CHAP, iv.] THE VASCULAR MECHANISM. 347 



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

 the spinal cord is divided in the thoracic 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 bulbar 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 not 

 so certain, of increase, i. e. constriction ; dilation of various cutane- 

 ous 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 spinal bulb 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 in part 

 at least regarded as an extreme form of inhibition. An example 

 of it occurs in the above experiment of section of the thoracic cord. 

 For some time after the operation the vaso-dilator nervi erigentes 

 (which have no special connection with the bulbar vaso-motor 

 centre) cannot be thrown into activity as part of a reflex action ; 

 their centre remains for some time inactive. After a while 

 however it recovers, and erection of the penis through the- nervi 

 erigentes may then be brought about by suitable stimulation of 

 sensory surfaces. Hence the question may fairly be put whether 

 the effects of cutting and injuring the structures which we have 

 spoken of as the bulbar vaso-motor centre, are not in reality 

 simply those of shock, whether the vascular dilation which follows 

 upon sections of the so-called bulbar vaso-motor centre, does not 

 come about because section of or injury to this region exercises a 

 strong depressing influence on all the vaso-motor centres situated 

 in the spinal cord below. Owing to the special function of the 

 spinal bulb in carrying on the all-important work of respiration, 

 a mammal whose bulb has been divided cannot be kept alive for 

 any length of time. We cannot therefore put the matter to the 

 simple experimental test of extirpating the supposed bulbar vaso- 

 motor centre and seeing what happens when the animal has 

 completely recovered from the effects of the operation : we have 

 to be guided in our decision by more or less indirect arguments. 

 And against the argument that the effects are those of shock, 

 we may put the argument, evidence for which we shall meet with 

 in dealing with the central nervous system, that when one part of 



