238 CIRCULATION OF THE BLOOD 



system, and it is very probable that their chief center, like the centers of the 

 other vegetative functions, lies in the medulla oblongata. 



The chief center for the vasoconstrictor nerves is positively known to lie 

 in the medulla. In the rabbit it occupies on each side a small prismatic 

 space; in man in a cross section taken at the level at which the facial nerve 

 passes off, it appears as one or more aggregations of gray matter on the 

 median side of the facial tract. From this center the vasoconstrictor nerves 

 descend chiefly, but not exclusively, in direct paths along the cord and pass 

 out in the nerve roots already mentioned (page 232). As we have seen, the 

 center is under tonic stimulation. If by transecting the cord, its influence 

 be cut off, the vascular tonus falls and as a consequence the blood pressure 

 becomes considerably less. 



But the vascular tonus is not entirely obliterated by this operation. On 

 the contrary, it has been shown that throughout the entire length of the 

 spinal cord, with the exception only of the cervical region, and the lowest 

 part of the lumbar, there are centers for the vasoconstrictor nerves which can 

 be stimulated both reflexly and by asphyxiation. These centers appear to be 

 less excitable than the vasomotor center in the medulla (although this is 

 denied). They do not react so promptly as that center; but their activity 

 lasts longer; and in virtue of their greater endurance they appear to be of 

 no less importance for the maintenance of vascular tonus. 



Experiment has shown also that after destruction of a large part of the 

 spinal cord, the tonus of the vessel may be gradually restored. The vessels 

 which receive their constrictor nerves from the destroyed part of the spinal 

 cord are at first entirely paralyzed, and they are dilated to their maximal 

 extent. But gradually their tonus returns; they react to local application of 

 cold and heat much the same as in the normal condition, but are not influ- 

 enced by distant parts of the body (Goltz and Ewald). Either the vascular 

 wall itself must have the property of contracting in a tonic manner, when it 

 is entirely isolated from the central nervous system, or this tonic contraction 

 is caused by ganglion cells strewn along the peripheral course of the vasomotor 

 nerves, which then serve as vasomotor centers of a third order. A definite 

 decision between these two alternatives is not possible, and the inherent prob- 

 ability of the one or the other naturally shapes itself according to one's 

 inclination to ascribe greater or less importance to the peripheral ganglia. 



The fact remains however that vessels entirely isolated from the central ner- 

 vous system can recover their tonus. And the action of the vasodilator nerves 

 favors the idea that vascular tonus is, at least in part, of peripheral origin. We 

 know of no muscles whose contraction could cause the vessels to dilate. Dilata- 

 tion must be caused therefore by diminishing the activity of the circular muscle 

 fibers i. e., vasodilator nerves must be a kind of inhibitory nerves. They can 

 even exercise their characteristic influence upon the vessels when all the vaso- 

 constrictor nerves to the same part of the body have been cut. In other words, 

 a certain tonus of the vessels remains after section of the constrictor nerves, 

 which however is entirely obliterated by stimulation of the vasodilator nerves. 



We can form the following conception, therefore, of the innervation of the 

 blood vessels. The musculature of the vessels is under the influence of the 

 central nervous system and of peripheral structures. In the former is found 



