.c. 



v.c. 



d,c. 



r.sc. 



Fig. i86. 



Diagram illustrating the arterial system of a mammal as seen from the ventral side. Portions 

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

 Af Dorsal aorta; d.c, dorsal (internal) carotid; i, innominate; l.c, left {common) carotid; 

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

 pulmonary ; r.sc, right subclavian ; S, systemic ; v.c, ventral (internal) carotid. 



[Note. — The growth of the neck between arches III and IV brings about a wide separation of 

 these arches in the adult, with a corresponding lengthening of the common carotid arteries.] 



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

 already described for the Reptilia but there is one striking peculiarity 

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

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

 (Fig. i86, 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 



