558 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



for determining the flow of blood to the various organs and tissues of 

 the body, and thus becomes a means of indirectly influencing their func- 

 tional activity. 



In certain instances stimulation of spinal nerves will not only pro- 

 duce contraction of the arterioles of different parts of the bod}^ through 

 reflex stimulation of the vaso-motor centre, but will also produce dilata- 

 tion of the arterioles in the vascular area supplied by that nerve ; thus, 

 for example, if in a rabbit under the influence of curare the central stump 

 of the great auricular nerve be stimulated with an induction current, the 

 blood pressure will be increased through constriction of the general vas- 

 cular areas, while inspection of the ear will show that its vessels have 

 become largely dilated. So, also, as already described under the section 

 on Digestion, when the chorda tympani nerve is stimulated, we not only 

 have increased secretion of saliva, but we have, also, an increase of the 

 supply of blood to the glands. Such nerves as the chorda tympani and 

 the great auricular, with numerous others, are spoken of as vaso-dilator 

 nerves, from the fact that their stimulation leads not to contraction of 

 arterioles, but to an increase in their calibre ; they, therefore, are of op- 

 posite function to the vaso-motor nerves. The vaso-dilator nerves, as a 

 rule, come from the cerebro-spinal system; the vaso-constrictor nerves 

 are, as a rule, branches of the sympathetic S3^stem. The explanation of 

 the functions of the vaso-dilator system of nerves is somewhat simplified 

 by the following observation : It has been stated that when the sciatic 

 nerve of a frog is divided, the vessels of the parts supplied by that nerve 

 dilate. Such dilatation is, however, usually transient. Twenty-four hours 

 after the section of the nerve the vessels ma}' be found to have quite 

 regained their normal calibre, even if still cut off from the central nerv- 

 ous sj'stem. Such a fact, which must, of course, be due to the regained 

 power of contraction of the circular muscular fibres, can only be ex- 

 plained through the assumption that the walls of the vessels are supplied 

 with nervous ganglia which are themselves capable of originating im- 

 pulses sufficient to produce varying degrees of contraction of the circular 

 muscular fibres of the arterioles. Normally, the impulses originated by 

 these peripheral ganglionic cells are dominated by the influences coming 

 from the central vaso-motor centre. When the vaso-motor nerve of a 

 part is divided these centres are suddenly deprived of this dominating 

 influence, the muscular fibres are paralyzed, and the arteries dilate. When 

 the shock of the operation has passed off, the peripheral ganglionic cells 

 themselves acquire the power of governing the degree of contraction of 

 the muscular fibres of the arteries and the vessels then regain their nor- 

 mal tone. When stimulation of the auricular nerve or of the chorda 

 tympani produces dilatation of the vessels of the parts supplied by these 

 nerves, the effect may be explained by assuming that the impulses com- 



