THE CIRCULATION 



299 



tance, are all purely mechanical conditions dependent solely on the 

 structure and workings of the cardiac pump and the tubes of the circula- 

 tion. So much in the animal economy depends on the functions of the 



FIG. 159 



Section through the wall of an artery: a, endoth-lial c-l!s of intima; b, subendothelial con- 

 nective-tissue; c, internal elastic membrane; d, elastic connective-tissue; e, elastic fibers in the 

 substance of the media; f, nuclei of involuntary muscle fibers; g, external elastic membrane; 

 h, adventitia; i, connective- tissue; j, vasa vasorum. (Bates.) 



heart and on the relations of the blood-pressure that the pulse is of great 

 practical and theoretical importance. The knowledge to be gained from 

 it is somewhat lessened by the fact that what is felt is the resultant 



FIG. 160 



Tortoise cardiograms by the isolated suspension-method to show the tonus. The larger waves 

 are the (vase-motor?) variations in tone. To be read from left .to right. The time-line is in 

 seconds. (See also Fig. 157.) 



effect, the balance, of several combined influences, and it may be hard to 

 accord this effect to its several true causes. Thus, for example, a low 

 peripheral resistance from relaxed capillaries may give the same sort of 



