284 PHYSIOLOGY CHAP. 



solitary cells, which from their form and character must be also 

 regarded as belonging to this group. 



The chief characteristic by which the cells of the grey matter 

 of the cord are distinguished from those of the inter-vertebral 

 and sympathetic ganglia is the branching of their processes into a 

 vast number of very fine filaments, similar to the ramifications of 

 the delicate fibrils by which the axis-cylinders of the peripheral 

 nerves terminate in the tissues. 



The nerve-fibres of the columns of the white matter of the 

 cord have medullated sheaths, but no sheath of Schwann. They 

 vary considerably in diameter; the largest are in the ventral 

 roots and outer parts of the lateral columns ; those of the dorsal 

 roots and columns are smaller ; smaller still those of the anterior 

 commissure and the parts of the lateral columns near the grey 

 matter. 



The general direction of the fibres is transverse in the roots, 

 longitudinal in the columns, oblique in the commissures, but in the 

 grey matter the medullated fibres interlace in all directions, both 

 individually and when collected into bundles, while its fine non- 

 medullated fibres form an inextricable felt-work. 



The white matter is traversed by a number of radial septa, 

 along which the marginal vessels penetrate into the cord. These 

 septa consist of neuroglia which also supports the medullated 

 nerve-fibres in a loose network and forms a denser net in the 

 grey matter. The neuroglia is particularly rich in the substantia 

 gelatinosa, which surrounds the central canal of the cord and 

 covers the cap of the dorsal horn. It is epiblastic in origin, 

 and as it consists of keratin, it is very resistant to artificial 

 digestion. 



II. As the minute structure of the cord and the still unsettled 

 questions of anatomy are not the business of the physiologist, we 

 must confine ourselves to such facts as are more particularly of 

 physiological interest. 



One of the best-established anatomical facts is that the fibres 

 of the ventral roots represent the processes or axis-cylinders of 

 ganglion cells that lie in the grey matter of the ventral horn of 

 the same side (Deiters, 1865). Some of these root fibres, however, 

 pass through the ventral commissure and form connections with 

 the cells of the ventral horn on the opposite side. 



The fibres of the dorsal roots, on the contrary, are not directly 

 connected with the cells of the dorsal horn, but are processes of the 

 spinal ganglion cells. These cells usually have a single process, which 

 divides after a short course into two branches, one of which passes 

 to the periphery through the spinal nerves, while the other branch 

 passes to the cord as the dorsal or posterior root. Almost all the 

 fibres of this root divide on reaching the cord (Eamon y Cajal and 

 Kolliker) into two main branches, one ascending, the other 



