THE VASOMOTOR NERVES. 541 



cially perhaps of the cutaneous nerves. And there is abundance 

 of evidence to show that similar results can be obtained in man. 

 The pressor effect manifests itself by a rise in general arterial pres- 

 sure, if a sufficiently large region is involved, and by a diminution 

 in size of the organ involved. On the other hand, depressor effects 

 may also be obtained from stimulation of many of the large nerve 

 trunks. If one stimulates the central end of the sciatic nerve, 

 for example, one obtains a pressor effect on the circulation in most 

 cases, but under certain conditions a marked depressor effect fol- 

 lows the stimulation.* The simplest explanation of such a result 

 is that the nerve trunks contain afferent fibers of both kinds. 

 When we apply our electrodes to a nerve we stimulate every fiber 

 in it and the actual result will depend upon which group of fibers 

 exerts the stronger action, and this may vary with the condition 



Fig. 225. Plethysmographic curve of forearm. The volume of the arm was recorded 

 by means of a counter-weighted tambour and the record shows the pulse waves. A problem 

 in mental arithmetic the product of 24 by 43 caused a marked constriction of the arm. 



of the nerve, the condition of the center, the anesthetic used, etc. 

 Under normal conditions no such gross stimulation occurs. The 

 pressor fibers are stimulated under some circumstances, the de- 

 pressor fibers under others. For instance, when the skin is exposed 

 to cold it is blanched not by a direct, but by a reflex, effect. The 

 low temperature stimulates the sensory (cold) fibers in the skin, 

 and the nerve impulses thus aroused reflexly stimulate the vaso- 

 constrictor center, or a part of it, and cause blanching of the skin. 

 Exposure to high temperatures, on the contrary, flushes the skin, 

 and in this case we may suppose that the sensory impulses carried 

 by the heat nerves inhibit the tone of the vasoconstrictor center 

 and cause dilatation or flushing of the skin. So far as man is 

 concerned, experiments made with the plethysmograph show very 

 *See Hunt, " Journal of Physiology," 18,381, 1895. 



