n6 THE POCKET ANATOMY 



each lateral half of the cord, united by an intervening pro- 

 cess, the posterior or grev commissure. 



Each crescent has two cornua, an anterior, thick and short, 

 not reaching the surface of the cord ; and a posterior, long 

 and slender, reaching to the postero-lateral fissure, just before 

 reaching which it becomes enlarged, caput cornu, and becomes 

 less opaque (substantia gelatinosa of Rolando). The size of 

 the grey crescents varies in different parts of the cord, being 

 largest in the cervical and lumbar enlargements. In the 

 upper dorsal region there is a projection of the grey matter 

 on the outer side of the crescent between the anterior and 

 posterior cornu, called the intermedia-lateral tract or lateral cornu. 



Central canal : extends through the whole length of the 

 cord in the middle of the posterior or grey commissure. It 

 is lined with a spheroidal ciliated epithelium. Opens above 

 into the floor of the 4th ventricle, and is continued below into 

 the upper part of the filum terminale. 



The white matter : encloses the grey matter in each lateral 

 half of the cord, except where the posterior cornu comes to 

 the surface. The portion of white matter between the pos- 

 terior or grey commissure and the anterior median fissure 

 constitutes the anterior or white commissure. 



THE MEMBRANES OF THE SPINAL CORD. 



The DURA MATER is the most external membrane, and is 

 continuous with that investing the brain ; but it does not 

 form the periosteum of the vertebrae, nor has it any sinuses, 

 but is separated from the bones by areolar tissue and a 

 plexus ot veins. It is connected above with the edge of the 

 foramen magnum ; at the top of the sacrum it becomes im- 

 pervious, and is continued as a slender cord to blend with the 

 periosteum of the coccyx. This membrane gives sheaths to 

 all the spinal nerves. 



The PIA MATER is less vascular, thicker, and more fibrous 

 than that investing the brain. It has an external fibrous 

 layer of longitudinal bundles having a fold, the linea splendens, 

 dipping into the anterior fissure, and a smaller one passing 

 into the posterior fissure. The pia mater ends in a slender 

 cord, the filum terminale, which is within the prolongation of 

 the dura mater. A process of pia mater, the ligamentum 

 denticulatum , passes outwards towards the dura, to which it is 

 attached by twenty-two tooth-like processes situated between 

 the origins of the spinal nerves ; its pial origin is continuous, 

 and lies between the anterior and posterior nerve-roots. 



The ARACHNOID is placed outside the pia mater, and loosely 



