1128 THE CEREBRO-SPINAL FLUID. [BOOK m. 



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 pigmen- 

 tary. 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 blood vessels 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 plexuses of the lateral ventricle, are called 

 the choroid plexuses of the third ventricle. 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. 



695. 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 cases 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 complete 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, colourless 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 p.c. Of these by far the greater part, *8 or '9 p.c., is 

 supplied by salts, the total quantity of which as well as the 

 relative amount of the several constituents being about the same 

 as obtain in blood and lymph. The comparative deficiency of 

 solids is due to the scantiness of the proteids, which rarely exceed 

 1 p.c. These are chiefly globulin and a form of albumose, or even 



