650 



STRUCTURE OF THE SPINAL CORD 



[CH. XLV. 



middle and lower part of its cervical portion, whence arise the large 

 nerve-roots for the formation of the brachial plexuses and the supply 

 of the upper extremities ; it again enlarges at the lowest part of its 

 dorsal portion and the upper part of its lumbar, at the origins of the 

 large nerves which, after forming the lumbar and sacral plexuses, are 

 distributed to the lower extremities. The chief cause of the greater 

 size at these parts of the spinal cord is increase in the quantity of 

 grey matter; the white part of the cord (especially the lateral 

 columns) becomes gradually and progressively smaller from above 

 downwards, because a certain number of fibres coming down from the 

 brain pass into the spinal grey matter at different levels. 



Grey matter. The grey matter of the cord consists of nerve- 

 fibres, most of which are very fine and delicate, of nerve-cells with 

 branching processes, and of an extremely delicate network of the 

 primitive fibrillae of axis-cylinders and of dendrites. This fine plexus 

 is called Gferlactis network, and is mingled with the meshes of 

 neuroglia. The neuroglia of the grey matter resembles that of the 

 white, but instead of everywhere forming a close network to support 

 the nerve-fibres, here and there it is in the form of a more open 

 sponge-work to support the nerve-cells. It is 

 especially developed around the central canal, 

 which is lined with columnar ciliated epi- 

 thelium, the cells of which at their outer ends 

 terminate in fine processes, which join the 

 neuroglia network surrounding the canal, and 

 form the substantia gelatinosa centralis. It is 

 also developed at the tip of the posterior cornu 

 of grey matter, forming what is known as the 

 substantial, gelatinosa lateralis of Eolando, which 

 is much enlarged in the upper cervical region. 



Groups of cells in the grey matter. The 

 multipolar cells of the grey matter are either 

 scattered singly or arranged in definite groups 

 (see fig. 400). 



(1) Anterior horn cells. In the cervical and 

 lumbar enlargements there are several groups 

 of large multipolar cells in the anterior horn ; 

 in bhe thoracic region these are reduced to two, 

 a mesial and a lateral group. The larger groups correspond with 

 segments of the limbs, and in the cervical cord there is one special 

 group from which the phrenic nerve arises for the supply of the 

 diaphragm. The axons pass out by the anterior nerve-roots of the 

 same side, but a few axons pass to the antero-lateral column of the 

 same side, and by the white commissure to that of the opposite side. 

 In birds, a few axons are stated to pass to the posterior roots. 



FIG. 400. Section of half the 

 spinal cord to show the 

 principal groups of cells in 

 the grey matter ; a, groups 

 of cells in the anterior 

 horn ; c, Clarke's column ; 

 i, intermedio-lateral group ; 

 m, middle cell column ; p, 

 scattered cells of the pos- 

 terior horn. (Diagrammatic 

 after Schafer.) 



