280 



VETERINARY PHYSIOLOGY 



These facts prove that the vaso-motor nerves may be divided 

 into two classes : 



1st. Vaso-constrictor. 



2nd. Vaso-dilator. 



A. Vaso-eonstrietor Nerves, The fact that section of these at 

 once causes a dilatation of the arterioles proves that they are con- 

 stantly transmitting impulses from the central nervous system. 



Course. Thecourseof these 

 fibres has been investigated 

 by section and by stimulation 

 (fig. 130). 



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 along the 

 various sympathetic nerves to 

 the viscera, or back through 

 the grey ramus (see fig. 75, 

 p. 151), into the spinal nerve, 

 and run in it to their ter- 

 minations. 



B. Vaso-dilator Nerves. A 

 good example of such a nerve 

 is to be found in the chorda 

 tympani branch of the facial 

 nerve, which sends fibres to 

 the subm axillary and sub- 

 lingual salivary glands. If 

 this nerve be cut, no change 

 takes place in the vessels of 



FIG. 130. Diagram of the Distribution 

 of Vaso-motor Nerves. The continu- 

 ous line shows the vaso-constrictors, 

 the dotted line the vaso-dilators. 

 C.N., cranial nerves ; Vag., vagus ; 

 T.S., thoracic sympathetic ; A.S., 

 abdominal sympathetic ; N.L., nerves 

 to the leg. 



the gland, but, when it is 

 stimulated, the arterioles di- 

 late and allow an increased 

 flow of blood through the 

 capillaries. These fibres, therefore, instead of increasing the 

 activity of muscular contraction, diminish or inhibit it. They 



