CEREBRO-SPINAL. AXIS. 185 



interior of the tube, while the white matter is exterior; but 

 in the brain there are extensive additional developments of 

 grey matter on the convoluted surface of the cerebrum and 

 cerebellum. 



139. Large blood-vessels appear to be inadmissible in the 

 substance of the cerebro-spinal axis, for, instead of having 

 vessels ramifying through it as in other textures, it is closely 

 invested with a vascular membrane called pia mater, which 

 consists of thickly meshed branches of arteries and veins 

 united with a little connective tissue; and from this mem- 

 brane, which dips down into every fissure, multitudes of 

 small arteries enter all over the surface, so minute that, in a 

 section of the brain, the only vessels visible to the naked 

 eye appear as scattered spots of blood no larger than marks 

 made with the point of a pin. 



Over the pia mater there is a delicate transparent serous 

 membrane, called the arachnoid, with its serous surface 

 turned outwards, and stretched across the various inequalities 

 of surface, without dipping into them. It is adherent to the 

 pia mater over great part of the surface of the brain, but on 

 the spinal cord is disposed as a loose bag. 



Superficial to the arachnoid is placed the dura mater, 

 an- exceedingly tough fibrous membrane, which, within the 

 cranium, serves as periosteum to the interior of the skull, as 

 well as for an envelope to the brain; but in the spinal canal 

 is separated from the vertebrae by a space containing loose 

 adipose tissue and large veins. Its outer surface is rough, 

 but the inner is polished and clothed with epithelium, com- 

 pleting with the opposed arachnoid membrane the boundaries 

 of a serous cavity, called the arachnoid space, in contradis- 

 tinction to the subarachnoid space between the arachnoid and 

 pia mater. 



The spinal arachnoid is attached to the dura mater on 

 each side, not only by the sheaths with which it clothes the 

 nerves, but by a series of attachments, one between each 

 pair of successive nerves, constituting the ligamentum denti- 

 culatum. When it is added that the subarachnoid space 

 contains a considerable amount of watery secretion, the 

 cerebro-spinal fluid, principally in the spinal canal, the 

 student will perceive that the spinal cord has in this, and 



