316 MANUAL OF PHYSIOLOGY. 



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 in- 

 creasing the action of the contractile elements, and the other 

 inhibiting it. 



3. Direct communication between this vaso-motor centre and the 

 contractile elements in the middle coat of the bloodvessels 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, 

 (a.) 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. '(.) 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, (y.) Special 

 influences, which are associated with the functions of the different 

 parts and organs, are only called into operation during the per- 

 formance of the function, whatever it may be. These impulses 

 probably are conveyed by the same nerves as excite the various 

 forms of functional activity, namely, ordinary peripheral nerves. 



These three sets of influences are variously brought about in 

 different parts, and thus we find that section or stimulation of 

 the different nerves gives vaso-motor effects which appear contra- 

 dictory. 



Section of a sensory nerve causes temporary vaso-motor paraly- 

 sis, owing to the tonic constrictor influence being cut off. Stimu- 

 lation of the peripheral stump causes vaso-constriction from 

 excitation of the same fibres. 



The stimulation of a motor nerve-fibre causes an increase in the 



