730 THE BRAIN. 



with it the thin non-nervous epithelium which alone represents at the place 

 the original wall of the medullary canal, is thrust inward at the transverse 

 fissure of the brain, beneath the fornix, to form the velum interpositum, thus 

 supplying a roof to the third ventricle, and that it thence projects into each 

 lateral ventricle as the choroid plexus of each side, reaching from the fora- 

 men of Monro in front along the edge of the fornix to the tip of the descend- 

 ing horn. The velum being a fold of the pia mater consists theoretically of 

 two layers, and between the upper dorsal layer and the lower ventral layer 

 lies a thin bed of connective tissue carrying arteries forward from the hind 

 edge of the corpus callosum, and similarly carrying veins backward ; these 

 vessels supply the choroid plexus with an abundant supply of blood. In the 

 choroid plexus the folded pia mater is developed into a number of villus- 

 like processes, the primary processes bearing secondary ones. Each process 

 consists, like a villus, of a basis of connective tissue, in which the bloodvessels 

 end in close-set capillary loops, covered with an epithelium. The epithelium, 

 though continuous with the rest of the epithelium lining the lateral ventricle, 

 and thus, as we have said, shutting off the lateral from the third ventricle 

 (except at the foramen of Monro), and though like it derived from the wall 

 of the original medullary canal, is different in structure. Over the ventricle 

 generally the epithelium consists of ordinary short columnar, apparently 

 ciliated cells, with more or less transparent cell substance ; the cells over the 

 choroid plexus are cubical, often irregular in form, and their cell substance 

 is loaded with granules, some of which are pigmentary. They have very 

 much the appearance of " active " secreting cells ; and indeed a branched 

 process of the plexus may be compared to an everted alveolus of a secreting 

 gland, with the epithelium outside and the bloodvessels within. It cannot 

 be doubted that these cells play an important part in secreting into the cavity 

 of the ventricle fluid which, passing thence by the foramen of Monro into 

 the third and so into the fourth ventricle, finds its way by the foramen of 

 Majendie into the subarachnoid space. 



As the velum overhangs the third ventricle it sends down vertically two 

 longitudinal linear fringes, which, resembling in structure the choroid plex- 

 uses of the lateral ventricle, are called the choroid plexuses of the third ven- 

 tricle. From the roof of the fourth ventricle there hangs down on each side 

 a similar linear fringe, the choroid plexus of the fourth ventricle, which is 

 especially developed at its front end beneath the overhanging cerebellum. 

 These subsidiary choroid processes doubtless assist in furnishing cerebro- 

 spinal fluid, but their share is small compared with that of the main choroid 

 plexuses of the lateral ventricle. 



607. The cerebro-spinal fluid. The specimens of cerebro-spinal fluid 

 which have been examined as to their composition are not quite comparable 

 with each other, since while some (such as those obtained from cases where 

 a fracture of the base of the skull has placed the subarachnoid space at the 

 base of the brain, where it is largely developed, in communication with the 

 external meatus, and the fluid escapes by the ear) may be regarded as 

 normal, others (such as those obtained from case* of hydrocephalus where 

 the ventricles contain an unusual quantity of fluid, or from cases of spinal 

 malformations) must be considered as abnormal. In most of the more com- 

 plete analyses, the fluid examined has belonged to the latter class; and the 

 following statements apply, strictly speaking, to them alone. 



With this caution, we may say that cerebro-spinal fluid is a transparent 

 colorless or very slightly yellowish fluid, of faint alkaline reaction, free from 

 histological elements. The specific gravity is about 1010 or less, the amount 

 of solids being on an average 1 per cent. Of these by far the greater part, 

 0.8 or 0.9 per cent., is supplied by salts, the total quantity of which a? well 



