18 



THE SPINAL CORD. 



efferent " cells, and the latter as " sensory or afferent." but it is doubtful if this physiological 

 distinction will hold good. In the spinal cord the cells of the anterior horn, those of 

 Clarke's column, and many of the cells of the lateral cell-column and of the posterior horn 

 belong 1 ' to the first category, while many of the solitary cells, the small cells of the substance 

 of Rolando, and some of those of the lateral cell-column, belong to the latter. 



lug. 1.3. CENTRAL CANAL SHOWING ITS KPITHKLIUM (E. A. S. ) 



A, from a child of six. Magnified 150 diameters. A', some of the ciliated cells, highly magnified. 



B, section of the isthmus from the lumbar cord of an adult (set. 33), showing the central canal in the 

 middle surrounded by the substantia gelatinosa centralis. f.a., anterior median fissure ; p.m.c. , 

 posterior white column ; a. c. , anterior white commissure. Magnified 30 diameters. 



Commissures. The anterior commissure (fig. 15, B, a.c.) consists of medul- 

 lated nerve-fibres which pass on each side, some into the anterior white column, 

 others into the anterior horn of grey matter. Their course is not strictly transverse, 

 many fibres which enter the ventral part of the commissure at one side leave it at 

 the dorsal portion on the other side. There is thus an oblique decussation at the 

 middle line (fig. 15). This decussation is most distinctly seen in the comparatively 

 short and wide commissure of the lumbar region, and in the upper part of the cervical 

 region. In the latter situation it appears as a continuation of the decussation of the 

 pyramids of the medulla oblongata, to be afterwards described. In addition to the 

 transverse there are a few longitudinal bundles of fibres in the region of this commis- 

 sure. The fibres are often somewhat displaced by vessels which pass into the grey 

 substance from the anterior fissure. 



The posterior commissure also contains medullated fibres running trans- 

 versely or with a slight obliquity, but there is a large amount of neuroglia between 

 them, and this gives the commissure a grey aspect. In this commissure is 

 contained the central canal of the spinal cord surrounded by an area free from 

 medullated nerve-fibres, and occupied by a substance which, like the superficial 

 neuroglia and the substance of Rolando, has in the hardened condition a reticulated 

 structure and in the fresh state a gelatinous aspect. It is termed substantia gelatinosa 



