330 THE NERVOUS REGULATION OF THE HEART 



The depressor nerve possesses a very characteristic and important 

 function. It is sensory in its nature and conducts impulses solely from 

 the heart to the nucleus of the vagus and the cardiac and vasomotor 

 centers. It must be obvious, therefore, that the effects ordinarily 

 obtained with the help of this nerve, can only be elicited by stimulat- 

 ing either the intact nerve or its central end. Concerning its function, 

 it may be stated in brief that it gives rise to reflexes which are centered 

 upon the cardiac and vasomotor mechanisms. The former produce 

 a reduction in the frequency of the heart and the latter, a fall in arterial 

 blood pressure. But their action upon the heart may be destroyed 

 by dividing the vagus distally to its point of union with the depressor 

 fibers. Naturally, the drop in pressure persists even after the division 

 and is then frequently associated with an increase in the frequency 

 of the heart. 1 



The foregoing data show very clearly that the depressor nerve 

 plays an important part in varying the resistance in the vascular 

 channels against which the heart must act. To illustrate, if the ar- 

 terial tension is too high, an impulse is set up in this organ which, on 

 being relayed to the cardiac and vasomotor centers, produces a re- 

 duction in the rate of the heart and a fall in the blood pressure. Ob- 

 viously, this reflex lessening of the peripheral resistance places the 

 cardiac muscle in a much more favorable position to contract with- 

 out strain. 



By connecting this nerve with a string galvanometer, Einthoven 2 

 has shown that sensory impulses are generated synchronously with 

 every contraction of the heart, but naturally, this fact does not signify 

 that the "depressor-reflex" is elicited an equal number of times. No 

 doubt, these impulses remain subminimal as a rule, and although trans- 

 mitte.d to the medulla, serve here merely the purpose of maintaining the 

 tonicity of the cardiac center. It has also been proved by Koster 

 and Tschermak 3 that electrical variations may be produced in this 

 nerve by increasing the intra-aortic pressure artificially. Inasmuch 

 as this nerve ramifies extensively upon the ascending portion of the 

 aorta, it may be surmised that these sensory impulses arise chiefly in 

 consequence of the mechanical stimulation resulting from the disten- 

 tion of this blood-vessel and, in a lesser degree, also from the disten- 

 tion of the heart itself. 



1 Bayliss, Jour, of Physiol., xiv, 1893, 303. 



2 Pfliiger's Archiv, cxxiv, 1908, 246. 



3 Ibid., xciii, 1903, 24; also see: Eyster and Hooker, Am. Jour, of Physiol., xxi, 

 1908, 373. 



