CHAP, iv.] THE VASCULAR MECHANISM. 283 



longer seen, though all the rest of the nervous system be left intact. 

 Nay, more, by partially interfering with the bulb, we may partially 

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

 the centre in question within the bulb 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 bulb, the same stimulation will produce the same rise as 

 before ; the vaso-motor centre has not been interfered with. Pro- 

 ceeding downwards, however, and removing the bulb piecemeal 

 by successive transverse sections a level is soon met with, beyond 

 which removal of the nervous substance causes an obvious dim- 

 inution in the effect produced by the stimulation of the sciatic ; 

 this marks the upper limit of the centre. Proceeding still further 

 downwards with successive slices, stimulation of the sciatic pro- 

 duces less and less rise of blood pressure, until at last a level is 

 reached, at which even strong stimulation of the sciatic or any 

 other afferent nerve produces no effect at all on blood pressure ; 

 this marks the lower limit of the centre. In this way the lower 

 limit of the bulbar 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 quadri- 

 gemina. We may add that 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. But 

 we will reserve what we have to say as to the structural features 

 of this centre until we come to study the spinal bulb in detail. 



155. The above experiments appear to afford adequate evi- 

 dence that, in a normal state of the body, the integrity of the 

 bulbar vaso-motor centre is essential to the production and dis- 

 tribution of those continued constrictor impulses by which the 

 general arterial tone of the body is maintained, and that an 

 increase or decrease of vaso-constrictor action in particular arteries, 

 or in the arteries generally, is brought about by means of the same 

 bulbar vaso-motor centre. But we must not therefore conclude 

 that this small portion of the spinal bulb is the only part of 

 the central nervous system which can act as a centre for vaso-con- 

 strictor fibres ; and, so we have seen, there is no evidence at 

 present that the vaso-dilator fibres are connected with either this 

 or any other one centre. In the frog reflex vaso-motor effects may 

 be obtained by stimulating various afferent nerves after the whole 

 spinal bulb has been removed, and, indeed, even when only a com- 

 paratively small portion of the spinal cord has been left intact, and 

 connected, on the one hand, with the afferent nerve which is being 

 stimulated, and, on the other, with the efferent nerves in which 

 run the vaso-motor fibres, whose action is being studied. In the 

 mammal such effects do not so readily appear, but may with care 



