634 



NERVOUS SYSTEM. (NERVOUS CENTRES. THE MENINGES.) 



fibrous tissue, which cannot confer elasticity. 

 And if a portion removed from the cord be 

 stretched, it will be found to possess very little 

 elasticity ; but if the cord be held up by the 

 filiform prolongation, and a slight jerking move- 

 ment be communicated to it, it may be made 

 to dance about as if by the elastic reaction of the 

 filliform process. The movement which may 

 be thus produced is very well calculated to 

 deceive, and Dr. Macartney must have founded 

 his opinion upon that experiment alone, omitting 

 to try the effect of stretching a detached portion 

 of the process. The fact is that when the cord is 

 suspended in this way, the pia mater becomes 

 stretched, and its anterior and posterior por- 

 tions are approximated and the cord flattened ; 

 when it is raised with a jerk, this tension of 

 the pia mater is diminished, and the cord re- 

 turns to its previous form until it falls again, 

 stretches the pia mater, and becomes once 

 more flattened, producing a degree of reaction 

 which favours its elevation, but which alone 

 would be insufficient for that purpose. Thus 

 it appears that the elastic reaction, which Dr. 

 Macartney attributed to the filiform process, is 

 in reality due to the compression and conse- 

 quent flattening of the cord by the tension of 

 the pia mater. It should be stated, further, 

 that this process is not formed of pia mater 

 alone, but also of a continuation of the liga- 

 mentum denticulatum on each side to be 

 described by-and-bye. 



The pia mater is abundantly supplied by 

 bloodvessels, many of which are extremely 

 tortuous. These vessels are derived from the 

 anterior and posterior spinal arteries. Along 

 the anterior surface of the spinal cord in front 

 of the anterior median fissure there is a narrow 

 band of fibrous tissue which is stretched across 

 this fissure like a bridge, and occupies its 

 whole length. No such arrangement exists on 

 the posterior surface. 



The pia mater of the spinal cord possesses 

 considerable strength and density. The ner- 

 vous matter may by pressure be squeezed out 

 of it, leaving a hollow cylindrical membrane, 

 or it may be dissolved out by the action of 

 liquor potassae. In the quite recent state, 

 while the cord is as yet firm, the pia mater 

 .nay be readily dissected off, its adhesion to 

 the cord being through the medium of nu- 

 merous exceedingly minute capillary vessels. 

 On its exterior the pia mater adheres to the 

 visceral layer of the arachnoid membrane by 

 means of a loose fibrous tissue. 



Pia muter of the brain. In tracing the pia 

 mater of the spinal cord upwards, it will be 

 found gradually to become much thinner and 

 more delicate as it passes from the medulla ob- 

 longata to the hemispheres of the cerebellum 

 and cerebrum. In connexion with these latter 

 parts it becomes of extreme tenuity, and owes 

 its physical tenacity chiefly to the intimate con- 

 nexion of the visceral layer of the arachnoid 

 membrane with it. The cerebral pia mater is al- 

 most exclusively composed of numerous ramifi- 

 cations of minute vessels which are accompanied 

 by white fibrous tissue in small quantity. These 

 vessels divide and subdivide to the last degree 



of minuteness, and are admirable objects for 

 examining the structure of capillary vessels. 

 The pia mater adheres closely to the whole 

 surface of the brain, cerebellum, and connect- 

 ing parts, and numberless vessels pass from it 

 into the nervous substance in contact with it. 

 On the surface of the brain it dips down into the 

 sulci or furrows between the convolutions, and 

 adheres to the superficial grey matter. Wher- 

 ever there is a depression or fissure of the 

 brain, the pia mater is found dipping into it. 

 It likewise sinks into the fissures between the 

 laminae of the cerebellum. 



We shall obtain, however, a very inadequate 

 notion of the extent of the pia mater, if we 

 confine our examination of it to the exterior of 

 the brain and cerebellum. At certain situations 

 this membrane is continued into the cavities 

 or ventricles of these organs, where it doubtless 

 fulfils some office connected with the support 

 and nutrition of certain parts of them. These 

 situations are four in number, as follow : on 

 each side, the fissure between the crus cerebri 

 and the middle lobe of the brain, behind, the 

 transverse fissure between the cerebellum and 

 cerebrum, and, lastly, the inferior extremity of 

 the fourth ventricle. 



Choroid plexuses of the lateral ventricles. - 

 These are apparently folded processes of the 

 pia mater which enter the inferior part of the 

 lateral ventricles on each side, and are continued 

 upwards and forwards to their middle portions, 

 where they become continuous with each other 

 in the foramen commune anterius, and with 

 a middle process, the velum. Each choroid 

 plexus forms a somewhat cylindrical process, 

 which, when traced from below upwards and 

 from behind forwards, will be found to follow 

 the direction of the lateral ventricle as far for- 

 wards as the apex of the horizontal portion of 

 the fornix, gradually diminishing in thickness, 

 and assuming the character of a simple mem- 

 branous expansion. It projects freely into the 

 cavity of the ventricle, having no connection 

 with the walls of that cavity excepting along 

 the margins of the fissure, at which it enters, 

 where the membrane of the ventricle adheres 

 to it, being probably reflected upon it. 



Fig. 363. 



Choroid plexus of lateral ventricle in the Sheep, 

 nhewinri a villous process, highly magnified, and 

 the epithelium. 



(After Valentin.) 



a, villus j b, epithelium ; c, nucleus of epithe- 

 lium. 



Very numerous and tortuous bloodvessels are 

 contained in these processes, forming a plexus 

 which has given name to the folds themselves. 



