614 PHYSIOLOGY. 



column and direct cerebellar tract; the finest are in the pos- 

 terior median column. 



Classification. The fibers of the cord are classified into two 

 great classes: intrinsic and extrinsic. 



Intrinsic. This class of fibers originates in and terminates in 

 the cord, thereby uniting the levels of gray matter. Fixed by their 

 lower extremities upon a given point of gray substance, they follow 

 an ascending course, to become lost by their extremities in a more or 

 less elevated part of the gray column. Thus they are fibers of union 

 or association for the purpose of establishing communication between 

 the different levels of the gray substance of the cord. 



Extrinsic. These fibers in the gray matter proceed to the gan- 

 glia of the brain after having traversed the medulla oblongata, pons, 

 and crura. They unite the cells of the gray substance of the spinal 

 cord to the upper nerve-centers. They are long and gradually 

 diminish in number from the top to the bottom of the cord. 



Degeneration occupies their whole extent. Some are centripetal 

 and undergo an ascending degeneration. They are contained in the 

 column of Goll, the direct cerebellar bundle, and Gowers's tract. 

 The others are centrifugal fibers, and undergo a descending degen- 

 eration. They are localized in the crossed pyramidal and bundle of 

 Turck. They are the last ones to appear in the foetus. 



The roots of the nerves arrive at the central gray substance and 

 plunge into it after having passed between the fibers of the peri- 

 pheral white substance. But few of them take part in the constitu- 

 tion of trie cortical white matter. 



Neuroglia. In addition to the fibers just discussed the white 

 matter of the cord contains neuroglia. From the neuroglia project 

 extremely fine prolongations. These penetrate the cord to form 

 within its thickness an infinity of partitions of extreme thinness. 

 These are united to the adventitious tissue of the vessels and to the 

 tissue which serves as a basement membrane to the epithelium of 

 the ependyma. Thus there is formed (on transverse section) a poly- 

 gonal network which isolates little colonies of nerve-elements one 

 from the other. This sort of framework has been compared to a 

 sponge in whos'e interstices are found the fibers and cells of the cord. 



Neuroglia does not belong to the category of connective tissues. 

 It is a special formation which is derived from the primitive epiblast. 

 In the central gray substance the neuroglia does not seem to be any 

 more than amorphous matter with some few cellular elements. The 

 gelatinous substance of Eolando is composed of abundant neuroglia 



