THE BLOODA'ASCULAR SYSTEM 



113 



regulated both by changes in degree of contraction of the arterial 

 walls and also by local closing and opening of the capillaries them- 

 selves. In the return of the blood the veins are largely passive, 

 acting merely as closed channels connecting the 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 tone in the walls of the vessels and thereby the 

 pressure of the blood (cf. p. 64). 



Many of the pecuHar features of the mammalian circulation 

 which at first sight do not appear to 

 be general, but are so in reality, de- 

 pend on the circumstance that the 

 complete partition of the organs is a 

 final stage of a general progressive de- 

 velopment, observable in air-breath- 

 ing vertebrates, in which the lungs and 

 their vascular connections become 

 perfected for pulmonary respiration. 

 On the other hand, the vascular 

 system in its earlier embryonic condi- 

 tion more especially in its aortic por- 

 tion, is arranged according to the type 

 of branchial respiration as found in 

 fishes. In this condition the blood is 

 sent forward from the heart through 

 a ventral aorta. The latter is con- 

 nected with a series of paired bran- 

 chial aortic arches, traversing the 

 rudimentary gill structures and thus 

 passing upward around the sides of 

 the primitive pharynx. The dorsal aorta is formed by the junction 

 of the branchial aortic arches, and passes backward as a main dis- 

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

 itself is formed primarily on a plan similar to that in fishes, where 

 all the blood is received by a 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 



Fig. 64. Plan of the branchial 

 aortic arches. The adult mammalian 

 vessels- are indicated in black (sys- 

 temic) or shaded (pulmonary) : 1-6, 

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

 pulmonary _ artery; c.e., external 

 carotid; c.i., internal carotid; d.a., 

 ductus arteriosus (Botalli) ; i., in- 

 nominate artery; s.d., right sub- 

 clavian; S.S., left subclavian. (From 

 Weber, after Boas.) 



