1104 PHYSIOLOGY 



lessening the resistance to the flow of blood there, affects too small 

 a vascular area to have any marked influence on the total resistance 

 of the circulation and therefore on the arterial blood pressure. If the 

 spinal cord be divided on a level with the region of the first dorsal 

 nerve, the blood pressure sinks considerably. In the dog it may fall 

 from 120 mm. Hg. to 40 or 50 mm. Hg. The heart after the section 

 beats more rapidly than before, so that the fall of pressure must be 

 ascribed to a change affecting the blood-vessels and lowering the 

 resistance to the flow of blood. Since a maximal effect on the blood 

 pressure is produced by section of the cord at this level, one may 

 conclude that the tonic constrictor impulses to all the vessels of the 

 body pass through this segment of the cord before leaving it to be 

 distributed to the arterial walls. The source of these impulses may 

 be made out by studying the effect of sections through different levels 

 of the nervous system. Division of the cord at about the first or 

 second lumbar nerve causes no effect on the blood pressure. On making 

 a section at the sixth dorsal root a considerable fall of pressure is pro- 

 duced, almost, but not quite, as great as that observed after section at 

 the first dorsal segment : stimulation of the lower end of the cut cord 

 causes almost universal vascular constriction and a large rise of blood 

 pressure. On the other hand, the fall of pressure is maximal when the 

 section is carried through the first dorsal segment or through any part 

 of the cervical cord. Section of the crura cerebri, or of the brain-stem 

 at the upper border of the fourth ventricle, leaves the blood pressure 

 unaffected. Destruction of a small region of the medulla situated on 

 each side of the middle line in the neighbourhood of the facial nucleus, 

 i.e. in the forward prolongation of the lateral columns after they have 

 given off their fibres to the decussating pyramids, causes an immediate 

 and maximal lowering of the blood pressure. 



We must therefore conclude that all the vessels in the body are 

 kept in a state of tonic contraction by impulses arising in this portion 

 of the medulla oblongata, travelling down the cord as far as the dorsal 

 region, and then passing out of the cord by the dorsal and upper 

 lumbar nerves. This conclusion is confirmed by the fact that, whereas 

 stimulation of the anterior roots of the cervical and lower lumbar 

 and sacral nerves has no influence on the blood pressure, a rise of 

 arterial pressure can be obtained by stimulating any of the anterior 

 roots from the first or second dorsal to the second or third lumbar. 

 The same effect is produced by stimulation of the white rami com- 

 municantes from these roots to the sympathetic system, or by excita- 

 tion of the sympathetic system itself. 



The portion of the medulla concerned with the sending out of the 

 tonic vaso-constrictor impulses is spoken of as the vaso-motor centre. 

 In this region it is exposed to and played upon by afferent impulses 



