152 LECTURE VI. 



of passiveness which maintains the (so-called active) 

 hyperaemia. 



If we now pass from the muscular to the elastic con- 

 stituents of the vessels, we meet with a property which 

 is of very great importance, on the one hand in the 

 veins, the activity of which is in many cases to be wholly 

 referred to their elastic elements, on the other hand, in 

 the arteries, and particularly in the aorta and its larger 

 branches. In these the elasticity of the walls has the 

 effect of compensating for the loss which the pressure of 

 the blood experiences from the systolic dilatation of the 

 vessels, and of converting the uneven current produced 

 by the jerking movements of the heart into an even one. 

 If the walls of the vessels were not elastic, the stream 

 of the blood would unquestionably be rendered very 

 much slower, and at the same time, pulsation would 

 take place throughout the whole extent of the vascular 

 apparatus as far as the capillaries, for the same jerking 

 movement which is communicated to the blood at the 

 commencement of the aortic system would 'continue even 

 into the smallest ramifications. But every observation 

 we make in living animals teaches us that within the 

 capillaries the stream is a continuous one. This equable 

 onward movement is effected by the elasticity of the 

 walls of the arteries, in virtue of which they return the 

 impulse which they receive from the in-rushing blood 

 with the same force, and by this means maintain a regu- 

 lar onflow of the blood during the time occupied by the 

 following diastole of the heart. 



If the elasticity of the vessel be considerably dimin- 

 ished, without its becoming stiff and immoveable (from 

 calcareous incrustations) in the same degree, the dilata- 

 tion, which it undergoes from the pressure of the blood, 

 is not again compensated ; the vessel remains in a 



