CHAP, i.] THE SPINAL COED. 713 



but superficial characters of which we shall speak later on, the 

 grey matter of the brain presents no histological features so 

 different from those of the grey matter of the cord, as to justify 

 us in concluding that the one is capable and the other incapable 

 of developing the impulses, which we call volitional, out of the 

 molecular nutritive changes of its substance. We are, there- 

 fore, led to the conclusion that the fuller automatic activity of 

 the brain is due to the intrinsic changes of its substance being 

 so much more largely assisted by the influx of various afferent 

 impulses and influences, notably those of the special senses. To 

 this question, however, we shall have to return later on. 



470. In treating of the vascular system we saw that the 

 central nervous system exercised through the vaso-motor nerves 

 such an influence on the muscular coats of the blood vessels as to 

 maintain, what we spoke of as 'tone,' section of vaso-constrictor 

 fibres leading to " loss of tone." We saw further, that arterial 

 tone, though normally dependent on the general vaso-motor 

 centre in the bulb, could be kept up by the cord itself, that for 

 instance a tone of the blood vessels of the hind limbs could be 

 maintained by the isolated dorso-lumbar cord. This mainte- 

 nance of arterial tone may be spoken of as one of the " auto- 

 matic " functions of the spinal cord. We have also seen that 

 plain muscular fibres, other than those of the arteries, notably 

 the fibres forming sphincters, such as the cardiac and pyloric 

 sphincters of the stomach, the sphincter of the bladder, and 

 especially the sphincter of the anus, also possess tone, and that 

 the tone of these sphincters is also dependent on the spinal cord, 

 or 011 some part of the central nervous system. We need not 

 repeat the discussions concerning these mechanisms and other 

 instances of the spinal cord exercising an automatic influence 

 over various viscera ; we have referred to them here, since they 

 serve as an introduction to a question which has been much de- 

 bated, and which has many collateral and important bearings, 

 namely the question whether the spinal cord exercises an auto- 

 matic function in maintaining a tone of the skeletal muscles. 



The question is not one which, like the case of arterial tone, 

 can be settled off hand by a simple experiment. Most observers 

 agree that the section of a motor nerve does not produce any 

 clearly recognizable immediate lengthening of a muscle supplied 

 by the nerve, in the same way that section of a vaso-constrictor 

 nerve undoubtedly gives rise to a relaxation of the muscular 

 fibres in the arteries governed by it ; and it has been inferred 

 from this that skeletal tone does not exist. But there are sev- 

 eral facts to be taken into consideration before we can come to 

 a just decision. 



The skeletal muscles have been described as being placed " on 

 the stretch " in the living body. If a muscle be cut away from 

 its attachments at each end, it shortens ; if ib be cut across, it 



