THE VASOMOTOR NERVES. 543 



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. 226). 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.* Anatomical studies show 

 that in the heart the fibers arise in part at least in the walls 

 of the ventricle, and physiological experiments indicate that 

 the nerve plays an important regulatory role.f When, for in- 

 stance, blood-pressure rises above normal limits it may be sup- 

 posed that the endings of this nerve in the heart are stimu- 

 lated 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. { In this 

 position the effect of supranormal aortic pressures may more 

 readily effect a stimulation of their endings and cause a fall of 

 pressure. 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. 



Vasoconstrictor Centers in the Spinal Cord. From the description of the 

 vasoconstrictor mechanism given above the probable inference may be made 

 that throughout the thoracic region the cells of origin of the preganglionic 

 fibers may, under special conditions, act as subordinate vasoconstrictor centers 

 capable of giving reflexes and of exhibiting some tonic activity. Numerous 

 experiments tend to support this inference When the spinal cord is cut in 



* 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, u Archiv f . die gesammte Physiologie, " 93, 24,. 

 1902. 



