352 



PHYSIOLOGY 



CHAP. 



longer after-effect, and are more quickly tired than the constrictors. 

 Simultaneous stimulation showed the functional predominance of 

 the constrictors over the dilators, of which, however, the excitation 

 is manifested bv an after-effect in agreement with Baxt's 

 experiments on the cardiac nerves. When, on the contrary, the 

 constrictors are stimulated with a weak, and the dilators with a 

 strong, current, the latter predominate. 



Certain experiments of Lepine, and later on of Bernstein, 



'-, V 



FIG. 161. Plethysmograms of hind-limb of cat during electrical excitation of divided sciatic, 

 by induction shocks of varying frequency. (Bowditch and Warren.) The sciatic was excited 

 in the intervals comprised between the two vertical lines = 15". At A, with 1 shock at each 

 second, a slight vascular contraction, followed by marked dilatation, was obtained. At B, with 

 4 shocks at each second, the contraction was larger, with a larger succeeding vascular 

 dilatation. At C, with 16 shocks per second, there was marked contraction with very small 

 dilatation. At D. with 64 shocks per second, there was a very marked contraction, with no 

 subsequent vascular dilatation. 



indicate that the atonic or hypertonic state of the vessels previous 

 to electrical excitation of their respective nerves leads to the 

 preponderance of the constrictors over the dilators, and vice versa. 

 They found, namely, that the stimulation of the sciatic produced 

 constriction or dilatation of the vessels, according as the extremities 

 had previously been warmed or cooled with hot or cold water. 



Ostroumoff further showed that a different rhythm, or a 

 varying intensity of electrical excitation, led to the predominance 



