260 NERVOUS SYSTEM. 



the cord, but simply covers their surfaces. Magendie point- 

 ed out a longitudinal, incomplete, cribriform, fibrous septum 

 in the cord, passing from the inner layer of the arachnoid to 

 the pia mater. A similar arrangement is found in certain 

 situations at the base of the skull. 1 



The pia mater of the encephalon is a delicate fibrous 

 structure, exceedingly vascular, seeming to present, indeed, 

 only a skeleton net-work of fibres for the support of the ves- 

 sels going to the nervous substance. This membrane cov- 

 ers the surface of the encephalon immediately, follows the 

 sulci and fissures, and is prolonged into the ventricles, where 

 it forms the choroid plexus and the velum interpositum. From 

 its internal surface, small vessels are given off which pass 

 into the nervous substance. 



The pia mater of the encephalon is continuous with the 

 corresponding membrane of the cord ; but in the spinal 

 canal, it is thicker, stronger, more closely adherent to the 

 subjacent parts, and its blood-vessels are by no means so 

 numerous. In this situation, many of the fibres are arranged 

 in longitudinal bands. This membrane lines the anterior 

 sulcus and a portion of the posterior sulcus. It is sometimes 

 spoken of as the neurilemma of the cord. 



At the foramina of exit of the cranial and the spinal 

 nerves, the fibrous structure of the pia mater becomes con 

 tinuous with the nerve-sheaths. 



Between the anterior and posterior roots of the spinal 

 nerves on either side of the cord, is a narrow ligamentous 

 band, the ligamentum denticulatum, which assists in holding 

 the cord in place. This extends from the foramen magnum 

 to the terminal filament of the cord, and is attached, inter- 

 nally, to the pia mater, and externally, to the dura mater. 



It is not necessary to enter into a detailed description of 

 the arrangement of the blood-vessels, nerves, and lymphatics 

 of the membranes of the brain and spinal cord, or of the vas- 

 cular arrangement in the substance of the cerebro-spinal axis, 



1 MAGENDIE, op. cit., p. 14. 



