MINUTE STEUCTUEE. 497 



ventricle ; and inferiorly, at the extremity of the conns medullaris, it 

 becomes enlarged, shaped like the letter "J", and extends backwards 

 to the surface of the cord, being covered in only by pia mater and 

 connective tissue. This canal, [though minute, is an ol^ject of con- 

 siderable interest as a typical part of the structure of the cord, it being 

 the permanent remains of the cavity of the cylinder formed by the spinal 

 cord at the earliest period of its development. It is more distinctly 

 seen in fishes, reptiles, and birds than in mammals. 



MINUTE STRUCTUEE. 



The substance of the spinal cord consists of a large proportion of 

 nervous substance, supported in a delicate framework of connective 

 tissue, and containing numerous minute blood-vessels. The white 

 matter presents nerve-fibres, but is almost destitute of nerve-cells : the 

 grey matter contains both elements. 



Connective tissue takes part in the structure of the cord to a very 

 considerable extent. It forms a complete covering surrounding the 

 white substance beneath the pia mater, and from this covering trabeculge 

 of connective tissue extend into the white substance (fig. 351, e, e). A 

 nari'ow layer of gelatinous connective tissue lies upon the surface, fills up 

 its inequalities, and sends prolongations with the fibrous septa. These, 

 with the exception of those above described (p. 493), are irregular, and 

 extend a variable depth into the cord, some reaching the grey substance. 

 They divide and ramify, and their ultimate subdivisions are continuous 

 with a peculiar delicate matrix in w'hich the nerve fibres are embedded, 

 and which is termed by Yirchow the now-oglia. It contains many 

 minute granule-like nuclei, and is supposed to be derived, in part 

 at any rate, from peculiar changes in connective tissue cells (see 

 p. 136). In the posterior median column the connective tissue is 

 remarkably abundant. In the grey matter there is also much connec- 

 tive tissue, especially in the immediate neighbourhood of the central 

 canal. , Whether the smallest cells of the grey substance are really 

 nervous or belong to the connective tissue is still undecided, but it is 

 certain that many of the nuclei belong to the connective tissue. 



WMte substance. — The fibres are in greatest part longitudinal ; the 

 principal exceptions being those contained in the commissure, and in 

 the roots of the nerves. The longitudinal fibres are finer in the poste- 

 rior columns, and posterior parts of the lateral columns, than in other 

 parts, and the deepest fibres are smaller than those placed more super- 

 ficially. (Kolliker.) The larger fibres are about y^Vo ^^ch in dia- 

 meter, the smaller are about -^i\\ of that size. A few branching nerve- 

 cells exist in the neighbourhood of the grey matter. 



Grey substance. — The fibres of the grey substance are for the 

 most part not more than one-half the diameter of their continuations 

 in the white substance and in the nerve-roots, but among them there 

 are a few of larger size. They are very various in their direction, 

 and are arranged in a complete network, except those which are con- 

 nected with the roots of the nerves, which pass in definite directions, 

 arranged in compact groups. The fine network appears to be, in part, 

 at any rate, composed of the fine branching processes of the nerve cells. 



The nerve-cells of the grey matter are of two kinds. Firstly, there 

 are very large branched cells, from -^^ to -^^ of an inch in size, con- 



VOL. II. K K 



