THE VASOMOTOR NERVES. 605 



medulla, the impulses along one path exciting the center, while 

 those along the other inhibit its tone, or, as explained below, excite 

 a vasodilator center. Among the many depressor effects that 

 have been observed on stimulation of afferent nerve fibers one has 

 aroused especial interest — namely, that caused by certain afferent 

 fibers from the heart or from the aorta. These fibers in some 

 animals — the dog, for instance — run in the vagus nerve, but in 

 other animals, the rabbit, they form a separate nerve, the so-called 

 depressor nerve of the heart — discovered by Ludwig and Cyon 

 (1866). So far as the effect in question is concerned the physiolog- 

 ical evidence indicates that the fibers arise from the descending 

 aorta and it might be more appropriate to speak of them as the 

 depressor nerve of the aorta.* In the rabbit this nerve forms a 

 branch of the vagus, arising high in the neck by two roots, one 

 from the trunk of the vagus and one from the superior laryngeal 

 branch. It runs toward the heart in the sheath with the vagus 

 and the cervical sympathetic. The nerve is entirely afferent. 

 If it is cut and the peripheral end is stimulated no result follows. 

 If, however, the central end is stimulated a fall of blood-pressure 

 occurs and also perhaps a slowing of the heart beat (see Fig. 255). 

 The latter effect is due to a reflex stimulation of the cardio-inhib- 

 itory center and may be eliminated by previous section of the 

 vagi. The fall of blood-pressure is explained by supposing that 

 the nerve, when stimulated, inhibits, to a greater or less extent, the 

 tonic activity of the vasoconstrictor center, f Physiological ex- 

 periments indicate that the nerve plays an important regulatory 

 role. I When, for instance, blood-pressure rises above normal 

 limits, it may be supposed that the endings of this nerve in the 

 aorta or heart are stimulated by the mechanical effect, and the 

 blood-pressure is thereby lowered by an inhibition of the tone of the 

 constrictor center. Moreover, it has been shown by Einthoven 

 that every heart beat sends up this nerve a series of nerve impulses, 

 that is, when the nerve is cut and the ends are connected witli 

 a string-galvanometer, electrical variations occur synchronous 

 with the heart beat (Fig. 280). To explain this result we can 

 only assume that each heart beat stimulates sensory endings 

 in the heart itself or in the aorta, and that the nerve impulses 

 thus transmitted to the medulla probably play a role in main- 

 taining the tonic activity of some of its centers, perhaps, as 

 Einthoven suggests, the tonic activity especially of the cardio- 

 inhibitory center. 



* See Eyster and Hooker, "American Journal of Physiology," 21, 373, 

 1908; also Koster and Tschermak, "Archiv f. die gesammte Physiologie, " 

 93, 24, 1902. 



t See Porter and Beyer, "American Journal of Physiology," 4, 283, 1900; 

 also Baylies, "Journal of Physiology," 14, 303, 1893. 



J Sewall and Steiner, ".Journal of Physiology," 6, 162, 1885. 



