272 THE COURSE OF VASO-MOTOR FIBRES. [BOOK i. 



possibility of the widening being due to vaso-dilator impulses 

 reaching the organ from without ; in such instances it is sug- 

 gested that the widening is due to a local effect of the products 

 of the activity of the organ. To this point we shall return. With 

 regard to vaso-constrictor fibres also the evidence that they are 

 supplied to muscles is, in like manner, not beyond dispute. 

 Section or stimulation of the nerves induces it is true changes in 

 the temperature of the muscles as it does in that of the skin. 

 But, as we urged just now, to argue from this that changes in the 

 blood supply have taken place is not wholly safe ; moreover the 

 changes in temperature observed are slight. Again, the fact that 

 when the nerve of a muscle is divided the blood vessels of the 

 muscle widen, somewhat like the blood vessels of the ear after 

 division of the cervical sympathetic, has been brought forward as 

 indicating the presence of vaso-constrictor fibres carrying the kind 

 of influence which we called tonic, leading to an habitual moder- 

 ate constriction. Neither arguments can be regarded as abso- 

 lutely conclusive. The knowledge we possess at present leaves 

 us in fact in doubt whether the blood-flow through the muscles, 

 though these form so large a part of the body, is really governed 

 by the central nervous system. 



The two parts of the body undoubtedly and pre-eminently sup- 

 plied by vaso-constrictor fibres proceeding from and governed by 

 the central nervous system are on the one hand the skin and on 

 the othar hand the abdominal viscera. As we shall see, the vari- 

 ations in the blood supply to the skin are more strikingly of use 

 to the body at large, in regulating the temperature of the body 

 for instance, than they are to the skin itself. ^Fhe variations in 

 the blood supply to the abdominal viscera also serve important 

 general purposes ; they play their part in the regulation of the 

 temperature of the body, and through them the viscera serve as 

 a reservoir to which blood may without harm be shunted when 

 occasion demands. It would appear as if the vaso-constrictor 

 mechanism were chiefly used for the general purposes of the 

 economy. 



Accepting the view that the presence of vaso-dilator fibres in 

 the nerves going to muscles is not definitely proved and disregard- 

 ing the scanty and more or less obscure vaso-dilators of the sciatic 

 and other spinal nerves, we find that in special cases only, in 

 cases where it would seem that special means are needed to 

 secure an ample flow of blood through a particular part, unmis- 

 takably vaso-dilator fibres are present. 



The Course of Vaso-motor Fibres. 



147. Both the vaso-constrictor and the vaso-dilator fibres 

 have their origin in the central nervous system, the spinal cord 



