CH. XLIX.] CONDUCTION IN THE COED 705 



each hemisphere is related to the opposite half of the body, not only 

 in regard to motion but in regard to sensation also. The main 

 difficulty of investigators has been to determine exactly where the 

 crossing occurs; in man especially, many of the impulses cross 

 shortly after their entry into the cord, and then travel up to the 

 higher centres by paths on the opposite side of the cord to that by 

 which they enter. In such investigations on man, various injuries 

 to the cord have to be carefully studied, and it is such careful study 

 that has led to this conclusion. 



For fifty years, physiologists, stimulated by the work of Brown- 

 Sequard, have attempted to trace the upward paths of afferent 

 impulses through the spinal cord. By experiments on animals, the 

 secondary ascending degenerations which follow injury can be fol- 

 lowed with exactitude, and traced into bulb, cerebrum, cerebellum, 

 or other parts. But even though such animals may be long under 

 observation, they cannot tell us, and we can only with difficulty 

 and ill-success guess, how their sensations are affected by the lesion. 



On the other hand, the clinical observer may expend much 

 time and trouble in determining the nature of the loss of sensation, 

 but it is usually impossible to verify the anatomical position and 

 nature of the lesion. In many cases where a microscopical examina- 

 tion has been made, the disease has been of a progressive nature, or 

 the patient has died from complications, which detract from his 

 suitability for an examination of this kind. 



Nevertheless, by a combination of the experimental and clinical 

 methods, we have now arrived at some accuracy on these points, and 

 it is impossible to overestimate in this direction the value of the 

 psycho-physical examination of patients which has within the last 

 few years been made by Head, Eivers, and Sherren. 



In order, however, to examine the sensations of a patient, it is 

 necessary to know first how to classify the sensations which are the 

 result of cutaneous stimulation. To do this, we must somewhat 

 anticipate what we shall go into more fully in the chapter on the 

 Cutaneous Senses. It has been proved that, scattered over the 

 external surface of the body, are a number of spots, some which are 

 more susceptible to one form of stimulus than others. The spots are 

 of four kinds, known as touch spots, pain spots, heat spots, and cold 

 spots, and these correspond to the four kinds of sensations which we 

 experience as the result of cutaneous excitation. They correspond 

 to different kinds of end-organs in the skin, and the impulses are 

 carried to the central nervous system by different groups of fibres. 



Dr Head found, from an experiment he made by cutting a sensory 

 cutaneous nerve in his own arm, that although the patch of skin 

 supplied by the nerve was entirely destitute of sensation, the under- 

 lying parts were still sensitive to pressure and to pain. These deep 



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