456 



VETERINARY PHYSIOLOGY 



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proves that they are constantly transmitting vaso- constrictor 

 impulses from centres in the nervous system. 



(1) Course of the Xerves. — The course of these fibres has 



been investigated by section 

 and by stimulation (lig. 

 191). 



They leave the spinal 

 cord, chiefly in the dorsal 

 region, by the anterior roots 

 of the spinal nerves, pass 

 into the sympathetic ganglia, 

 where they have their cell 

 stations, and then, as non- 

 medullated fibres, pass, either 

 (a) along the various sym- 

 pathetic nerves to the vis- 

 cera, or (6) back through 

 the grey ramus (see fig. 46, 

 p. 106) into the spinal 

 nerve, in which they run 

 to their terminations. 



(2) Mode of Action of 

 the Mechanismi. — This 

 mechanism is constantly in 

 action, maintaining the tonic 

 contraction of the arterioles. 

 («) Reflex Stimulation. — 

 If any afterent nerve be 

 stimulated the effect is to 

 cause a general constriction 

 of arterioles, and thus to 

 raise the general arterial 

 pressure. A central mech- 

 anism therefore exists cap- 

 able of reflex excitation. In ordinary conditions, so many 

 afferent nerves are constantly being stimulated, that it is not 

 easy to say how far the tonic action of this centre is reflex 

 and dependent on the stream of afferent impulses. 



(6) Direct Stimulation. — The centre may undoubtedly 



Fig. 191. — Diagram of the Distribu- 

 tion of Vaso- motor Xerves. The 

 continuous line shows the vaso- 

 constrictors, the dotted line the 

 vaso-dilators. C'.A^., cranial nerves; 

 Vag., vagus; T.S., thoracic sym- 

 pathetic; A.S., abdominal sym- 

 pathetic ; N.L., nerves to the leg. 



