THE SPINAL CORD. 397 



occupying above the dorsal region the larger part of the lateral columns, 

 and in the remainder of the cord their posterior half. 



Similar experiments have been performed by Ott* on the spinal cord 

 of the rabbit in the cervical region, with results essentially like those 

 above detailed, excepting that the effects of paralysis were exhibited 

 in the anterior limbs as well as the posterior, and that there was 

 evidence of a certain amount of decussation of the motor tracts in the 

 cervical portion of the cord. 



Descending degenerations of the pyramidal tract, according to Charcot, 

 do not usually extend to the motor nerves or nerve roots. From this 

 it is inferred that the pyramidal fibres terminate in the gray substance 

 of the anterior horns, while the nerve roots consist of new fibres 

 originating from the gray substance. The nerve root therefore degen- 

 erates only when divided beyond the point of its emergence from the 

 anterior horn. 



Crossed Action of the Spinal Cord. 



The spinal cord, as a medium of communication between the brain 

 and the peripheral organs, exerts a crossed action. Sensitive impres- 

 sions received by the integument on one side of the body are con- 

 ducted through the cord to the opposite side of the brain ; and motor 

 impulses originating on one side of the brain pass to the nerves and 

 muscles on the opposite side of the body. This is established both by 

 experiment on animals and by pathological observation in man ; since 

 lesions on the right side of the brain cause paralysis, both of sensation 

 and motion, on the left side of the body, and vice versa. These two 

 functions may be paralyzed either together or separately, according to 

 the locality and extent of the injury to the brain ; but when the paraly- 

 sis is distinctly confined to one side of the body, the alteration of ner- 

 vous tissue upon which it depends is found after death on the opposite 

 side of the brain. 



Decussation of the Motor Tracts. It may be said, in general terms, 

 that the transmission of voluntary motor impulses, in the spinal cord, 

 takes place continuously upon the same side. That is, if a transverse 

 section of one lateral half of the cord be made at any point in the 

 lumbar, dorsal, or cervical region, a paralysis of voluntary motion is 

 produced on the same side for all parts below the level of the injury. 

 This observation, first made by Galen, ( has been confirmed by all sub- 

 sequent experimenters. Each side of the body therefore derives its 

 power of voluntary motion from the pyramidal tract in the corre- 

 sponding half of the spinal cord. But at the decussation of the 

 pyramids, in the medulla oblongata, these tracts cross to the opposite 

 side, afterward continuing their course, through the tuber annulare and 

 crura cerebri, to the brain. A lesion of the motor tract accordingly 



* American Journal of the Medical Sciences. Philadelphia, October, 1879. 

 f De Locis Affectis. Liber III., Cap. xiv. 



