IBCT. 117.] NERVOUS SYSTEM. 2 J 9 



single lamella, which also lies close to the inner surface of the 

 dura matey; but, unlike that of the cord, it is, in many places 

 adherent or even blended with the pia mater, especially upon all 

 the convolutions and the projecting parts at the basis of the 

 brain ; and where this intimate adhesion does not exist, the 

 two membranes are connected by numerous processes. Accord- 

 ingly in the case of the brain also, there is no continuous sub- 

 arachnoid space, but numerous larger and smaller spaces, only 

 partially communicating with each other. The larger of them, 

 which arc found between the cerebellum and the medulla oblonqata 

 and below the pons, the cerebral peduncles, the Sylvian fissure, etc., 

 or at least the former, pass directly into the sub-arachnoid space 

 of the cord, whilst the smaller, corresponding to the sulci, over 

 which the arachnoid extends like a bridge, may, perhaps, commu- 

 nicate with each other, but the majority of them at least are not 

 connected with the above-mentioned larger spaces. The arach- 

 noid is nowhere connected with the lining of the cerebral ventri- 

 cles. Its structure is the same as that of the spinal arachnoid, 

 and its outer surface is likewise furnished with an epithelium. 



The pia mater of the brain is more vascular but more delicate 

 than that of the cord, and invests all the elevations and depressions 

 of the surface of the brain, with the sole exception of the 

 bottom of the fourth ventricle, which it spans over, as the tela 

 chorioidea inferior, in order, then, to be reflected at the inferior 

 surface of the vermis inferior and tonsillar. Thepj'a mater pene- 

 trates into the interior of the brain only at one place, viz., at the 

 transverse fissure of the cere! rum, where, enveloping the vena 

 magna Galeni and the pineal gland, it enters beneath the splenium 

 corporis ccdlosi, and forms the tela chorioidea superior with the 

 plexus chorioideus ventriculi teriii ; then passing under the corpus 

 cal'osum, it gives rise to the choroid plexuses of the lateral ven- 

 tricles, which, between the crus cerebri and the middle lobe, are 

 connected with the pia mater at the base of the brain. Besides 

 its numerous vessels, the pia mater contains some indistinctly 

 fibrous and connective tissue without elastic fibres, and occasion- 

 ally, at the base of the brain, pigment cells. 



The t< he chorioidece and plexus chorioidei consist almost wholly 

 of vessels, and have, on their free surfaces, a simple pavement 

 epithelium, the cells of which mostly contain some pigment 

 and fat-granules, and in the mammalia as also in the frog, are 

 ciliated. 



All parts of the cavities of the brain which are not connected 



