56 BIOLOGICAL PHYSICS 



cavities, or spaces. For this purpose, it appears to us that 

 the only mechanisms, or organisms, existing within the 

 central nervous system, available for the purpose, are the 

 choroid plexuses of the two lateral, third, and fourth 

 ventricles, which, in fact, are the only non-nervine 

 structures within the great central cavities of the brain, 



FIG. i. THE LATERAL VENTRICLES OPENED BY REMOVAL OF THE 



MIDDLE PART OF THE CORPUS CALLOSUM, AND THE DESCENDING 

 CORNU EXPOSED ON THE RIGHT SIDE. \. 



a, 6, anterior and posterior parts of the great longitudinal fissure ; c, section of the 

 anterior part of the corpus callosum ; d, posterior part of the same; e, the left 

 choroid plexus ; _/, the fornix ; g, the anterior ; A, the posterior, and q, the 

 descending cornu of the lateral ventricle ; k, k, corpora striata ; /, /, optic thalami ; 

 n, n, right and left hippocampus minor ; o, posterior pillar of the fornix ; v, the 

 fimbria into which it passes ; f, on the cornu ammonis or hippocampus major ; 

 //, on the medullary substance of the cerebral hemisphere ; r, part of the grey 

 cortical substance showing the white stria of Vicq-d'Azyr ; s, taenia semicircularis; 

 y, eminentia collateralis. 



and which represent, or rather, are inflections of the pia 

 mater with its vascular and connective tissues (Figs. I 

 and 2). We therefore claim that the function, or at any 

 rate the main function, of these organs or structures is 

 that of secretion, or excretion as it might be called, into 

 the central cavities of the brain, in which they spread out,, 

 of an internal modicum of cerebro-spinal fluid, and that 

 a constant and physiologically suitable and graduated 



