The Vascular System. 



85 



capillaries with the heart. The control of the muscular action of 

 the heart and arteries through the vagus and sympathetic nerves is 

 an important element in maintaining pressure and tone in the 

 vascular system (cf. p. 62). 



Many of the peculiar features of the mammalian circulation 

 which at first sight do not appear to be general, but arer^so in 

 reality, depend on the circumstance that the complete partition 

 of the organs is a final stage of a general progressive development, 

 observable in air-breathing vertebrates, in which the lungs and their 

 vascular connections becom; per- 

 fected for pulmonary respiration. 

 On the other hand, the vascular 

 system as it appears in the embryo, 

 more especially its aortic portion, 

 is arranged according to the type 

 of branchial respiration as found 



fishes. In this condition the 



m 



blood is sent forward from the 

 heart through a ventral aorta. The 

 latter is connected with a series of 

 paired branchial aortic arches, 

 traversing the rudimentary gill struc- 

 tures, and thus passing upward 

 PRIMITIVE around the sides of 



AORTIC ARCHES, ^^e primitive pha- 

 rynx. The dorsal 

 aorta is formed by the junction of the 

 branchial aortic arches, and passes 

 backward as a main distributing 

 vessel on the ventral side of the axial support. The heart itself 

 is formed primarily on a two-chambered plan, similar to that, in 

 fishes, where all the blood is received by single atrium and is 

 delivered forward to the gills by a single ventricle. 



The definitive condition of the chief arterial vessels is arrived 

 at by an extensive modification of the branchial plan. As indicated 

 in the accompanying diagram (Fig. 47) the arch condition is retained 

 by the aorta and by the pulmonary artery, and it is interesting to 

 note also that the primary connections of these vessels, represented 

 in the embryo by an open canal, the ductus arteriosus (Botalli), is 



Fig. 47. Plan of the branchial aortic 

 arches. The adult mammalian vessels 

 are indicated in black (systemic) or 

 shaded (pulmonary). 1-6. primary 

 arches; ao., aorta; a. p., pulmonary 

 artery; c.e., external carotid; c.i., in- 

 ternal carotid;^ d.a., ductus arteriosus 

 (Botalli); i., innominate artery; s.d., 

 right subclavian; s.s., left subclavian. 

 (From Weber, after Boas.) 



