THE HEART REFLEXES 



193 



both the rate of the heart beat and the duration of its different phases may re- 

 main almost entirely unchanged (Bayliss and Starling, 0. Frank, Hunt, et at.). 

 And yet we are not to suppose that the resultant effect is the algebraic 

 sum of the two when acting separately. For upon stimulation of the two 

 together, if the vagus effect predominates during stimulation, a characteristic 

 after-effect of the accelerator comes on when stimulation has ceased (Fig. 68). 

 The two nerves are not therefore to be regarded as true antagonists; for if 

 they were, stimulation of the two ought to give the same result as stimulation 

 of neither; that is, the peculiar after-effect of the accelerator ought not to 

 appear (Baxt). In view of these facts, it seems probable that the inhibitory 

 and " promoting fibers " have different modes of connection with the cardiac 

 muscle fibers. 



12. THE HEART REFLEXES 



The efferent cardiac nerves are roused to action reflexly both by the affer- 

 ent nerves of the heart itself, and by other nerves, and the heart is variously 

 influenced. 



On the anterior as well as on the posterior wall of the ventricle run 

 numerous nerves, which, on stimulation of their central cut ends, reflexly 

 raise or lower the blood pressure, and 

 accelerate or retard the rate of the heart 

 beat (Wooldridge). The heart itself 

 therefore through its own afferent nerves 

 can set in action mechanisms by which 

 the circulator} 7 apparatus can be changed 

 in one sense or the other,, according to 

 the momentary requirements. 



The depressor nerve discovered by 

 Ludwig and Cyon, which runs as a sepa- 

 rate nerve in the rabbit, is the most im- 

 portant of the nerve trunks conveying 

 fibers from the cardiac plexus. It rises 

 as a rule by two roots, one from the 

 cervical portion of the vagus, the other 

 from the superior laryngeal, and runs 

 parallel with the vagus to the cardiac 

 plexus (Fig. 69). According to Koster 

 and Tschermak, the depressor does not 

 come from the heart but from the aorta. 

 Because of its great importance for the 

 action of the heart, we shall discuss it in 

 this connection. 



Stimulation of its peripheral end 

 has no effect whatever. Stimulation of 

 the centra] end produces a fall in blood pressure and retardation of the heart 

 beat (Fig. 70). If the vagi are cut the latter effect is wanting, but the fall 

 in pressure occurs just as before. The retardation is therefore due to reflex 



FIG. 69. The depressor nerve of the rab 

 bit, after Ludwig and Cyon. 1, sympa- 

 thetic; 2, hypoglossal; 3. descending 

 branch of the hypoglossal; 4, branch 

 from the cervical plexus; 5, vagus; 6, 

 superior laryngeal ; 7, first and 8, second 

 root of the depressor. 



