CHAP, iv.] THE VASCULAR MECHANISM. 327 



pyramids. This space is largely occupied by a mass of grey 

 matter, called by Clarke the antero-lateral nucleus, and containing 

 large multipolar cells; but it is by no means certain that this 

 group of nerve cells really acts as the centre in question. 



177. The above experiments appear to afford adequate 

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

 medullary vaso-motor centre is essential to the production and 

 distribution of those continued constrictor impulses by which the 

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

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

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

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

 that this small portion of the medulla oblongata is the only part of 

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

 strictor fibres ; and, as we have seen, there is no evidence at pre- 

 sent 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 

 medulla has been removed, and indeed even when only a compara- 

 tively 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 

 and under special conditions be obtained. Thus in the dog, when 

 the spinal cord is divided in the dorsal region, the arteries of the 

 hind limbs and hinder part of the body, as we have already said, 

 172, become dilated. This one would naturally expect as the 

 result of their severance from the medullary vaso-motor centre. 

 But if the animal be kept in good condition for some time, a 

 normal or nearly normal arterial tone is after a while re-estab- 

 lished ; and the tone thus regained may, by afferent impulses 

 reaching the cord below the section, be modified in the direction 

 certainly of diminution, i.e. dilation, and possibly, but this is by 

 no means so certain, of increase, i.e. constriction; dilation of 

 various cutaneous vessels of the limbs may be readily produced 

 by stimulation of the central stump of one or another nerve. 



These remarkable results, which though they are most striking 

 in connection with the lower part of the spinal cord hold good 

 apparently for other parts also of the spinal cord, naturally suggest 

 a doubt whether the explanation just given above of the effects 

 of section of the medulla oblongata is a valid one. When we 

 come to study the central nervous system, we shall again and 

 again see that the immediate effect of operative interference with 

 these delicate structures is a temporary suspension of nearly all 

 their functions. This is often spoken of as 'shock' and may be 

 regarded as an extreme form of inhibition. An example of it occurs 

 in the above experiment of section of the dorsal cord. For some 



