396 SPECIAL HISTOLOGY. 



are continuous with the pia mater at the base of the brain, between the 

 cms cerebri and the inferior lobe. With respect to its intimate struc- 

 ture, the cerebral pia mater contains so many vessels, that in parts the 

 connective tissue which forms the matrix appears as a subordinate con- 

 stituent. It is rarely, as in the spinal cord, distinctly fibrous, for the 

 most part more homogeneous, approaching in character " Reichert's mem- 

 branes," or immature connective tissue, with a few nuclei and without 

 elastic fibres. Occasionally, however, the pia mater also contains 

 reticular connective tissue, as around the vena Craleni, the pineal gland, 

 the larger vessels, and also on the cerebellum. Fusiform pigment-cells 

 also occur here, as in the spinal cord, particularly on the medulla ob- 

 longata, arid pons Varolii, but also, more anteriorly, at the base of the 

 brain as far as within the fissure of Sylvius, in which situation I have 

 noticed them even in the m. adventitia of smaller arteries. 



Those portions of the pia mater which are in relation with the ven- 

 tricles, the telce chorioidece and plexus chorioidei, do not differ in their 

 structure from other portions of the membrane, except that, especially 

 in the plexus, they are composed almost wholly of vessels, and are fur- 

 nished with an epithelium at those points where they are not adherent 

 to the walls of the ventricles. This epithelium consists of a single 

 layer of roundish polygonal cells, 0-008-0-01 of a line in diameter, and 

 0-003-0-004 of a line in thickness, and usually containing together with 

 the rounded nucleus, yellowish granules, often in great numbers, and 

 one or two, dark, round oil-drops of 0'001-0*002 of a line in size. Ac- 

 cording to Henle, almost all these cells send out, from the angles towards 

 the layer of connective tissue of the plexus, short, slender, acuminate, 

 transparent, -and colorless processes, like spines ; and according to 

 Valentin ("Physiol.," 2d ed., part 2, p. 22), in the Mammalia they also 

 support cilia. The epithelium is succeeded by a thin layer of apparently 

 homogeneous, connective tissue, beneath which is a very close inter- 

 lacement of larger and smaller vessels, between which no formed con- 

 nective tissue can be perceived, but only a clear, homogeneous, inter- 

 stitial substance. 



All the portions of the ventricles which are not lined by the con- 

 tinuations of the pia mater, that is to say, the floor of the fourth 

 ventricle, the aqueductus Sylvii, the floor and the sides of the third 

 ventricle, the ventriculus septi lucidi, the roof of the lateral ventricles, 

 the anterior and the posterior cornua, and a considerable part of the 

 descending cornu, in the embryo also the cavity in the olfactory bulb, 

 and the canal in the spinal cord, have a special lining membrane, the 

 so-termed ependyma ventriculorum (Fig. 151). This is a simple tessel- 

 lated epithelium, which, according to Purkinje and Valentin (Mull. 

 "Arch.," 1836; Val. "Repert.," 1836, p. 156), is said to exhibit ciliary 



