THE ARACHNOID MEMBRANE. 



187 



membrane (perivascular sheath). The diameter of the (lymphatic) canal thus 

 formed may be considerably larger than that of the contained vessel. A similar 

 sheath, derived from the pia mater, accompanies the vessel into the substance of the 

 brain. At its commencement it is loose and funnel-shaped and can be injected from 

 the subarachnoid cavity. On the cerebrum the inner layer of the pia mater is more 

 closely adherent to the cortical substance of the convolutions, than on the cerebellum, 

 where a distinct space traversed by fibres exists between the two. 



Nerves. Purkinje described a retiform arrangement of fine nerve-fibres in 

 the pia mater ; these are derived, according to Kolliker and others, from the 

 sympathetic, and from the third, fifth, sixth, facial, pneumogastric, glossopharyngeal, 

 and accessory nerves. Most of the fibres are destined in all probability for the 

 blood-vessels. 



The spinal pia mater is supplied by nerves from the grey rami communicantes of 

 the sympathetic. 



THE ABACHNOID MEMBRANE. 



The arachnoid is a delicate membrane which is situated outside the pia mater, 

 and invests the brain and spinal cord much less closely than that membrane. It 

 passes over the various eminences and depressions on the cerebrum and cerebellum, 



Fig. 131. SECTION OP THE POS- 

 TERIOR AND LOWER PARTS OF 

 THE BRAIN WITHIN THE SKULL, 

 TO EXHIBIT THE SUBARACHNOID 

 SPACE, AND ITS RELATION TO 



THE VENTRICLES. (After Key 

 and Retzius. ) 



The section was made in the 

 frozen state, the cavities having 

 been previously filled with injec- 

 tion. 



1, 1', atlas vertebra ; 2, odon- 

 toid process of the axis, 2' ; 3, 

 third ventricle ; 4, fourth ventri- 

 cle ; C. C, corpus callo.sum ; C', 

 gyrus fornicatus ; C, cerebellum ; 

 t, tentorum ; p, pituitary body ; 

 c.c. , central canal of the cord ; 

 f.M. in the cerebello-medullary 

 part of the subarachnoid space, is 

 close to the foramen of Magendie 

 by which that space communicates 

 with the fourth ventricle. 



without dipping down into 

 the sulci and smaller 

 grooves. Beneath it, be- 

 tween it and the pia mater, is a space (subarachnoid space) in which is a considerable 

 quantity of fluid (subarachnoid fluid), and in which are seen the larger blood-vessels 

 passing obliquely towards the brain. 



The subarachnoid space is larger and more evident in some places than in 

 others. Thus, in the longitudinal fissure, the arachnoid does not descend to the 

 bottom, but passes across, immediately below the edge of the falx, at some distance 

 above the corpus callosum. In the interval thus left, the arteries of the corpus 

 callosum run backwards along that body. At the base of the brain and in the 

 spinal canal there is a wide interval between the arachnoid and the pia mater. In 

 the base of the brain, this subarachnoid space extends in front over the pons and 

 the interpeduncular recess as far forwards as the optic nerves, and behind it forms 



