DEPRESSOR NERVE. 321 



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 vasomotor centres, are intimately connected with 

 the centres which preside over the functional activity of various 



organs and parts, and are also closely related to the nerves com- 

 ing from all parts of the circulatory apparatus. From these 

 centres impulses of two distinct kinds may emanate, the one 

 increasing the action of the contractile elements, and the other 

 inhibiting it. 



3. Direct communication between this vasomotor centre and the 

 contractile elements in the middle coat of the blood vessels is kept 

 up by means of efferent nerve channels of different sorts, some 

 bearing stimulating (vaso-constrictor), others inhibitory (vaso- 

 dilator) impulses, these being conveyed by nerve fibres which run 

 side by side in the same nerve cord. 



4. The activity of the contractile elements of any given vascu- 

 lar area may be altered by impulses arising from different sources. 

 () Local influences under ordinary circumstances are brought 

 but little into play, but, if cut off from the nervous centres, are 

 capable of controlling the local blood supply by changing the 

 degree of arterial constriction, (ft) Central influences from the 

 medulla are habitually in action, affecting all the vessels and 

 keeping up the vascular tone. These impulses are variously modi- 

 fied by changes occurring in distant parts of the circulatory appa- 

 ratus, and can be regarded as a general regulating mechanism. 

 They probably pass through the sympathetic chain, (f) Special 



