MICROSCOPICAL STRUCTURE OF SPINAL CORD. 



1035 



Those of the first class, or intermedio-lateral cells, are associated with the formatio reticu- 

 laris and its condensation, the lateral horn, and hence are often spoken of as the group or 

 colimm of the lateral horn. These cells form a slender tract of small closely packed elements 

 that is represented through almost the entire length of the cord, although best marked in the 

 upper third of the thoracic region and partially interrupted in the cervical and lumbo-sacral 

 segments. Where the formatio reticularis is condensed with a distinct lateral horn, as in the 

 thoracic region, the cells occupy the projection, but elsewhere lie within the base of the gray net- 

 work. As a continuous cell-column the tract extends from the lower part of the eighth cervical 

 segment to the upper part of the, third lumbar, being most conspicuous at the level of the third and 

 fourth thoracic nerves (Bruce). Practically suppressed in the cervical region between the eighth 

 and third segments, above the latter the column reappears along with the formatio reticularis. 

 Below, it is again seen within the third and fourth sacral segments. The nerve-cells are multi- 

 polar or fusiform in outline, from .015-045 mm. in their longest diameter, contain little pigment, 

 and are provided with a variable number of dendrites, of which two are usually larger than the 

 others. These arise from opposite poles of the cell and send branches, for the most part, into the 

 adjacent white matter. The axones pass directly into the lateral columns and become ascending 

 or descending fibres ; a few axones, however, enter the anterior column of the same side (Ziehen). 



The cells of the second class, or intermediate cells, are irregularly disposed and only in the 

 upper part of the cord present a fairly distinct middle group (VValdeyer). They are polygonal 

 or fusiform in outline, small in size (seldom exceeding .025 mm.) 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 small number of isolated nerve-cells are usually to be found within the white matter, out- 

 side but in the neighborhood of the gray core. These, the outlying cells of Sherrington,' by 

 whom they have been studied,' occur most frequently in the vicinity of the more superficially 

 placed cell-columns. Within the anterior columns they lie in the paths of the fibres proceeding 

 to the anterior white commissure ; in the lateral colunuis they are in proximity to the intermedio- 

 lateral group of the lateral horn and formatio reticularis and to the cells of the substantia 

 Rolandi ; and in the posterior columns, where they are relatively numerous, they are associated 

 with the fibre-tracts leading to the column of Clarke. The outlying cells are regarded as 

 elements displaced from their usual position during the course of the differentiation and growth 

 of the white and gray matter. Similar displacement sometimes affects the cells of the spinal 

 ganglia, which then may be encountered within the cord. 



The Neuroglia of the Gray Matter. — As ir other parts of the cord, so in 



the gray matter the neurogha is everywhere present as the supporting framework of 



the nervous elements, the 



Fig. 890. 



Posterior median 



septum 



Paramedian septum 

 subdividing 

 posterior column 



Lateral 

 column 



cells and fibres. The gen- 

 eral structureof neuroglia 

 having been described 

 (page 1004), it only re- 

 mains to note here the 

 special features of its 

 arrangement within the 

 gray matter. In general, 

 the felt-work of the neu- 

 rogliar fibrils is more 

 compact than that per- 

 meating the white matter, 

 being somewhat denser 

 at the periphery than in 

 the deeper parts of the 

 gray matter. There is, 

 however, no hard boun- 

 dary between the sup- 

 porting tissue of the two, 

 since numerous glia fibrils 

 extend outward from the 

 frame-work of the gray matter to be lost between the nerve fibres of the adjoining 

 columns. This feature is marked in the anterior horn, where the glia fibrils form 

 septa of considerable thickness that diverge into the surrounding columns ; further 



Anterior median fissure 



Anterior column 



Transverse section of cord slightly magnified, showing general arrangement 



of neuroglia. X 10. 



^Proceedings Royal Society, vol. 30, 1890. 



