712 THE BRAIN. 



Clinical histories, moreover, agree, at least to a large extent, in showing 

 that when the lesion is confined to one-half of the cord, the sensations 

 affected in the parts below the level of the lesion are chiefly or even exclu- 

 sively those of the crossed side. But there is not entire accordance, espe- 

 cially as to the crossing being complete. And with regard to the muscular 

 sense there is a distinct conflict of opinion ; the majority of cases seem to 

 show that in unilateral disease or injury to the cord, the muscular sense in 

 company with the voluntary movements fails on the same side ; but cases 

 have been recorded in which the muscular sense in company with other 

 sensations seemed to be affected on the crossed side ; it must be remem- 

 bered, however, that it is very difficult to appreciate a deficiency of muscu- 

 lar sense mingled with deficiencies in other sensations, and we should a 

 priori expect the muscular sense to run parallel with motor impulses. 



When, however, we appeal to clinical histories or indications as to the 

 several paths within the spinal cord taken by these several impulses, the 

 answer is a most uncertain one, as indeed might be expected from the too 

 often diffuse character of the lesions of disease ; and it is perhaps not too 

 much to say that no satisfactory deductions at all can be made. 



596. Whether, then, we turn to experiments on animals or to the study 

 of disease, the teachings with regard to sensation, in contrast to those with 

 regard to voluntary movement, are in the highest degree uncertain and 

 obscure. A few general reflections will perhaps help us to appreciate the 

 value of such facts as we possess. 



We have seen reason to think that in every movement, whether voluntary 

 and of cortical origin, or involuntary and started either as a simple spinal 

 reflex or through the working of some part or other of the brain, the motor 

 impulses, which sweep down the motor fibres to the muscles, issue mar- 

 shalled and coordinated from the gray matter of the cord (for the sake of 

 clearness we may omit the cranial nerves), from what we have called the 

 motor mechanisms of the cord. Analogy would lead us to suppose that 

 the afferent impulses, forming the bases of the several kinds of sensations, 

 similarly left the afferent fibres to join the gray matter of the cord in what 

 we may call the sensory mechanism. And such anatomical leading as we 

 possess seems to support this view; with the exception of the median pos- 

 terior tract, to which we will return immediately, all the fibres of a poste- 

 rior root seem to end in the gray matter not very far from the entrance of 

 the root. We have seen that a coordinate reflex movement may be carried 

 out by at least a few segments of the cord ; that a reflex movement may be 

 started by stimuli of various kinds and therefore presumably by afferent 

 impulses of various kinds ; and that impulses forming the basis of the 

 muscular sense are essential to the coordination of the movement. All 

 our knowledge goes to show that in reflex movement carried out by a few 

 segments of the cord, the whole chain of events between the arrival of the 

 afferent impulses along the posterior root and the issue of efferent impulses 

 along the anterior root may be carried out by gray matter, and gray matter 

 alone. We may further infer that, while on the one hand the same pro- 

 cedure might obtain not through a few segments only but along the whole 

 length of the cord, there would be an advantage, especially in respect to 

 the rapidity of transmission, in employing internuncial tracts of fibres be- 

 tween the several segments, the advantage being greater the more distant 

 the segments which have to work together. 



We might further suppose that it would be of advantage to possess some 

 direct path between the cerebral cortex and the spinal sensory mechanism 

 immediately connected with the posterior root, such as is afforded by the 

 pyramidal tract between the cortex and the spinal motor mechanism 



