THE VASOMOTOR NERVES. 



539 



graphs, the vasoconstrictor fibers emerge from the cord over a 

 definite region, and they exhibit constant tonic activity. It has 

 been shown, moreover, that if the cord be cut anywhere in the 

 cervical region all of the constrictor fibers lose their tone; a great 

 vascular dilatation results in both the splanchnic and skin areas. 

 We may infer from this fact that the vasoconstrictor paths originate 

 from nerve cells in the brain and that their tonic activity is to 

 be traced to these cells. Such a group of cells exists in the medulla 

 oblongata, and forms the vaso- 

 constrictor center. The axons 

 given off from these cells de- 

 scend in the cervical cord and 

 terminate at various levels in 

 the anterior horn of gray mat- 

 ter in the region from the upper 

 thoracic to the upper lumbar 

 spinal nerves. A spinal neuron 

 continues the path as the pre- 

 ganglionic vasoconstrictor fiber 

 which terminates, as already 

 described, in some sympathetic 

 ganglion, whence the path is 

 further continued by the post- 

 ganglionic fiber. This arrange- 

 ment of the constrictor paths is 

 indicated schematically in Fig. 

 224. The exact location of the 

 group of cells that plays the im- 

 portant role of a vasoconstrictor 

 eenter has not been determined 

 histologically. Its region has, 

 however, been delimited roughly 

 by physiological experiments. If 

 the brain is cut through at the 

 level of the midbrain there is no 

 marked loss of vascular tone in 

 the body at large. If, however, 

 similar sections are made farther 

 and farther back a point is 



reached at which vascular paralysis begins to be apparent and a 

 point farther down at which this paralysis is as complete as it 

 would be if the cervical cord were cut. Between these two points 

 the vasoconstrictor center must lie. The careful experiments of 

 this kind made by Dittmar* are now somewhat old. According 

 * " Berichte d. Sachs. Akademie, Math.-phys. Klasse, " 1873, p. 449. 



Fig. 224. Schema to show the path 

 of the vasoconstrictor fibers from the vaso- 

 constrictor center to the blood-vessel and 

 the mechanism for the reflex stimulation 

 of these fibers: v. c, The vasoconstrictor 

 center; 1, the medullary neuron on the 



vasoconstrictor path ; 2, the spinal neuron 

 (preganglionic fiber) ; 3, the sympathetic 

 neuron (postganglionic fiber) ; a, the arte- 



riole ; 4, the sensory fibers of the posterior 

 root making connections by collaterals 

 with the vasoconstrictor center; 5, an in- 

 tercentral fiber (efferent) acting upon the 

 vasoconstrictor center. 



