452 SPECIAL HISTOLOGY. 



tissue, with a few nuclei and without elastic fibres. Occasion- 

 ally, however, the pia mater also contains reticular connective 

 tissue, as around the vena Galeni, 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 

 oblongata, and 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 ventricles, the tela chorioidece and plexus chorioidei, do 

 not differ in their structure from other portions of the 

 membrane, except that, especially in the plexus, they are com- 

 posed almost wholly of vessels, and are furnished with an 

 epithelium at those points Avhere they are not adherent to the 

 walls of the ventricles. This epithelium consists of a single 

 layer of roundish polygonal cells, O008 — 001'" in diameter, 

 and - 003 — 0004" 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 — 

 - 002'" in size. According 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 colourless 

 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 interlacement of larger and smaller vessels, between 

 which no formed connective tissue can be perceived, but only a 

 clear, homogeneous, interstitial substance. 



All the portions of the ventricles which are not lined by 

 the continuations of the pia mater, that is to say, the floor of 

 the fourth ventricle, the aqueductus St/lvii, 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 

 ventricu/orum (fig. 151). This is a simple tesselated epithe- 

 lium, which, according to Purkinje and Valentin (Mull. ' Arch./ 



