1 2 8 ELEMENTS OF HISTOLOGY. [Chap. xvi. 



spongy tissue the sub-arachnoid tissue (Key and 

 Retzius) the sub-arachnoid space is subdivided into 

 a labyrinth of areolse. On each side of the cord, 

 between the anterior and posterior nerve roots, 

 extends a spongy fibrous tissue, called ligamentum 

 denticulatum, between the arachnoidea and pia. By 

 it the sub-arachnoidal space is subdivided into an 

 anterior and posterior division. 



165. The sub-dural and sub-arachnoidal spaces do 

 not communicate with one another (Luschka, Key 

 and Retzius). 



The dura mater, as well as the arachnoid, sends 

 prolongations on to the nerve roots; and the sub- 

 dural and sub-arachnoidal spaces are continued into 

 lymphatics of the peripheral nerves. 



All three membranes contain their own system of 

 blood-vessels and nerve-fibres. 



166. The cord itself (Fig. 82) consists of an outer 

 or cortical part composed of medullated nerve-fibres : 

 the white matter, and an inner core of grey matter. 

 On a transverse section through the cord the 

 contrast of colour between the white mantle and 

 the grey core is very conspicuous. The relation 

 between the white and grey matter differs in diffe- 

 rent parts; it gradually increases in favour of the 

 former as we ascend from the lumbar to the upper 

 cervical portion. The grey matter presents in every 

 transverse section through the cord more or less the 

 shape of a capital H ; the projections being the 

 anterior and posterior horns or cornua of grey 

 matter, and the cross-stroke being the grey com- 

 missure. In the centre of this grey commissure 

 is a cylindrical canal lined with a layer of columnar 

 epithelial cells ; this is the central canal ; the part 

 of the grey commissure in front of this canal is 

 the anterior, the rest the posterior, grey commissure. 

 The shape of the whole figure of the grey matter 



