338 EFFECTS OF YASO-MOTOE ACTIONS. [BOOK i. 



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-dilator 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. 



171. 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 

 of the body are in a state of tonic, i.e. of moderate, contraction, 

 and it is the narrowing due to this contraction which forms a 

 large item of that peripheral resistance which we have seen to be 

 one of the great factors of blood pressure. The normal general 

 blood pressure, and therefore the normal flow of blood, is in 

 fact dependent on the ' general tone ' of the minute arteries. 



In the second place local vaso-motor changes in the condition 

 of the minute arteries, changes, that is to say, of any particular 

 vascular area, have very decided effects on the circulation. These 

 changes, though local themselves, may have effects which are both 

 local and general, as the following considerations will shew. 



Let us suppose that the artery A is in a condition of normal 



