PHYSIOLOGICAL STUDIES OK THK CHINOOK SALMON. 



443 



Table VIII. — Effects on the blood pressun and upon the heart rate produced by compressing the ventral 



aorta to the point of complete closure. 



In one case, No. 213, a pressure of 67 millimeters before compression of the 

 aorta was increased to 172 millimeters during compression. During this high pres- 

 sure the heart rate and the pulse pressure remain practically the same as before 

 compression, the rate increasing only 4 heats per minute. These tests reveal a 

 latent power of the heart quite enough to double the blood pressure, and, therefore, 

 to double the efficiency of the circulation if there is any coordinating mechanism by 

 which the salmon may call into activity this latent or potential heart energy, a fact 

 which remains to be seen. 



NERVOUS REGULATION OF THE HEART. 



My experiments have proved that the heart responds to vagus stimulation and 

 to reflexes. It was noticed over and over again that during the dissections made in 

 the process of experiments the heart was often very irregular. An example is given 



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Mim. 



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Fig. 6.— Experiment Aug. 22. 1902, No. 209, male: length, 918 millimeters. 

 Showing the fall in ventral aortic pressure and the irregular heart rate fol- 

 lowing cutaneous stimulation. 



in figure 6, where cutting away the margin of the opercle in the process of exposing 

 the vagus nerves led to well-marked cardiac inhibitions, with fall of blood pressure. 

 These inhibitions can be brought about reflex ly by stimulating various parts of the 

 skin. Direct stimulation of the vagus nerve with rapidly interrupted induction cur- 

 rents produced marked slowing of the heart rate with fall of blood pressure during 

 the continuance of the stimulation. If the currents used were strong enough the 

 slowing of the heart passed over into complete inhibition. The heart escaped 

 from inhibition with single strong contractions, these contractions coming at long 



