CHAP. iv.J THE VASCULAR MECHANISM. 335 



indicating the presence of vaso-constrictor fibres carrying the kind 

 of influence which we called tonic, leading to an habitual moderate 

 constriction. Neither arguments can be regarded as absolutely con- 

 clusive. 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 

 supplied by vaso-constrictor fibres proceeding from and governed 

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

 and on the other hand the abdominal viscera. As we shall see, the 

 variations 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. 

 The 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, unmistakably 

 vaso-dilator fibres are present. 



The Course of Vaso-motor Fibres. 



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

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

 the brain, but it will be desirable to speak of the course of the 

 two sets separately. 



Vaso-constrictor Fibres. In the mammal, so far as we know 

 at present, all the vaso-constrictor fibres for the whole body take 

 their origin in the middle region of the spinal cord, or rather, 

 leave the spinal cord by the nerves belonging to this middle 

 region. Thus in the dog the vaso-constrictor fibres, not only for 

 the trunk but for the limbs, head, face and tail, leave the spinal cord 

 by the anterior roots of the spinal nerves reaching from about the 

 second thoracic to the fourth lumbar nerve, both inclusive, though 

 some few may pass by the first thoracic and by the fifth lumbar. 



Those for the head and neck leave the spinal cord as we have 

 seen, 166, chiefly by the* second and third thoracic nerves, 

 though some leave by the fourth and a variable small number by 



