CH. XLIX.] CONDUCTION IN THE CORD 707 



and not by the posterior columns. It certainly is the case that in 

 the condition called syringomyelia (a disease of the grey matter of 

 the cord), sensations of heat, cold, and pain are lost, but this is due 

 to the disease cutting through the crossing fibres which convey the 

 impulses in question. Head has pointed out that disease strictly 

 limited to the grey matter does not produce loss of any kind of 

 sensation, except by interfering with those paths which pass through 

 its substance. 



We have seen that afferent impulses pass into the cord by 

 peripheral nerves (the primary or peripheral level) in certain com- 

 binations from the protopathic, epicritic, and deep systems. In the 

 spinal cord these are sorted out and travel up in new combinations, 

 and it is possible that before they finally reach the cortex a fresh 

 sorting may take place in higher cell-stations before they ultimately 

 arrive at the seat of consciousness. The first rearrangement occurs 

 at the secondary level, that is on entrance into the cord, and a 

 further sorting at the third level, but of this but comparatively 

 little is known at present. We shall here only deal with the 

 rearrangement at the second level, as derived from a study of spinal 

 cord disease. 



We may make a rough comparison of what occurs, to what takes 

 place in the correspondence which flows in from all quarters to a 

 busy man, such as a Secretary of State. The letters will come from 

 all quarters, and deal with numerous topics ; some will be private 

 letters, some will be advertisements, some will be official, some 

 begging letters, and so forth. This mass of correspondence will be 

 sorted out by minor officials, the advertisements and the begging 

 letters will probably never reach the busy officer of State, and he will 

 therefore not be conscious of their existence unless he examines the 

 waste-paper basket. But the private letters and the official letters 

 will be sorted out into separate bags, whether they come from England 

 or from outlying parts of the Empire, and these ultimately reach his 

 eye. In the same way, the impulses that give rise to pain, whether 

 from cutaneous or deep structures, will all be combined and travel up 

 one path. Those due to heat or to cold, whether protopathic or 

 epicritic, in other paths; those which are tactile or motorial, in 

 another. And so a localised spinal lesion may interrupt all the 

 fibres subserving the sensation of heat without interfering with those 

 which underlie sensations of cold, and so forth. 



Tactile, painful, and thermal impulses, and those associated with 

 tactile localisation, cross in their passage through the spinal cord at 

 varying levels soon after entrance. But the sensory impulses which 

 underlie the recognition of passive position and movement, and finer 

 tactile discrimination, do not cross within the limits of the spinal 

 cord ; they pass up the posterior column on the side of entrance, and 



