THE AUTOMATIC ACTIONS OF THE SPINAL CORD. 579 



fluences which flow into the centre from without. The automatism of the 

 spinal cord as a whole resembles, in this respect, that of the respiratory cen- 

 tre rather than that of the heart. It has for its basis doubtless the intrinsic 

 molecular changes of the gray matter, on whose remarkable constitution we 

 dwelt in a previous section ; the metabolic events of this substance are so 

 ordered as to give rise to discharges of energy ; but the discharge appears to 

 be also intimately dependent on the inflow into the gray matter of afferent 

 impulses and influences. The normal discharges of efferent impulses from 

 the cord undoubtedly take place under the influences of these incoming im- 

 pulses ; and it may be doubted whether the gray matter of the cord would 

 be able in the absence of all afferent impulses, to generate any sustained 

 series of discharges out of its merely nutritive intrinsic changes. The auto- 

 matic activity of the cord is fed not only by intrinsic nutritive events, but 

 also by extrinsic influences. 



In this feature we may, however, find perhaps the reason why the auto- 

 matic activity of the spinal cord is so limited as compared with that of the 

 brain. In spite of certain striking but superficial characters of which we 

 shall speak later on, the gray matter of the brain presents no histological 

 features so different from those of the gray matter of the cord as to justify 

 us in concluding that the one is capable and the other incapable of devel- 

 oping 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. 



510. In treating of the vascular system we saw that the central ner- 

 vous system exercised through the vasomotor nerves such an influence on the 

 muscular coats of the bloodvessels 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 

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

 instance, a tone of the bloodvessels 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 sphinc- 

 ters of the stomach, the sphincter of the bladder, and especially the sphinc- 

 ter 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 exercising an automatic influence 

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

 an introduction to a question which 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 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 con- 

 sideration before we can come to a just decision. 



