56 



ZOOI.IMJY. 



of the left ventricle on the fluid, and is a phenomenon produced 

 in a great measure by the pressure exercised by the finger. 

 It is called tin- pulse, and is most usually felt at the wrist 

 (Fig. 32), the radial artery being favourably 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 IK. t 

 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 haemorrhage 

 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. 



But there are other circum- 



stances influencing the motion of 

 the blood in the veins. These 

 vessels, and more especially those 

 of the limbs, are provided with 

 valves (b), permitting the blood 

 b to flow towards the heart, but 

 preventing its reflux toward the 

 capillaries. Every info-mi ({<{ 

 compression of these vessels con- 

 trilnttcs to f//( return of the 

 blood to the heart. 



105. The dilatation of thi- 

 ckest caused by respiration facili- 

 tatcs the return of the blood to- 

 wards the heart. By expiration 

 the movement of the blood in the 

 veins is momentarily interrupted; and the double motion 



i, 



Fig. 37. Vein, laid open. 



