CHAP. IL] THE BRAIN. 1103 



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 grey matter, and grey matter 

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

 same procedure 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 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 connected 

 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 contrast to what 

 takes place in the motor tracts, relays of grey matter are con- 

 tinually being interpolated. 



The median posterior tract, since it gathers up representatives 

 of successive nerves, presents itself as the nearest approach to 

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

 in the bulb, and is not continued on directly to the cortex. And 

 possibly it does play a somewhat analogous part, in so far as it 

 serves as a special connection between the brain and the whole 

 series of spinal nerves. But we are wholly ignorant as to what it 

 really does ; and whatever be the exact nature of the part which 

 it plays, it probably has relations not to one kind of sensation 

 only, but to all the different kinds of sensation. It has indeed 

 been supposed by some to be especially a tract for the impulses 

 of the muscular sense ; but neither experiment nor clinical study 

 affords adequate proof of this view. The condition known as 

 locomotor ataxy, the salient feature of which is loss or impairment 

 of muscular sense, is associated with disease of the posterior root 

 and of its entrance into the cord, not with disease confined 

 exclusively to the median posterior column. Moreover the tract 

 cannot carry all the impulses of muscular sense, since some of 

 them must pass at once into the grey matter, to take part in the 

 coordination of reflex movements, and must therefore travel by 

 fibres which do not form this tract. Similarly is there no 

 adequate proof of the tract being an exclusive channel for 

 tactile or for painful sensations. 



We may also perhaps urge similar considerations with regard 

 to the cerebellar tract, which though starting from a relay of grey 

 matter is thence onward to the cerebellum a continuous tract. 

 This tract also has been supposed to carry impulses of a par- 



F. 70 



