Chap, iv.] THE VASCULAR MECHANISM. 275 



very speedily becoming connected with nerve cells, others run- 

 ning a very long course before they so end. 



§ 148. Vaso-dilator Fibres. Some of these appear to run 

 much the same course as the vaso-constrictors. Such are the 

 vaso-dilator fibres running in spinal nerves like the sciatic and 

 brachial, those which seem to be present in the splanchnic, and 

 certain fibres of the cervical sympathetic which in some animals 

 at least act as vaso-dilators towards certain parts of the mouth 

 and face. With regard to these, the evidence of whose existence, 

 as we have seen, is at least in most cases, difficult, special or 

 indirect, we have at present no proof that their general course is 

 essentially different from that of the constrictors. 



The more distinct and notable vaso-dilators however do run a 

 different course. These are found in the nerves coming from the 

 cranial and sacral regions of the central nervous system whence, 

 as we have seen, no vaso-constrictor fibres are known to issue. 

 Thus the vaso-dilator fibres for the sub-maxillary gland running 

 in the chorda tympani may be traced as we have seen back to 

 the facial or seventh nerve ; and the continuation of the chorda 

 tympani along the lingual nerve to the tongue contains vaso-dila- 

 tor fibres for that organ ; when the lingual is stimulated, the 

 blood vessels of the tongue dilate owing to the stimulation of the 

 conjoined chorda tympani fibres. The ramus tympanicus of 

 the glossopharyngeal nerve contains vaso-dilator fibres for the 

 parotid gland, and it appears probable that the trigeminal nerve 

 contains vaso-dilator fibres for the eye and nose and possibly for 

 other parts. The vaso-dilator fibres which pass into the nervi 

 erigentes, leave the sacral region of the cord by the anterior roots 

 of the sacral nerves, the particular nerves differing in different 

 animals ; thus in the dog and cat they pass by the first, second 

 and third, in the rabbit by the second, third and fourth, in man 

 by the third, fourth and fifth sacral nerves. 



In these instances the vaso-dilator fibres, as they leave the 

 central nervous system, are, like the vaso-constrictor fibres, fine 

 medullated fibres, but unlike the majority at least of the vaso- 

 constrictors they retain their medulla for the greater part of 

 their course and only lose it near their termination in the tissue 

 whose blood vessels they supply. 



The Effects of Vaso-motor Actions. 



§ 149. A very little consideration will shew that vaso-motor 

 action is a most important factor in the circulation. In the first 

 place the whole flow of blood in the body is adapted to and 

 governed by what we may call the general tone of the arteries 

 of the body at large. In a normal condition of the body, the 

 muscular fibres of a very large number of the minute arteries 



