454 THE SPINAL CORD. 



stance alone is followed by almost complete paralysis of the parts below 

 the level of the injury. In the dorsal region, injury of the anterior 

 columns produces a greater amount of paralysis than that of the lateral 

 columns; in the cervical region, on the other hand, this relation is 

 reversed, the lateral columns taking a more important part in voluntary 

 transmission than the anterior. 



It is evident, accordingly, that in the spinal cord the transmission of 

 sensitive and motor impulses does not take place with the same sim- 

 plicity as in the nerves and nerve-roots. The various nervous tracts, 

 as well as the white and the gray substance, are associated in such a 

 manner as to make of the cord a single organ, more or less complicated 

 in structure, which cannot be separated, so far as our present knowledge 

 extends, into completely independent parts. 



Crossed Action of the Spinal Cord, 



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

 brain and the external parts, exerts a crossed action. That is, the 

 sensitive impressions received by the integument on one side of the 

 body are conducted through the cord to the opposite side of the brain ; 

 and the voluntary motor impulses which originate on one side of the 

 brain pass to the nerves and muscles on the opposite side of the body. 

 This is established both by experiments upon animals and by patholo- 

 gical observations in man ; since injury or disease situated upon the 

 right side of the brain is known to cause paralysis, both of sensation 

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

 These two nervous functions may be paralyzed either together or 

 separately, according to the locality and extent of the injury to the 

 brain substance ; but when the paralysis is distinctly confined to one 

 side of the body, the alteration of nervous tissue upon which it depends 

 is found after death to be seated upon the opposite side of the brain. 



The crossing or decussation of the motor and sensitive tracts, from 

 side to side, takes place in the following manner : 



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

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

 itself, takes place continuously upon the same side. That is, if a trans- 

 verse 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 upon the same side for all parts situated below the level of 

 the injury. This observation, which was first made by Galen, has been 

 confirmed by all subsequent experimenters. But in the cervical region, 

 the lateral columns gradually preponderate in importance, as the organs 

 of transmission for voluntary motion, over the anterior columns ; and 

 on approaching the level of the medulla oblongata, their fibres pass in 

 direction forward and inward, until they reach the inner and anterior 

 part of the cord. At the medulla oblongata, these fibres cross the 

 median line, as distinct bundles of considerable size passing obliquely 

 upward, to form the anterior pyramids of the opposite side. This 



