MEMBRANES OF THE NERVOUS CENTRES. 



469 



Fig. 312. 



or even blackish color. They are also found in the medulla ob- 

 longata and the pons Yarolii, and still more anteriorly. 



The vascular plexuses, just named, in the ventricles are composed 

 mostly of vessels, and are covered by an epithelium where they 

 are not adherent to the walls of the ventricles. This consists of a 

 single layer of roundish polygonal cells, y^j^ to j^W f an i nc ^ 

 in diameter, and ? oV <j to 3 oV <y of an inch thick; containing, besides 

 the rounded nucleus, many yellowish granules, and one or two dark 

 round oil-drops measuring T 2ouu to soso f an mCQ - It i g not 

 probable that they are ciliated, as asserted by Valentin. Under- 

 neath the epithelium is a simple membrane ; and next, the vessels 

 connected by a hyaline homogeneous 

 substance (p. 108). 



All the portions of the ventricles 

 not covered by the continuations of 

 the pia mater, have a special lining 

 membrane, the ependyma ventriculo- 

 rum. This is a simple conoidal epi- 

 thelium (Fig. 312) ; and it is separated 

 from the brain -substance by a fila- 

 mentous layer T5 4 OII to 5 i^ of an inch 

 thick, of embryonic areolar tissue. 

 Virchow and Kolliker did not find it 

 to exhibit ciliary motion, as asserted 

 by Purkinje and Valentin. 



2. The arachnoid membrane does not consist of two lamella, as 

 usually described ; but of a single one, the internal one of authors. 

 This is an extremely delicate transparent membrane, corresponding 

 in extent to the dura mater. It is made up of lamellae of fasciculi 

 of white fibrous tissue, surrounded by fine elastic fibres. In the 

 spinal canal it is loosely adherent to the pia mater, by fasciculi of 

 areolar tissue, so that a space, called the subarachnoid space, exists 

 between it and the latter membrane, and which is filled with the 

 cerebro-spinal fluid. In the cranium it is much more adherent to 

 the pia mater. Thus, there is no continuous subarachnoid space 

 upon the brain, but numerous larger and smaller spaces only par- 

 tially communicating. The larger of these spaces (between the 

 cerebellum and medulla oblongata, under the pons Varolii, the 

 crura cerebri, and the fossa Sylvii), open directly into the subarach- 

 noid space of the spinal cord; while the remaining ones do not. 



The ependyma in man. A. From the 

 corpus striatum ; 1, from the surface ; 2. 

 from the side ; a, epithelial cells ; 6. 

 nerve-fibres lying beneath. B. Epithe- 

 lium-cells from the commissura mollis. 

 Magnified 350 diameters. (Kttliker.) 



