606 STKUCTDRE OF THE SPINAL CORD. [cu. XLII. 



nerve-fibres, and a posterior commissure behind the central canal 

 consisting also of medullated nerve-fibres, but with more neuroglia, 

 which gives the grey aspect to this commissure (fig. 450, B). 

 Each half of the spinal cord is marked on the sides (obscurely at 

 the lower part, but distinctly above) by two longitudinal furrows, 

 ivhich divide it into three portions, columns, or tracts, an anterior, 

 lateral, and posterior. From the groove between the anterior and 

 lateral columns spring the anterior roots of the spinal nerves 

 (fig. 450, B and c, 5) ; and just in front of the groove between the 

 lateral and posterior column arise the posterior roots of the same 

 (B, 6) : a pair of roots on each side corresponds to each vertebra. 



White matter. The white matter of the cord is made up 

 of medullated nerve-fibres, of different sizes, arranged longitudi- 

 nally, and of a supporting material of two kinds, viz. : (a) ordi- 

 nary fibrous connective-tissue with elastic fibres, which is con- 

 nected with septa from the pia mater which pass into the cord 

 to carry the blood-vessels. (6) Neuroglia ; the processes of the 

 neuroglia-cells are arranged so as to support the nerve-fibres 

 which are without the usual external nerve sheaths. 



The general rule respecting the size of different parts of the 

 cord is, that each part is in direct proportion to the size and 

 number of nerve-roots given off from it. Thus the cord is very 

 large in the 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 down- 

 wards, 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 fibrillse of axis-cylinders and of dendrites. This 

 fine plexus is called Gerlach's 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 net- 

 work 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 



