VASCULAR SYSTEM 



203 



In man, and to some extent in the anthropoids, the skin 

 muscles have undergone more or less complete retrograde 

 metamorphosis, until in man the power of even moving the 

 scalp backwards and forwards is an accomplishment denied to 

 all but the few. But the development of facial muscles capable 

 of conveying innumerable shades of expression is, like his speech 

 perhaps, peculiar to man. 



All these mimetic movements of the face are performed by 

 striped voluntary muscles. Another kind of movement on a 

 small scale is executed by smooth muscle-fibres which by their 

 contraction (either from internal or external stimuli) can make 

 the hairs move and the follicle as a whole rise up like a papilla, 

 constituting the so-called " goose-flesh," or like the wrinkling 

 of the scrotum which results from contraction of the dartos 

 muscle. 



III. Vascular System. 



The human vascular system is an enclosed one, and like the 

 higher Vertebrates the circulation is twofold. In reptiles the 



FIG. 108. Aortic valves. (From Ranke.) a, hall open; b, closed. 



blood passes into the systemic circulation by both aortic arches, 

 and in birds by the right arch, but in man and all mammals the 

 aorta springs from the left side of the heart. 



In man and horses the greater part of the systolic period is 

 occupied by systole of the ventricle, only about one-third by 

 auricular systole. 



The corpora Aurantii, small nodules on the centre of the 

 free border of each semi lunar cusp, are peculiar to man and 

 occur in no other animal. 



An apex-beat like that of man is best seen in carnivora ; 



