STRUCTURE OF THE GREY SUBSTANCE OF THE SPINAL CORD. 355 



law of Bell, which had been applied more or less successfully 

 to the white columns of the spinal cord, to the grey substance, 

 and maintained that the larger cells of the anterior cornua 

 were motor, and the smaller cells of the posterior horns the 

 sensory elements, notwithstanding that every tyro was aware 

 that neither the conditions requisite for voluntary move- 

 ments, nor those for sensation, are present in the spinal cord 

 separated from the medulla oblongata. Hence neither the 

 elementary parts that give the impulse for movement, nor those 

 in which sensory impressions are perceived, can be seated 

 in the spinal cord. Of the various centric manifestations of 

 activity that exist in the cerebro-spinal organ, two only are 

 present in the spinal cord, the reflectorial and the automatic. 

 It is therefore very tempting to consider these two different 

 modes of central activity to be associated with the two forms 

 of cells of the spinal cord, so that the more important re- 

 flectorial actions are attributable to the more numerous cells 

 provided with nerve processes, and the automatic actions to 

 those which have only protoplasmic processes. In favour of 

 this hypothesis is the circumstance also that the former of 

 these cells, as has been rendered more than probable, stand in 

 direct relation, by means of the nerve-process through the 

 anterior roots, with muscles ; as well as the observation of 

 M. Schultze, according to which the fibrils constituting both 

 the nerve-process and the protoplasmic processes of these cells 

 do not originate in them, but only undergo rearrangement, a 

 fact that at least rendered in some measure intelligible, from a 

 morphological point of view, the hitherto completely unknown 

 process of the transference of the condition of excitation from 

 one nerve fibre to another. 



After having now acquired some knowledge of the elementary 

 constituents of the grey substance, it only remains to describe 

 more minutely its several portions. The median portion of the 

 grey substance of the spinal cord, the so-called grey commissure, 

 is composed of several distinct histological divisions. Situated 

 slightly in front of the exact centre is the central canal (fig. 

 226, c), lined by columnar epithelial cells (fig. 226, d), which 

 rest on a membrane composed of connective tissue tolera- 

 bly free from nerve fibres (fig. 226, e). Anteriorly to this, 



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