896 THE CENTRAL NERVOUS SYSTEM 



from the clinical observations which, however imperfect, are in such 

 questions our surest guide, it can only be this, that in man some 

 of the sensory impulses, and particularly those connected with the 

 cutaneous sensations of pain and temperature, decussate, in part at 

 least, in the cord. But there is also evidence that tactile afferent im- 

 pulses, including those coming from the muscles and related to the 

 muscular sense, and, it may be, some of the impulses associated with 

 pain, 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 certainly 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; stimula- 

 tion 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. But apropos of our present problem, we may 

 say that there is very clear proof from the pathological 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 syringo-myelia, a condition in which cavities are formed in the grey 

 matter of the cord secondary to a new growth of the neuroglia surround- 

 ing the central canal, a frequent symptom is the loss in a certain region 

 of sensibility to pain and to changes of temperature, while tactile 

 sensibility is unaffected (dissociation of sensations). Again, in loco- 

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

 derangement 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 either give rise to impressions belonging to 

 the group of tactile sensations, or produce no effect in consciousness, and 

 which, according to the most widely accepted doctrine, serve as the 

 basis of the muscular sense, and play an important part in the main- 

 tenance of equilibrium (p. 941), pass up in the posterior column. It 

 may also conduct tactile impressions 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 (gummata) in 



