272 



NORMAL HISTOLOGY. 



the opposite anterior column. A few cells of type II those whose axones are not 

 prolonged into nerve-fibres but soon break up into elaborate end-arborizations 

 confined to the gray matter are found within the gray matter of the posterior cornua. 

 The nerve-cells of the pars intermedia, the gray matter which connects the horns 

 and lies opposite the gray commissure, may be divided broadly into two classes, the 

 lateral and the middle cells, that occupy respectively the outer border and the more 

 central area of this part of the gray matter. Those of the first class, or intertnedio- 

 lateral cells, are associated with the formatio reticularis and the lateral horn and, 

 hence, are also called the group or column of the lateral horn. Where there is a 

 distinct lateral horn, as in the thoracic region, the cells occupy this projection, but 

 elsewhere lie within the base of the gray network. The cells are multipolar or 

 fusiform, from 15-45 A* m diameter, and provided with a variable number of dendrites. 

 The axones pass directly into the lateral column and become ascending or descending 

 fibres; a few axones may enter the anterior column of the same side. The cells of 

 the second class, the intermediate cells, are irregularly disposed and only in the upper 



Posterior median septum 



Paramedian septum 



"ividing posterior column 



Lateral column 



Anterior column 

 FIG. 318. Transverse section of spinal cord, showing general arrangement of neuroglia. X 9. 



part of the cord present a fairly distinct middle group. They are polygonal or 

 fusiform, small in size, and provided with irregular dendrites. The axones are 

 continued chiefly within the lateral column of the same side, although some pass to 

 the anterior column and a few probably cross to the opposite side. 



A few isolated nerve-cells are usually to be found within the white matter in the 

 vicinity of the more superficially placed cell-columns. These, the so-called outlying 

 cells, are regarded as elements displaced from their usual position during the differ- 

 entiation and growth of the white and gray matter. Similar displacement sometimes 

 affects the cells of the spinal ganglia, which then may be found within the cord. 



The neuroglia of the gray matter is everywhere present as a 

 delicate reticulum supporting the nerve-cells and fibres. The structure of 

 neuroglia having been described (page 70), the special features of its 

 arrangement may be noted. The feltwork of neuroglia fibrils within the 

 gray matter is, in general, more compact than within the white matter and, 

 further, somewhat denser at the periphery than in the deeper parts of the 

 gray core. There is, however, no sharp boundary between the supporting 

 tissue of the white and gray tracts, since numerous glia fibrils extend out- 

 wards from the framework of the gray substance to be lost between the nerve- 

 fibres of the adjoining columns. This feature is marked in the anterior 



