264 HUMAN PHYSIOLOGY 



2nd. Vaso-dilator. 



A. Yaso-constrictor Nerves. The fact that section of 

 these at once causes a dilatation of the arterioles proves 

 that they are constantly transmitting impulses from the 

 central nervous system. 



Course. The course of these fibres has been investigated 

 by section and by stimulation (Fig. 126). 



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. 145) into the spinal nerve, 

 and run in it to their terminations. 



B. Yaso-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 submaxillary and sublingual 

 salivary glands. If this nerve is cut, no change takes place 

 in the vessels of the gland, but, when it is stimulated, the 

 arterioles dilate 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 play the same part in regard to the muscular fibres of 

 the arterioles as the inferior cardiac branch of the vagus 

 does in regard to the cardiac muscular fibres. As examples 

 of vaso-dilator nerves we may take the gastric branches of 

 the vagus carrying vaso-dilator fibres to the mucous mem- 

 brane of the stomach, and the nervi erigentes carrying vaso- 

 dilator fibres to the external genitals. 



The vaso-dilator nerves of most parts of the body run side 

 by side with the vaso-constrictor nerves ; and, hence, curious 

 results are often obtained. If the sciatic nerve of a dog is 

 cut, the arterioles of the foot dilate. If the peripheral end 

 of the cut nerve is stimulated, the vessels contract. But, 

 after a few days, if the nerve be prevented from uniting, the 

 arterioles of the foot recover their tonic contraction, and if 

 the sciatic nerve is then stimulated, a dilatation, and not a 

 constriction, is brought about. The vaso-constrictor fibres 

 seem to die more rapidly than the vaso-dilator fibres which 

 run alongside of them. Under certain conditions the activity 



