894 THE CENTRAL NERVOUS SYSTEM 



subserve volitional movements, and division of the anterior portion 

 of the antero-lateral column may cause deeper and more permanent 

 paralysis than division of the pyramidal tract. 



In the dog total section of the pyramids is not followed by com- 

 plete paralysis of voluntary movements, and stimulation of the 

 cortical motor areas can still elicit characteristic movements. It is 

 obvious that impulses emanating from the cortex can reach the 

 motor nuclei of the cord by other routes than the long pyramidal 

 fibres, possibly by paths with several segments, of which, for 

 example, the rubro-spinal tract (p. 867) may be one. Just as an 

 important business house may find it useful or indispensable to 

 supplement or replace the common telegraph service by private wires 

 in the interest of more prompt and satisfactory communication with 

 its principal correspondents, while still utilizing the ordinary channels 

 to some extent, so the higher brains may be supposed to have 

 developed more and more the direct service of the pyramidal tract 

 to tighten the grip of the cortex upon the motor nuclei of the cerebro- 

 spinal axis, while still availing themselves, although in diminishing 

 degree as their evolution proceeds, of the more primitive indirect 

 paths. 



Decussation of the Sensory Paths. On the other hand, it is certain 

 that pathological or traumatic lesions, apparently involving the 

 destruction of one lateral half of the cord in man and experimental 

 jemisections in some mammals, are followed by symptoms which 

 suggest that some kinds of sensory impulses decussate chiefly in the 

 spinal cord viz., diminution or loss of sensibility to pain and to 

 changes of temperature on the opposite side below the level of the 

 lesion, with little or no impairment, and often increase of sensibility 

 (hypersesthesia) on the same side. Tactile sensibility is lost on the 

 side of the lesion, and likewise the muscular sense. 



The first general description of this symptom-complex was given by 

 Brown-Sequard. On the basis of clinical observations in man, he 

 came to the conclusion that unilateral lesions of the cord, equivalent 

 approximately to a semisection, are associated with muscular paralysis 

 below the level of the lesion on the same side, and loss of cutaneous 

 sensibility on the opposite side, while on the side of the lesions there 

 may be an augmentation of sensibility. He interpreted these facts as 

 meaning that the sensory path decussates soon after its entrance into 

 the cord. The sensory path from the left side is therefore spared by a 

 lesion of the left side of the cord, but interrupted by a lesion of the 

 right side of the cord. The left and right motor paths, having already 

 decussated in the bulb, are cut by lesions in the left and right halves of 

 the cord respectively. Long afterwards Brown-Sequard saw cause to 

 retract this interpretation of the facts observed by him, but the majority 

 of subsequent observers have considered his original hypothesis more 

 satisfactory than his later ones. While it may be true that in man it has 

 not been rigidly demonstrated that the symptoms are associated with 

 a clean-out lesion precisely limited to one-half of the cord, clinical 

 observation lias on the whole tended to confirm the view that an 



