1026 PHYSIOLOGY 



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 w.alls. 

 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 produced, 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. Destruc- 

 tion 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 oi 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 communicantes from these roots to the sympathetic system, or by 

 excitation 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 from all portions 

 of the body, from the higher centres of the brain and the cortex cerebri, and 

 especially by afferent impulses travelling by the vagi from the viscera of 

 the chest and abdomen. Whether in the absence of all afferent stimuli the 

 centre would be active is doubtful; all we know is that the sum of the 

 stimuli arriving at the centre produces a state of average continued activity, 

 which is responsible for the maintenance of arterial tone and for the regulation 

 of the arterial blood pressure. 



The centre may also be affected directly by changes in its blood supply, 

 or in the composition of the blood flowing through it. Thus anything which 



