THE CENTRAL NERVOUS SYSTEM 703 



temperature, seem to decussate in the cord. But there is also 

 evidence that some afferent impulses, including those coming from 

 the muscles and related to the muscular sense, decussate not in the 

 cord but in the bulb. 



The Paths for Different Kinds of Sensory Impressions. If 

 this is the state of our knowledge where the problem is merely 

 to determine the crossing-place of afferent impulses which are cer- 

 tainly known to cross, it is only to be expected that we should be 

 still more in the dark as regards the routes by which different kinds 

 of afferent impulses thread their way through the maze of conducting 

 paths in the neural axis to reach their planes of decussation and gain 

 the ' sensory crossway ' in the internal capsule. Some authors have 

 indeed cut the Gordian knot by assuming that any kind of sensory 

 impression may travel up any afferent path. Direct stimulation of 

 a naked nerve-trunk, it has been argued in favour of this view, gives 

 rise to a sensation of pain ; stimulation of the skin in which the 

 end-organs of the nerve lie gives rise to a sensation of touch or a 

 sensation of temperature, according as the stimulus is a mild 

 mechanical or a thermal one, the contact of a feather or of a hot 

 test-tube. Why, it has been asked, should we imagine that the 

 difference in the result of stimulation depends on a difference in 

 the nerve-fibres excited, and not on a difference in the kind of 

 impulses set up in the same nerve-fibres ? This is a question which 

 we shall have again to discuss (p. 757). But apropos of our present 

 problem, we may say that there is very clear proof from the patho- 

 logical side that a limited lesion in the conducting paths of the 

 central nervous system may be associated with defect or total loss 

 of one kind of sensation, while all the other kinds remain intact. 

 And there seems no other tenable hypothesis than that in such 

 cases the pathological change has picked out a particular group of 

 fibres, either collected into a single strand or scattered among 

 unaltered fibres of different function. For example, in locomotor 

 ataxia, a disease in which inco-ordination of movement and derange- 

 ment of the mechanism of equilibration are prominent symptoms, 

 degeneration in the posterior column of the cord is a most constant 

 lesion. And there is strong evidence that afferent impulses from 

 muscles and tendons, which give rise to impressions belonging to the 

 group of tactile sensations, and which, according to the most widely 

 accepted doctrine, serve as the basis of the muscular sense, and play 

 an important part in the maintenance of equilibrium (p. 731), pass 

 up in the posterior column, which may also conduct tactile im- 

 pressions from the skin. A case has been observed where a man 

 received a stab which divided the whole of one side of the cord and 

 the posterior column of the other side. Sensibility to touch was lost 

 on both sides of the body below the level of the injury, sensibility to 

 pain only on the side opposite to the main lesion. In another case, 

 in which some small syphilitic tumours (gummae) in the lateral 

 column on the left side caused marked degeneration in the left direct 



