THE VASOMOTOR NERVES. 563 



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. These fibers in some ani- 

 mals 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). 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 sympa- 

 thetic. 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. 234). The latter effect is due 

 to a reflex stimulation of the cardio-inhibitory center and may 

 be eliminated by previous section of the vagus. The fall of 

 blood-pressure is explained by supposing that the nerve, when 

 stimulated, inhibits, to a greater or less extent, the tonic activ- 

 ity of the vasoconstrictor center.* Physiological experiments 

 indicate that the nerve plays an important regulatory role.f When, 

 for instance, blood-pressure rises above normal limits it may be 

 supposed that the endings of this nerve in the heart are stimulated 

 by the mechanical effect, and the blood-pressure is thereby lowered 

 by an inhibition of the tone of the constrictor center. It is possible, 

 according to recent work, that the depressor fibers end in the walls 

 of the aorta outside the heart. J In this position the effect of 

 supranormal aortic pressures may readily effect a stimulation of 

 their endings and cause a fall of pressure. 



A most suggestive example of the regulating action of the depressor nerve 

 is given by Sewall. When the carotids in a rabbit are clamped a variable 

 and not very large rise of arterial pressure is observed. If, however, the 

 depressor nerves are first cut, clamping the carotids causes an extraordinary 

 rise of arterial pressure. When the carotids are closed we may suppose that 

 the resulting anemia of the medulla stimulates the vasoconstrictor center 

 and thus tends to raise arterial pressure, but this effect is neutralized because 

 as the pressure rises the depressor fibers of the heart are stimulated. It 

 seems evident that during life the depressor fibers must exert a very important 

 regulating effect upon the circulation. 



A similar nerve has been described anatomically in man, while 

 in animals like the dog, in which it is not present as a separate 

 anatomical structure, it probably exists within the trunk of the 

 vagus. If this latter nerve is cut in the dog and the central end 

 is stimulated a depressor effect is usually obtained. 



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

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



t Sewall and Steiner, "Journal of Physiology, " G, 162, 1885. 



t Koster and Tschermak, " Archiv f . die gesammte Physiologic, " 93, 24, 

 1902. 



