322 MANUAL OF PHYSIOLOGY. 



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 vasomotor effects which appear contra- 

 dictory. 



Section of a sensory nerve causes temporary vasomotor 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 

 flow of blood, or, in other words, is associated with a vaso-dila- 

 tor effect, probably dependent on the inhibitory effect of certain 

 centrifugal fibres which control the local agencies. 



Thus we must suppose that there exist local agents under the 

 control of the medullary centres, and that there are two distinct 

 efferent and afferent sets of exciting and inhibitory fibres passing 

 between the centre and periphery, along two perfectly distinct 

 routes ; one being in the direct track of the ordinary functional 

 nerve of the part, the other being in the sympathetic, wnich to 

 a great extent runs along the vessels themselves, and forms most 

 intricate networks capable of carrying impulses in all imaginable 

 directions. 



