THE SPINAL CORD AS A CONDUCTING PATH 



609 



way poured into the central nervous system are of different kinds and 

 may either remain within the domain of the cord or may be conveyed 

 onward to higher centers. The same holds true of the efferent im- 

 pulses. While some of them arise in the brain and neighboring parts, 

 some also originate in the motor cells of 

 the cord itself Obviously, therefore, the 

 conduction system of the cord is arranged 

 in the form of a long or projection system 

 and a short or reflex system. The latter 

 is the more primitive, and hence, we find 

 that it occupies a position next to the 

 gray matter, while the projection paths 

 correlating peripheral parts with the 

 brain, form the external shell of the 

 spinal white matter. 



The axons of the nerve cells uniting 

 these widely separated portions of the 

 nervous system, are of different lengths. 

 It is said that the motor neurons in 

 the anterior horn of the spinal gray 

 matter reach all the way to the periphery 

 and attain a length of 1 .0 m. The same 

 holds true of the motor cells of the cere- 

 bral cortex, the axons of which terminate 

 low down in the cord. In many cases, 

 however, two or three neurons are re- 

 quired to cover a distance of only a few 

 centimeters. In adult life, the axons of 

 the spinal white matter are surrounded 

 by medullary sheaths but not by neuro- 

 lemma. They differ, therefore, in this 

 regard from ordinary nerve fibers. They 

 are of different size and give off small 

 collaterals which connect with the gray 

 matter at different levels of the cord. 

 Externally, they are invested by a tube 

 formed by neuroglia tissue. 



The Methods Used for the Localiza- 

 tion of Spinal Conduction. — We have 

 previously seen that the white matter of 

 the cord is arranged as anatomically dis- 

 tinct bundles. The question may now 

 be asked whether these morphological 

 units also represent physiological entities. In other words, can it be 

 proven that the different fasciculi possess a different origin and desti- 

 nation so that their direction of conduction assumes a specific char- 

 acter? While the investigations pertaining to this topic cannot be 



Fig. 301. — Sections thkough 



DiFFEEENT REGIONS OF THE SpINAL 



Cord. 



A, At the level of the sixth cer- 

 vical nerve; B, at the mid-dorsal 

 legion; C, at the center of the 

 lumbar enlargement; D, at the up- 

 per part of the conus medullaris. 

 1. Posterior roots. 2. Anterior 

 roots. 3. Posterior fissure. 4. 

 Anterior fissure. 5. Central canal. 

 (After Schwalbe.) 



