458 VETERINARY PHYSIOLOGY 



section of the spinal cord high up, the animal be kept alive 

 for some days, the dilated arterioles again contract. If the 

 spinal cord below the point of section be now destroyed, 

 another marked fall of blood pressure occurs. This shows 

 that secondary vaso-constrictor centres exist all down the grey 

 matter of the spinal cord. Normally these are under the 

 control of the dominant centre, but when this is out of action 

 they then come into play. 



B. Vaso-dilator Mechanism. — A good example of a vaso- 

 dilator 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 be cut, no change 

 takes place in the vessels of the glands, 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 activitv of muscular contraction, 

 inhibit it. The gastric branches of the vastus carrviuCT vaso- 

 dilator fibres to the mucous membrane of the stomach, and 

 the nervi erigentes carrying vaso-dilator fibres to the external 

 genitals, are further examples of vaso-dilator nerves. 



1. Course of the Fibres. — 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 be cut, the arterioles of the foot dilate. 

 If the peripheral end of the cut nerve be stimulated, the 

 vessels contract. But after a few days, if the nerve be pre- 

 vented from uniting, the arterioles of the foot recover their 

 tonic contraction. If the sciatic nerve be now 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 of the vaso-dilator fibres 

 seems to be increased. Thus, if the sciatic nerve be stimu- 

 lated when the limb is warm, dilatation rather than con- 

 striction may occur. Again, while rapidly repeated and 

 strong induction shocks are apt to cause constriction, slower 

 and weaker stimuli tend to produce dilatation. 



The vaso-dilator nerves pass out chiefiy by the anterior 



