CHAPTER XXVI 

 THE CONTROL OF THE CIRCULATION (Cont'd) 



THE NERVE CONTROL OF THE PERIPHERAL RESISTANCE 



As already explained, the nerve control of the peripheral resistance 

 takes place through the action of vasoconstrictor and vasodilator nerve 

 fibers on the musculature of the arteriole walls. The vasoconstrictor 

 impulses like those in the vagus of the heart are tonic, so that when a 

 nerve containing such fibers is cut, the corresponding blood vessels un- 

 dergo dilatation (see page 135), and when their peripheral ends are stim- 

 ulated artificially, constriction occurs. On the other hand, the vasodi- 

 lator impulses do. not appear, at least under ordinary circumstances, to 

 be tonic, so that the cutting of such fibers does not cause vasoconstriction ; 

 their stimulation, however, causes marked dilatation. Vasomotor fibers 

 are contained in most of the efferent (motor) nerve trunks, and to 

 detect their presence the nerve must be either cut or stimulated and the 

 condition of the blood vessels of the innervated area observed. 



Methods for the Detection of Constriction or Dilatation 



Several methods, varying with the exact area under observation, can 

 be used for the detection of vasoconstriction or dilatation. In many cases 

 visual inspection is sufficient, as in the well-known experiment of Claude 

 Bernard on the blood vessels.in the ear of the rabbit (see Fig. 106). When 

 this is held with a light behind it, and the cervical sympathetic of the 

 corresponding side is cut, marked dilatation will become evident and 

 vessels will spring into view where previously there were none visible. 

 Visual inspection is usually also a satisfactory method of demonstrat- 

 ing vasodilatation or constriction in exposed glands, in mucous pas- 

 sages and in the vessels of the skin. 



Another comparatively simple method is the observation of the tem- 

 perature of the part, this being particularly useful when the vascular 

 area is one situated in the peripheral part of the body, such as the hand 

 or foot (see page 209). When dilatation occurs the temperature of the 

 part rises, because the warmer blood from the viscera flows with greater 

 freedom through the peripheral regions, where it is cooled off by radia- 

 tion. When a thermometer is placed between the toes of a dog or cat, a 



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