VASOMOTOR ACTIONS. 227 



of gray 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. 



163. The above experiments appear to afford adequate evidence that, in 

 a normal state of the body, the integrity of the medullary vasomotor 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 increase or decrease of vaso-constrictor action in particular arteries, 

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

 vasomotor centre. But we must not, therefore, conclude that this small por- 

 tion of the medulla oblongata is the only part of the central nervous system 

 which can act as a centre for vaso-constrictor fibres ; and, as 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 vasomotor effects 

 may be obtained by stimulating 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 vasomotor 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 ( 158), become 

 dilated. This one would naturally expect as the result of their severance 

 from the medullary vasomotor 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-established ; 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., dilatation and possibly, but this is by no means so cer- 

 tain, of increase i. e., constriction. Dilatation 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 connec- 

 tion with the lower part of the spinal cord, hold good apparently for other 

 parts also of the spinal core, naturally suggest a doubt whether the explana- 

 tion 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 func- 

 tions. 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 experi- 

 ment of section of the dorsal cord. For some time after the operation the 

 vaso-dilator nervi erigentes (which, as far as we know, have no special connec- 

 tion with the medullary vasomotor 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 still be brought about by suitable stimulation of sensory 

 surfaces. Hence the question may fairly be put whether the effects of cut- 

 ting and injuring the structures which we have spoken of as the medullary 

 vasomotor centre, are not in reality simply those of shock, whether the vas- 

 cular dilatation which follows upon sections of the so-called medullary vaso- 

 motor centre, does not come about because section of or injury to this^ region 

 exercises a strong inhibitory influence on all the vasomotor centres situated 

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

 oblongata in carrying on the all-important work of respiration, a mammal 



