922 TONE OF SKELETAL MUSCLES. [BOOK in. 



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, therefore, 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. 



597. 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 maintenance of arterial 

 tone may be spoken of as one of the " automatic " 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 on some part of the central 

 nervous system. We need not repeat the discussions concerning 

 these mechanisms and other instances of the spinal cord exer- 

 cising 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 debated, and which has many 

 collateral and important bearings, namely the question whether 

 the spinal cord exercises an automatic 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 several 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 it be cut across, it gapes. 



