60 



ZOOLOGY. 



Fig. 45.* 



102. When the finger is gently pressed against an 



artery resting on a firm 

 surface, as a bone, an 

 impulse or jet is felt to 

 strike the finger regu- 

 larly. This is due to the 

 action of the left ven- 

 tricle on the fluid, and 

 is a phenomenon pro- 

 duced in a great measure 

 by the pressure exer- 

 cised by the finger. It 

 is called the pulse, and 

 is most usually felt at 

 the wrist (Fig. 41), the 

 radial artery being fa- 

 vourably placed in this 

 respect. 



103. The blood does not reach all the organs with the same 

 swiftness. Distance from the heart is one cause : but it is not 

 the only one. The arteries run mostly in a tortuous manner, 

 and this causes, as is well known, a retardation of the fluid 

 circulating. By a vast increase of minute branches, the blood 

 is spread into many channels ; hence arises another cause of 

 retardation as regards the arteries. Finally, Nature, all-fore- 

 seeing, provides, by numerous anastomoses or junctions of 

 the vessels, for any accidental stoppage or obliteration of the 

 larger or smaller trunks. 



104. Course of the Venous Blood. The blood passes, 

 by means of the capillaries, from the arteries into the veins. 

 The impulsion it first receives from the heart determines 

 its course in the veins. This is proved experimentally, by 

 placing a ligature on the artery supplying certain veins, and 

 thus cutting off the action of the heart ; the hsemorrhage from 

 a punctured vein will cease, even although the vein be full of 

 blood, and will return when the action of the heart is allowed 

 to influence it, by removing the ligature from the artery. 



* a, body of the pump representing the auricle, and receiving the liquid 

 by the canal c; b, body of the pump representing the ventricle ; d, canal of 

 communication, representing the auriculo-ventricular orifice, furnished with 

 a sucker permitting the fluid to pass from a to b, but opposing its return ; 

 e y the sucker or valve, situated at the opposite orifice of the pump b, repre- 

 senting the semilunar valves of the aorta, and having the same action as the 

 preceding valve or sucker ; /, canal, representing the aorta. 



