SPINAL CORD. 279 



the gray substance has in cross-section roughly the form of 

 the letter H (Figs. 205-208), and, taken as a whole, consists 

 of two long columns laid parallel to one another and joined 

 together by the so-called gray commissure. Each of the col- 

 umns is thicker on its ventral than on its dorsal side. There 

 is therefore in cross-sections a wide ventral horn and a smaller 

 dorsal horn. In the lower cervical and the upper thoracic 

 regions of the cord there appears the lateral horn (Iractus inter- 

 mediolateralis). In the same regions processes of the gray sub- 

 stance extend into the white matter in such a way that a net- 

 like structure is formed, containing in its meshes bundles of 

 fibres from the white matter. This is known as theformatio 

 reticularis, and appears at the junction of the ventral and 

 dorsal horns (Fig. 205). 



From the ventral surface of the ventral horn, bundles of 

 nerve fibres run out through the white matter, forming the so- 

 called ventral root. Similar nerve bundles are present on the 

 dorsal side, making up the dorsal root. 



With small magnification there can be distinguished in the 

 thoracic region of the cord a well-defined group of cells known 

 as the column of Stilling- Clarke, or the nucleus dorsalis (Fig. 

 207). It can be observed also in the upper part of the lumbar 

 region. It occupies a position on the median side at the base 

 of the dorsal horn opposite the formatio reticularis. Another 

 conspicuous structure to be observed in the gray matter 

 throughout the whole length of the cord is the substantia 

 gelatinosa (Rolandi). This lies at the apex of the dorsal 

 horn, and consists of small spindle-shaped cells with less neu- 

 roglia than other parts of the gray matter (Weigert). This 

 is seen in preparations made by Weigert's method as a light 

 band across the end of the dorsal horn (Fig. 212). 



The gray commissure is a flat band of gray matter connect- 

 ing the two lateral gray masses. In its centre is the central 

 canal, which runs the whole length of the cord and is continu- 

 ous with the cavities of the brain and medulla. The diameter 

 of its lumen is usually about 1 mm. In embryos it is lined 

 with ciliated epithelium and surrounded by the substantia 



