The Bloodvessels. fi!> 



velocity in the venai cavse is half that in the aorta (Landois 

 and Stirling). 



The Duration of the Circulation depends upon the length 

 of time which it takes a red corpuscle to travel from a 

 given point and back to it again. In the horse the com- 

 plete circuit is performed in 31:5 seconds (Tierordt), and 

 is equivalent, according to the latter observer, to about 

 27 beats of the heart : this would give a pulse of 51 for 

 the horse, which is decidedly abnormal. According to 

 Munk, an amount of blood equal to that in the body passes 

 through the heart of a horse in fifty seconds, and of an 

 ox in forty seconds (M'Kendrick). The blood-cell spends 

 most of its time in the capillaries of the tissues and lungs. 



The volume of an organ depends upon the amount of 

 blood in it. This is well exemplified in certain patho- 

 logical conditions, such as engorgements of the spleen, 

 where the organ may be as much as ten times its normal 

 size. Physiologically we see the same fact demonstrated 

 in erection of the penis. 



We must now glance at the circulation through the 

 various systems of the body, and the methods by which it 

 is assisted. 



Arterial Circulation. — The walls of the arteries are both 

 elastic and muscular ; the former condition is predominant 

 in the large arteries of the bod}', where an elastic resistance 

 is necessary to provide for the extra bulk of fluid sent out 

 at each contraction of the left ventricle, and where, in 

 addition, the elastic recoil of the vessel converts an inter- 

 mittent into a continuous flow : the smaller vessels, on the 

 contrary, are principally muscular, for they act as a tap, 

 and regulate the blood supply to the part through thq aid 

 of certain nerves known as vaso-motor. 



We have previously drawn particular attention to the 

 fact that the pulsations in an artery represent a wave 

 resulting from the expansion and contraction of the larger 

 arterial walls, which movement travels very much faster 

 than the blood itself, but does not represent the progress 

 of the fluid in the tube. 



