214 VASO-MOTOR CENTRES. [BOOK i. 



motor fibres are actually in functional connection with it may perhaps 

 be doubted; but at all events the fibres passing to so many vascular 

 areas, and those of such magnitude and importance, are by means 

 of it brought into functional relationship with so many afferent 

 nerves of the body, that it may fairly be spoken of as the general 

 vaso-motor centre. 



But the use of this phrase must not be understood to imply 

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

 the central nervous system which can act as a vaso-motor centre. 

 In the frog reflex vaso-motor effects may be obtained by stimu- 

 lating various afferent nerves after the whole medulla has been 

 removed, and indeed even when only a comparatively 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-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 become dilated. This one would naturally expect as the 

 result of their severance from the general 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-esta- 

 blished ; and the tone thus regained may be modified in the direc- 

 tion certainly of dilation, and possibly, but this is by no means so 

 certain, of constriction by afferent impulses reaching the lumbar 

 cord. Erection of the penis through the nervi erigentes may then 

 be still brought about by suitable stimulation of sensory surfaces, 

 and dilation of various vessels of the limbs 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 lumbar cord hold good apparently for the 

 dorsal cord also and indeed for all parts 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. And the question may 

 fairly be put whether the effects of cutting and injuring the 

 structures which we have spoken of as the medullary vaso-motor 

 centre, are not in reality simply those of shock. The case of the 

 dog with the divided dorsal cord, and other similar cases, clearly 

 prove that parts of the spinal cord, other than the particular 

 region of the medulla oblongata of which we are speaking, may 

 act as vaso-motor centres. And we may very fairly at least put 

 forward the view, that the vascular dilation which follows upon 



