806 CUTANEOUS SENSATIONS. [BOOK in. 



tions 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 marshalled 

 and coordinated from the grey 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 grey matter of the cord in what we may call the sen- 

 sory mechanism. And such anatomical leading as we possess 

 seems to support this view ; with the exception of the median 

 posterior tract, to which we will return immediately, all the fibres 

 of a posterior root seem to end in the grey 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 mus- 

 cular sense are essential to the coordination of the movement. 

 All our knowledge goes to shew that in a reflex movement car- 

 ried 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 grey matter, and grey matter alone. We 

 may further infer that, while on the one hand the same proce- 

 dure might obtain not through a few segments only but along 

 the whole length of the cord, there would be an advantage, espe- 

 cially in respect to the rapidity of transmission, in employing 

 internuncial tracts of fibres between 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 pos- 

 terior root, such as is afforded by the pyramidal tract between 

 the cortex and the spinal motor mechanism immediately con- 

 nected with the anterior root. But no anatomical evidence of 

 such a tract is forthcoming; and, as we have before remarked, 

 along all the tracts which seem to be sensory in nature, in con- 

 trast to what takes place in the motor tracts, relays of grey 

 matter are continually being interpolated. 



The median posterior tract, since it gathers up representa- 

 tives of successive nerves, presents itself as the nearest approach 

 to such a sensory homologue of the pyramidal tract, though it 



