254 INNEKVATION. [CHAP. X. 



space exist. The ventricles of the brain contain a secretion of 

 very similar, if not identical characters, which Majendie describes 

 as communicating with that of the sub-arachnoid space through 

 an orifice at the inferior extremity of the fourth ventricle. This, 

 however, is extremely doubtful, as the fluid of the ventricles is 

 enclosed by a proper membrane which lines their cavity. 



The cerebro-spinal fluid obviously affords mechanical protection 

 to the nervous centres which it surrounds. The interposition of a 

 fluid medium between them and the walls of the cavities is well 

 adapted to guard the former against shocks communicated from 

 without. Its accumulation at the base of the brain is highly favour- 

 able for the protection of the large vessels and nerves situate there. 



The pia mater is the immediate investing membrane of the brain 

 and spinal cord. It is composed of white fibrous tissue and blood- 

 vessels. The former is most abundant in the pia mater of the cord, 

 the latter are most numerous in that of the brain. The principal 

 distinction, therefore, between the spinal and cerebral pia mater 

 is as regards strength and thickness ; the spinal being dense and 

 strong, the cerebral being very delicate, almost wholly composed 

 of minute blood-vessels, which are accompanied by white fibrous 

 tissue in small quantity. The spinal membrane forms a complete 

 sheath to the cord, and sends in processes which dip into its anterior 

 and posterior median fissures. It is continuous with the neurilemma 

 of the roots of the nerves on each side. At the inferior extremity 

 of the cord it tapers and terminates in a thread-like process (filum 

 terminate) which is inserted into the inferior extremity of the dura 

 mater. Superiorly it gradually diminishes in density as it passes 

 over the medulla oblongata to the cerebral and cerebellar hemi- 

 spheres. To the surface of these it adheres closely, and innu- 

 merable minute blood-vessels pass from it into the nervous sub- 

 stance. It sinks into all the sulci and fissures, and passes into the 

 lateral, the third and fourth ventricles. In the lateral and fourtli 

 ventricles it forms projecting processes or folds, somewhat fringed, 

 highly vascular, and invested by epithelium derived from the mem- 

 brane which lines the ventricles. These processes are called the 

 chor old plexuses. Into the third ventricle, it sends a lamellar fold 

 of triangular shape (velum interpositum) , which forms a roof to that 

 cavity and supports the fornix. 



Attention has lately been directed to some minute sandy particles, 

 globular in shape, which are frequently connected with the minute 

 vascular ramifications of the internal pia mater. They are found 

 chiefly in the choroid plexuses, and in that portion of the velum 



