92 NERVOUS SYSTEM. 



Recognise the subdivision of the cord into lateral halves by 

 the ventral fissure (open) and the dorsal fissure (filled with 

 pia mater), which dip in as far as the commissure that unites 

 the lateral halves. Within the white matter of the cord lies 

 the grey substance, the outline of which forms an irregular 

 letter H, being united across the middle line by the grey 

 commissure. Each half of the grey matter presents a narrow 

 ventral horn which terminates in a square end some distance 

 from the free surface. 



The dorsal horn, slight and fusiform in shape, attains to the 

 surface, where it is met by the dorsal root of the spinal nerve. 

 Nearly midway on the outer side of the grey substance the 

 lateral horn forms a pointed projection. 



The white substance is divided into three main anatomical 

 subdivisions : (1) The ventral column, between the ventral fissure 

 and the outer strands of the ventral nerve root fibres ; (2) The 

 lateral column, extending from this to the dorsal root ; and (3) 

 The dorsal column, between the dorsal root and the corresponding 

 fissure. The only other subdivision of the white substance is 

 occasionally caused by a thin septum of pia, which separates 

 the dorsal column into an inner GoWs (Gracile) column and 

 an outer Jlurdactis (Cuneate) column. (H) Examine the cells 

 of the ventral horn. Search for their axons running into the 

 strands of the corresponding nerve root. The large nerve cells 

 are mostly confined to this cornu. Inspect the lateral cornu, 

 in which small nerve cells can be recognised (Intermedia lateral 

 tract). Dorsally to this find the lateral reticular formation, 

 a network of fibres passing into the white matter. The dorsal 

 cornu is pointed and capped by the gelatinous substance of 

 Rolando. The dorsal root enters its apex in part only, most 

 of it making a sweep inwards to its mesial side. On the 

 mesial aspect of the neck of the dorsal horn near the com- 



