58 BIOLOGICAL PHYSICS 



of such mechanical support and hydraulic force as are 

 required for maintaining the patency and continuity of 

 these intra-spaces. 



Structurally the choroid plexuses are made up of 

 inflections, as we have said, or continuations of the pia 

 mater *, or more exactly of the pia-arachnoid meninges, 

 supported by their framework of fibre-elastic connective 

 tissue, and surrounded by inflections of the ventricular 

 endothelial linings thus formed, they traverse those 

 central intra-spaces of the brain known as the two lateral, 

 the third, and the fourth ventricles. As regards the 

 functions of these structures, their study may be said to 

 have hitherto been comparatively neglected, their non- 

 nervous textures apparently debarring them from that 

 special attention which their situation otherwise entitles 

 them to. 



The facts that they are highly organised bodies, or 

 textures, that they occupy a most important central 

 position amid the most highly organised structures of the 

 most vital organ of the body, and that they spring from, 

 and terminate in, the blood circulating media of the brain, 

 suggest that they must perform some still almost unknown 

 functions of a vitally important nature in the economy of 

 the great central nervous system, the discovery of which 

 must be regarded as of consequently nothing less than the 

 very greatest scientific interest, and physiological, patho- 

 logical, and chemical importance. 



The choroid plexuses being inflections of the pia- 

 arachnoid textures into the central cavities of the brain 

 where no lining pia mater is distributed, secrete or excrete, 

 into these cavities fluid sufficient to maintain the requisite 

 amount of moisture therein ; in other words, they distil, 

 into the intra-spaces of the central nervous system, fluid 

 sufficient to maintain, in conjunction with the inter- 

 meningeal fluid, or lymph, the equilibrium of the fluid 

 pressure within and without that system. A c< give and 

 take " distribution, or disposal, of the combined fluids, we 

 take it, existing between the central cavities and the 

 sub-arachnoid and sub-dural spaces surrounding the brain 

 and cord, and the connected peripheral nervous system ; 

 the foramina of Majendie, and the pineal gland, among 



