326 VASO-MOTOR CENTRE. [BOOK i. 



The condition of the central nervous system seems to determine 

 whether the effect of afferent impulses on the central nervous 

 system is one leading to an augmentation of vaso-constrictor 

 impulses and so to a rise, or one leading to a diminution of vaso- 

 constrictor impulses and so to a fall of blood-pressure. 



176. We have used the words ' central nervous system ' in 

 speaking of the above ; we have evidence however that the part 

 of the central nervous system acted on by the afferent impulses 

 is the vaso-motor centre in the medulla oblongata, and that the 

 effects in the way of diminution (depressor) or of augmentation 

 (pressor) are the results of afferent impulses inhibiting or aug- 

 menting the tonic activity of this centre or of a part of this centre 

 especially connected with the abdominal splanchnic nerves. The 

 whole brain may be removed right clown to the medulla oblongata, 

 and yet the effects of stimulation in the direction either of dimi- 

 nution or of augmentation may still be brought about. If the 

 medulla oblongata be removed, these effects vanish too, though 

 all the rest of the nervous system be left intact. Nay, more, by 

 partially interfering with the medulla oblongata, we may partially 

 diminish these effects and thus mark out, so to speak, the limits of 

 the centre in question within the medulla itself. Thus, in an intact 

 animal under urari, stimulation of the sciatic nerve with a stimulus 

 of a certain strength will produce a rise of blood-pressure up to 

 a certain extent. After removal of the whole brain right down 

 to the medulla oblongata, the same stimulation will produce 

 the same rise as before ; the vaso-motor centre has not been 

 interfered with. Directly, however, in proceeding downwards, the 

 region of the centre in question is reached, stimulation of the 

 sciatic produces less and less rise, until at last when the lower 

 limit of the centre is arrived at no effect at all on blood-pressure 

 can be produced by even strong stimulation of the sciatic or other 

 afferent nerve. In this way the lower limit of the medullary vaso- 

 motor centre has been determined in the rabbit at a horizontal 

 line drawn about 4 or 5 mm. above the point of the calamus 

 scriptorius, and the upper limit at about 4 mm. higher up, i.e. 

 about 1 or 2 mm. below the corpora quadrigemina. When trans- 

 verse sections of the brain are carried successively lower and 

 lower down, an effect on blood-pressure in the way of lowering it 

 and also of diminishing the rise of blood-pressure resulting from 

 stimulation of the sciatic, is first observed when the upper limit 

 is reached. On carrying the sections still lower, the effect of 

 stimulating the sciatic become less and less, until when the 

 lower limit is reached no effects at all are observed. The centre 

 appears to be bilateral, the halves being placed not in the middle 

 line but more sideways and rather nearer the anterior than the 

 posterior surface. It may perhaps be more closely defined as a 

 small prismatic space in the forward prolongation of the lateral 

 columns after they have given off their fibres to the decussating 



