264 



COMPARATIVE PHYSIOLOGY. 



It is not to be supposed that the change in any of these 

 cases is confined to any one vascular area invariably, but that 

 it is this or that, according to the nerve stimulated, the condi- 



j 



UU\JUU\J[JUU\JUUUlAJUU\^\JUlJlJULlUUU^ 



Fig. 220.— Curve of blood-pressure resulting from stimulation of the central end of 

 the depressor nerve. To be read from right to left. T indicates the rate at which 

 the recording surface moved, the intervals denoting seconds. At the current 

 was thrown into the nerve, and shut off at 0. The result appears after a period 

 of latency, and outlasts the stimulus (Foster) 



tion of the centers, and a number of other circumstances. 

 Moreover, it is importaat to bear in mind that with a fall of 

 blood-pressure in one region there may be a corresponding rise 

 in another. With these considerations in mind, it will be ap- 

 parent that the changes in the vascular system during the 

 course of a single hour are of the most complex and variable 

 character. 



The question of the distribution of vaso-motor nerves to 

 veins is one to which a definite answer can not be given. 



THE CAPILLARIES. 



The cells of which the capillaries are composed have a con- 

 tractility of their own, and hence the caliber of the capillaries 

 is not determined merely by the arterial pressure or any similar 

 mechanical effect. 



Certain abnormal conditions, induced in these vessels by 

 the application of irritants, cause changes in the blood-flow, 

 which can not be explained apart from the vitality of the ves- 

 sels themselves. 



Watched through the microscope under such circumstances. 



