THE NERVOUS SYSTEM. 395 



The arachnoid membrane of the brain differs from that of the spinal 

 cord, not so much in its structure as in its disposition. It is true, that 

 in this situation also, there is but one lamella demonstrable as a mem- 

 brane composed of connective tissue, which corresponds with the so- 

 termed visceral layer of the arachnoid of authors, and is also very closely 

 applied to the inner surface of the dura mater, but the arachnoid mem- 

 brane here is in much more intimate relation to the pia mater. That is 

 to say, instead of its being united with the latter, as in the cord, by scat- 

 tered fibres and lamellae, it is, in the brain, in many situations, as on all 

 the convolutions, and the projecting parts at the base of the brain, 

 adherent to and coalescent with it, and, elsewhere, where this is not the 

 case, united to it by numerous processes. For this reason, there exists, 

 in the brain, no continuous subarachnoid space, but numerous, larger 

 and smaller spaces, which only partially communicate. The larger of 

 these spaces between the cerebellum and medulla oblongata, and under 

 the pons Varolii, the crura cerebri, the fossa Sylvii, &c., open directly 

 into the subarachnoid space of the spinal cord, whilst the smaller, corre- 

 sponding to the sulci, and over which the membrane composed of connec- 

 tive tissue is stretched, are perhaps partially in communication with each 

 other, but, at all events most of them, not with the larger spaces just 

 mentioned. The arachnoid, as has been correctly stated by Henle, is 

 nowhere in connection with the lining membranes of the cerebral ven- 

 tricles. The structure of the membrane is the same as in the spinal 

 cord, except that the anastomosing fasciculi and spiral elastic fibres are 

 for the most part thicker, measuring as much as 0*01 or even 0-02 of a 

 line ; and the former frequently present, as it were, special and more 

 homogeneous sheaths of connective tissue, beneath which, fat- and pig- 

 ment-granules are often deposited. The outer surface is covered with 

 an epithelium in all respects like that of the dura mater. 



The pia mater cerebri is more vascular but more delicate than that of 

 the spinal cord, and covers all the elevations and depressions on the 

 surface of the brain, if not very closely yet quite exactly, with the 

 single exception of the floor of the fourth ventricle, above which it is 

 stretched across from the calamus scriptorius, as far as the nodulus, the 

 free border of the vela medullaria inferiora and the flocculi, forming 

 the tela chorioidea inferior, from which points it proceeds to invest the 

 under surface of the inferior vermiform process and of the tonsillce. 

 The pia mater penetrates into the interior of the brain only at one 

 point, viz., at the transverse fissure of the cerebrum, where it passes 

 beneath the splenium corporis callosi, investing the vena maana G-aleni, 

 as well as the pineal gland, forming the tela chorioidea superior, with 

 the plexus chorioideus ventriculi tertii; and passing beneath thefornix, 

 also constitutes the vascular plexuses of the lateral ventricles, which 



