ANATOMY AND PHYSIOLOGY OF NERVOUS SYSTEM. 617 



(transverse commissures) whose function seems to be to unite the 

 ceKs of the right side to those of the left side of each segment. The 

 transverse commissures are but from one to three centimeters in 

 extent. 



In addition to the spinal commissures just mentioned there are 

 two other kinds formed by the long fibers uniting the spinal cord 

 either to the cerebrum or cerebellum. They are known as the 

 cerebro-spinal and cerebelh-spinal fibers. 



Experimental physiology, pathological anatomy, and embryology 

 all agree very admirably in demonstrating that the apparently homo- 

 geneous cord is composed of distinct and specialized parts. These 

 parts are called systems, which, in the white substance, form sec- 

 ondary columns, or bundles. 



White Columns of the Cord. 



Flechsig ascertained that in the foetus the different bundles of 

 nerve-fibers did not all take on myelin layers at the same time. By 

 taking advantage of this fact he was able to trace trie bundles of fibers 

 with myelin and thus map out the different tracts of the spinal cord 

 and brain. Gudden extirpated an organ of sense and after waiting 

 a sufficient length of time was able to trace the course of the atro- 

 phied nerve-fibers. 



The nerve-fibers of the cord enveloping the central gray axis are 

 distributed in different bundles or columns. These have previously 

 been mentioned cursorily, but will now be discussed in detail. 



Anterior Column. The anterior co'umn comprises that area be- 

 tween the anterior median groove and the line of implantation of the 

 anterior roots of the spinal nerves. Its most internal fibers are com- 

 missural; they cross throughout the whole extent of the cord and so 

 ntribute in the formation of the white commissure. Other fibers 

 across at the same level to connect the large cells of the anterior 

 rns of the two halves of the spinal cord. 



The anterior column comprehends two bundles : one, internal 

 ext to the median groove), is known as Turck's bundle, or direct 

 pyramidal bundle; the other, external, comprises the remainder of 

 e anterior column and is known as the root-bundle of the anterior 

 umn, or antero-lateral ground-bundle. 



The bundle of Tiirck (pyramidal bundle, direct cerebral, direct 

 'tor) is formed of centrifugal fibers which descend from the brain 

 to the cord without decussating at the level of the medulla ob- 

 gata. Its fibers are longitudinal and travel along and through 



