240 THE CIRCULATION OF THE BLOOD 



and in the hypogastric plexus situated on the neck of the bladder in the latter. 

 It is therefore commonly assumed that, although not recognizable by histological methods, 

 such terminal cell stations must also exist in close association with all blood vessels 

 to which the vasodilator fibers run. Whether or not such peripheral cell stations exist, 

 there is a marked difference between the course of vasodilator and of vasoconstrictor 

 fibers. 



The Vasomotor Nerve Centers 



Our next problem is to trace these fibers farther into the central 

 nervous system, and find the location and study the characteristics of 

 the nerve centers from Avhich they are derived. We must postulate the 

 existence of both vasoconstrictor and vasodilator centers, but since there 

 is no adequate evidence at the present time which enables us to locate 

 the latter, we must confine our attention to the vasoconstrictor centers. 

 These exist at two levels in the cerebrospinal axis: (1) in the gray mat- 

 ter of the spinal cord, and (2) in the gray matter of the medulla 

 oblongata. 



The spinal, or as they are often called, the subsidiary vasoconstrictor 

 centers, are represented by certain cells of the lateral horn of gray mat- 

 ter in the thoracic portion of the spinal cord, from which the pregan- 

 glionic vasoconstrictor fibers above described are derived. The exact 

 location of the nerve cells composing the chief centers in the medulla has 

 not as yet been definitely made out; they undoubtedly lie near those of 

 the vagus center (see Ranson). The axons of the medullary cells de- 

 scend in the lateral columns of the spinal cord to end by synapses 

 around the cells of the subsidiary vasoconstrictor center in the lateral 

 horns. 



The experimental evidence which indicates the existence of chief and 

 subsidiary centers is quite definite. Thus, if the spinal cord is cut at the 

 lower cervical region (below the phrenic nuclei, so as not to interfere 

 with the movements of the diaphragm), the arterial blood pressure falls 

 profoundly, because the pathway connecting the two centers is broken. 

 After several days, however, the blood pressure will gradually rise again. 

 If after this has occurred, the spinal cord is destroyed by pushing a wire 

 down the vertebral canal, the arterial blood pressure will again fall, 

 indicating that the vascular tone which had been reacquired after sec- 

 tion of the pathway between the main and the subsidiary centers must 

 have been brought about by the development in the subsidiary centers 

 of an independent power of reflex tonic action. This experiment there- 

 fore demonstrates that in the intact animal the subsidiary centers do not 

 by themselves discharge tonic impulses. In other words, the subsidiary 

 centers ordinarily serve merely as transfer stations for the tonic im- 

 pulses coming from the chief center, but when these impulses no longer 



