130 ELEMENTS OF HISTOLOGY. [Chap. xvi. 



cervical and lumbar regions where the nerves of the 

 i>rachial and sacral plexus respectively join the cord, 

 this latter possesses a swelling, and the grey matter 

 is there increased in amount, the swelling being in 

 fact clue to an accumulation of grey matter, in which 

 an additional number of nerve-fibres originates ; but 

 the general shape of the grey matter is retained. 



167. The cornua of the grey matter are generally 

 thicker near the grey commissure; they become thinned 

 out into anterior and posterior edges respectively, 

 which are so placed that they point towards the 

 antero-lateral and postero-lateral fissures. The an- 

 terior horns are generally thicker and shorter than 

 the posterior ones, and, therefore, the latter reach 

 nearer to the suface than the former. 



168. The white matter is composed chiefly of 

 medullated nerve fibres running a longitudinal course. 

 They are arranged into columns, one anterior, one 

 lateral, and one posterior column for each lateral 

 half of the cord ; the two halves being separated by 

 the anterior and posterior median longitudincd 

 fissure. The anterior median fissure is a real fissure 

 extending in a vertical direction from the surface of 

 the cord to near the anterior grey commissure. It 

 contains a prolongation of the pia mater and in it 

 large vascular trunks. The posterior fissure is not 

 in reality a space, but is filled up by neuroglia. It 

 extends as a continuous mass of neuroglia in a vertical 

 direction from the posterior surface of the cord to the 

 posterior grey commissure. The exit of the anterior 

 or motor nerve roots, and the entrance of the posterior 

 or sensitive nerve roots are indicated by the anterior 

 lateral and posterior lateral fissures respectively. 

 These are not re-al fissures in the same sense as the 

 anterior median fissure, but correspond more to the 

 posterior median fissure, being in reality filled up with 

 neuroglia tissue, into which extends a continuation 



