834 



THE NERVE SYSTEM 



also as a fluted column having a continuous extent throughout the cord. This 

 gray column is drawn into ventral and dorsal ridges, connected respectively 

 with the ventral and dorsal nerve roots, while the white substance fills out the 

 irregularities and completes the nearly cylindrical outline of the cord (Fig. 608). 

 On viewing a trans-section, it is seen that the gray substance is so arranged as to 

 present,, in each lateral half of the cord, a crescentic or comma-shaped mass, the 

 concavity of which is directed laterad. The two lateral masses are connected 

 by a transverse bar or band of gray substance, termed the gray commissure (com- 

 missura cinerea [grisea]), and containing the central canal, which extends the 

 entire length of the cord. The .dorsal septum of the cord reaches quite to the 



gray commissure, there being but a few white com- 

 missural fibres in the dorsal zone. Ventrad. however, 

 a lamina of white substance, the commissura ventralis 

 alba, separates the gray commissure from the ventral 

 fissure (Fig. 606). 



In trans-sections of the cord it is seen that the 

 lateral crescentic gray masses, united across the middle 

 line by a gray commissure, have the aggregate appear- 

 ance of the letter H. Each crescentic mass presents 

 projections which are more or less pronounced accord- 

 ing to the segment of the cord under consideration. 

 Broadly stated and without reference to special levels, 

 the most marked projections are the dorsal and ventral 

 horns or cornua. 



The dorsal horn, directed dorsolaterad, is elongated 

 and narrow, and its apex is composed of a translucent, 

 V-shaped mass termed the caput gelatinosa Rolandi 

 (caput gliosum would be a better term). 



The attenuated apex of the dorsal cornu approaches 

 the surface of the cord along the line of entrance of 

 the dorsal nerve roots. The apex of the dorsal horn is 

 w r ider in the regions of the enlargements and the gelat- 

 inosa is most marked in the higher cervical segments. 

 The base or cervix of the dorsal horn is constricted 

 somewhat except in the thoracic portion, where its 

 greater breadth is due to the presence of the nucleus 

 dorsalis (Clarke's column}. 



The ventral horn is shorter, thicker, and more blunt, 

 and is separated from the ventral and lateral surfaces 

 of the cord by a tolerably thick lamina of white 

 substance. Its margin, in trans-sections, presents a 

 dentate or stellate appearance due to the emergence of 

 fascicles of efferent or ventral root axones on their way 

 to the ventral surface of tjie cord. 



What is known as the lateral horn projects as a lateral peninsular extension of 

 the central gray nearly on the line of the gray commissure. It is best marked as 

 a triangular projection in the upper thoracic segments. In the cervical enlarge- 

 ment it is merged with the greatly expanded ventral horn, but it again becomes 

 prominent in the upper cervical segments. 



The gray substance of the cord is not everywhere sharply demarcated from the 

 white owing to the invasion of myelinic and amyelinic nerve fibres. Facing the 

 lateral column, in the angle between the dorsal and ventral horns, small fascicles 

 of white fibres are embedded in the gray so that it is broken up in a peculiar 

 basket-work pattern termed the reticula or reticular formation. This gray network 



FIG. 607. Projection upon a 

 plane of the absolute and relative 

 extent of the gray and white sub- 

 stance of the cord as determined 

 by successive sectional areas. 

 Gray substance shown in black. 

 (Adapted from the measurements 

 of Stilling.) 



