STRUCTURE OF SPINAL CORD 465 



5. The Minute Structure of the Spinal Cord and 

 Spinal Ganglia. — The spinal cord consists, as already 

 described, of a central canal surrounded by grey matter 

 composed largely of nerve-cells, and arranged in two 

 crescent-shaped masses. The grey matter is surrounded 

 by white matter, consisting chiefly of medullated nerve- 

 fibres, and the whole is invested by the pia mater com- 

 posed of connective tissue. 



The pia mater carries the blood-vessels and lymphatics 

 which supply the substance of the cord ; it dips down 

 into and completely fills the narrow so-called posterior 

 fissure, and similarly lines the wider cavity of the anterior 

 fissure. At frequent intervals, all over the surface of the 

 cord, and in the fissures, the pia mater sends conspicuous 

 prolongations (see Fig. 143) into the substance of the 

 white matter, forming partitions or septa which run on 

 the whole towards the grey matter, and thus carry the 

 blood-vessels into the cord. These larger primary septa 

 give off" fine secondary septa, wliich still further subdivide 

 the white matter, and provide for the more intimate dis- 

 tribution of minute blood-vessels throughout its substance. 

 The inner ends of the septa are continued on into the grey 

 matter for purposes similar to those which they subserve 

 in the white matter. 



The spaces in the white matter between the septa 

 derived from the pia mater are filled by (i) medullated 

 nerve-fibres, whose structure has been previously described, 

 which run on the whole lengthwise or parallel to the long 

 axis of £he cord, and are supported by (ii) a fine felt-work 

 of extremely delicate fibres, which constitutes what is known 

 as the neuroglia, (i'fi}poi' = nerve, and 7Xta = glue) since 

 it binds the nerve-fibres together. The fibres of the 

 neuroglia are, in reality, processes from numberless 

 minute cells, in which the body of each cell is extremely 

 small, and the processes unusually numerous ; these cells 

 are known as neuroglia-cells (Fig. 147)- 



The processes of the neuroglia-cells are wrapped closely 



H H 



