382 



THE NERVOUS SYSTEM. 



iir 



cell processes running in various directions, most of them exhibiting 

 abundant ramifications, while some continue their course for a con- 

 siderable distance undivided, and assume at last the appearance of 

 axis cylinders. The largest and most remarkable are situated in the 

 anterior horns, where they reach the size of" from 

 67 to 135 mmm. in diameter ; many of them 

 being the largest known cells in the nervous 

 system. They are mainly arranged on each side 

 in three groups, namely, at the point, and at the 

 external and internal borders of the anterior horn. 

 Throughout the dorsal region there is a group of 

 similar cells at the base of each posterior horn, known 

 as the ''column of Clarke," extending from the 

 lower cervical region nearly to the lumbar enlarge- 

 ment. Here these cells disappear as a distinct 

 group, while those of the anterior horns increase 

 considerably both in numbers and size. Elsewhere 

 throughout the gray substance, but especially in 

 the posterior horns, the nerve cells are much 

 smaller, but similar in form, and provided with 

 branching prolongations. The anterior and pos- 

 terior horns are not therefore absolutely distin- 

 guished from each other by the size of their nerve 

 cells, but only by the relative abundance of the 

 larger and smaller varieties ; since a few large cells 

 are found in the posterior horns, and the smaller 

 cells exist in both regions. 



The nerve fibres of the gray substance are, in 

 general, much smaller than those of the white 

 substance, but otherwise present the same ana- 

 tomical characters. Most of them run horizon- 

 tally, in a transverse, antero-posterior, or radi- 

 ating direction. They consist, first, of fibres which 

 have penetrated the gray substance from the ante- 

 rior and posterior nerve roots ; secondly, of fibres 

 TRANSVERSE SECTIONS OF which cross the median line in the gray commis- 



THE SPINAL CORD IN , ,, . . 



MAN. i. Upper Cervi- sure, both m front and behind the central canal, 

 cai Region, ii. Lower forming a commissural connection between the two 



Cervical Region. III. , , 



Dorsal Region, iv. lateral halves of the gray substance ; and, thirdly, 

 Lumbar Enlargement, of fibres which run in a great variety of directions, 



V. Lower Extremity. , ,. , . , 



and of which the origin and terminations are un- 

 known. 



The White Substance. The white substance of the spinal cord con- 

 sists Of nerve fibres, the large majority of which run in a longitudinal 

 direction, forming tracts or "columns," designated, according to their 

 situation, as the anterior, lateral, and posterior columns of the cord. 

 In microscopic transverse sections of the cord, treated by hardening 



