394 THE NERVOUS SYSTEM. 



substance be divided, leaving only the posterior columns and the gray 

 substance, sensibility remains ; and Brown-Sequard has varied the mode 

 of procedure by dividing both anterior, lateral, and posterior columns 

 in the same animal at different levels, so that the continuity of the 

 cord as a whole is preserved by the gray substance, while all the longi- 

 tudinal tracts of white substance are divided. In this case sensibility 

 remains, although diminished in intensity. 



The transmission of sensitive impressions, therefore, takes place 

 through the gray substance. This substance, which is itself insensible 

 to direct irritation, forms the medium of communication between the 

 peripheral sensitive nerves and the brain above. It is not known 

 whether this communication be made by longitudinal fibres running 

 continuously through the gray substance, or by successive connections 

 of the nerve cells. 



The channels for voluntary motion in the spinal cord are mainly in 

 the posterior part of the lateral columns. These tracts have been shown 

 (page 385) to be continuous at the medulla oblongata with the anterior 

 pyramids and their prolongations above. They are therefore known 

 as the " pyramidal tracts ;" and they form the medium of communica- 

 tion between the brain and the origin of the motor nerves in the gray 

 substance of the spinal cord. This has been established by a variety 

 of investigations, carried on by different methods. It is certain, in the 

 first place, that the posterior columns take no direct part in the act of 

 voluntary motion, since after their complete section this power remains 

 unimpaired ; and according to Brown-Sequard, if all the rest of the cord 

 be divided, leaving the posterior columns untouched, voluntary motion 

 is lost in the parts below. There remain therefore only the lateral 

 and anterior columns of white substance which can serve as tracts of 

 communication for voluntary impulses. 



This question has received further elucidation from the study of 

 secondary degenerations in the spinal cord, first observed by Tiirck* 

 in 1851. These degenerations are similar to those of the spinal nerves 

 and nerve roots, when separated from their trophic centres. They take 

 place, both in the brain and spinal cord, in consequence of the destruction, 

 by a primary disorder, of certain nerve centres or the intervening parts ; 

 and they are therefore known as " secondary " degenerations. They 

 extend for long distances through the cerebro-spinal axis, involving 

 the tracts connected with the part primarily diseased ; and these 

 degenerated tracts can then be distinguished from the healthy white 

 substance by which they are surrounded. 



As in the nerves and nerve roots, secondary degenerations in the 

 spinal cord, may be ascending or descending. Ascending degenerations 

 are those which extend from the primary lesion upward to the brain 

 and are therefore centripetal. Descending degenerations extend from 

 the point of lesion downward through the cord, and are therefore 

 centrifugal. 



* Sitzungsberichte der Akademie der Wissenchaften. Wien, 1851, Band vi. } p. 288. 



