588 NERVOUS SYSTEM. 



sures, 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 the membrane is thicker, 

 stronger, more closely adherent to the subjacent parts, and its blood-vessels 

 are not so abundant. In this situation many of the fibres are arranged in 

 longitudinal bands. This membrane lines the anterior fissure and a portion 

 of the posterior fissure of the cord. At the foramina of exit of the cranial 

 and the spinal nerves, the fibrous structure of the pia mater becomes contin- 

 uous 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, internally, 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 vascular arrangement in the substance of the cerebro- 

 spinal axis, as these points are chiefly of anatomical interest. The circul 

 tion in these parts presents certain peculiarities. In the first place, the e 

 cephalon being contained in an air-tight case of invariable capacity, it has bee 

 a question whether or not the vessels be capable of contraction and dilatatio 

 or whether the quantity of blood in the brain be subject to modifications 

 health or disease. These questions may certainly be answered in the affir 

 ative. In infancy and in the adult, when an opening has been made in th 

 skull, the volume of the encephalon is evidently increased during expiration 

 and is diminished in inspiration. Under normal conditions, in the adult, it 

 is probable that the quantity of blood is increased in expiration and dimin- 

 ished in inspiration ; but it is not probable that the cerebro-spinal axis under- 

 goes any considerable movements. The important peculiarities in the cere- 

 bral circulation have already been fully considered in connection with the 

 physiology of the circulation. It has been shown that the encephalic capilla- 

 ries are surrounded or nearly surrounded by canals (perivascular canal-sys- 

 tem), which are connected with lymphatic trunks or reservoirs situated under 

 the pia mater. The system of canals may, by variations in its contents, serve 

 to equalize the quantity of liquid in the brain, as the blood-vessels are dis- 

 tended or contracted. 



Cephalo-Racliidian Fluid. The greatest part of the fluid in the craniu 

 and in the spinal canal is contained in the subarachnoid space. The ventri- 

 cles of the encephalon are in communication with the central canal of the 

 cord, and are also connected with the general subarachnoid space, by a narrow, 

 triangular orifice situated at the inferior angle of the fourth ventricle. By 

 this arrangement the liquid in the ventricles of the encephalon and in the 

 central canal of the cord communicates with the liquid surrounding the cer- 

 ebro-spinal axis, and the pressure upon these parts is equalized. 



