DEPRESSOR NERVE. 315 



Besides this chief vaso-motor centre it is probable that in the 

 higher animals, as certainly is the case in the frog, other centres 

 are distributed throughout the spinal cord, which seem to be able 

 to take the place of the great primary centre. For after the 

 spinal cord has been cut high up, the hinder extremities more or 

 less recover their vaso-motor power in a few days, and destruction 

 of the lower part of the spinal cord causes renewed vaso-motor 

 paralysis. In frogs this is very well marked, the centres being 

 less confined to the medulla than is the case in the more highly 

 organized animals. 



During recent times numerous investigations on the subject of 

 vaso-motor nerves have, no doubt, thrown much light on the 

 subject, but these inquiries have not made the nerve mechanism 

 by which the various vascular areas are governed so clear or so 

 obvious as might be wished. 



In order to explain and reconcile the various experimental 

 truths on this subject (too numerous to be mentioned here), we 

 must suppose that the vaso-motor nerve mechanisms are very 

 complex. The supposition of some such arrangements as the 

 following may help to simplify the matter in some degree to the 

 student. 



1. The bloodvessels have muscular elements which, though 

 commonly controlled by nerves, are capable of automatic activity. 

 A free supply of arterial blood is a sufficient stimulus for their 

 moderate action, and mechanical or other stimulus is capable of 

 exciting increased constriction. We know that such automatic 

 contractile elements exist in some of the lower animals (snail's 

 heart, hydra, etc.), and we have no reason to doubt their exist- 

 ence in mammals. Moreover, such a hypothesis obviates the 

 necessity of supposing that local nerve elements exist, which we 

 cannot recognize morphologically. 



2. In the medulla oblongata (in close relation to the centres 

 governing the respiratory, cardiac, intestinal, and other move- 

 ments subservient to the vegetative part of the economy) there 

 exist nerve centres which constantly exert an important influence 

 over the activity of the vessel muscles. These groups of nerve 

 cells, called the vaso-motor centres, are intimately connected with 



