428 THE EXPERIMENTAL PRODUCTION OF AN INTERNAL HYDROCEPHALUS. 



vexities of the cerebral hemispheres a very slow and less efficient distribution occurs. 

 Everywhere this extraventricular course is through the meshes of the subarachnoid 

 spaces a complicated, interrupted fluid-channel, completely clothed by low or 

 flat mesothelial cells. In the cisternal regions, and to a lesser extent about the 

 upper spinal cord, the mesh of this fluid channel is rather large, but over the hemi- 

 spheres it becomes quite small. The pial reflection of the mesothelial cells lining 

 the subarachnoid space is broken by the openings of the perivascular spaces (Robin) ; 

 into these cuffs the mesodermal cells continue for a short distance. By way of these 

 perivascular spaces a small but rather important fluid, representing probably the 

 elimination of the brain tissue, is added to the subarachnoid cerebro-spinal fluid. 



It seems quite well established that the cerebro-spinal fluid is returned directly 

 into the blood stream. This escape of fluid is wholly from the subarachnoid space; 

 all observers are agreed that the intra ventricular absorption is minimal. The most 

 important hypotheses regarding the mode of absorption of the fluid are those which 

 assume the Pacchionian villi as the pathways into the dural sinuses (Key and 

 Retzius, 1876) ; that the fluid reaches the cerebral capillaries by way of the peri- 

 vascular sheaths (Mott, 1910); or that a direct absorption into the capillaries of 

 the subarachnoid space occurs (Dandy and Blackfan, 1913, 1914). More recently 

 evidence that the arachnoid villi (normal projections of arachnoidea into the dural 

 sinuses) are the essential structures in the pathway of escape, has been presented 

 (Weed, 1914, a, b, c.). The direction of flow (toward subarachnoid space) in the 

 perivascular spaces argues against Mott's idea of drainage, while the absence of 

 capillaries in the arachnoid renders the hypothesis of Dandy and Blackfan unten- 

 able. The strongest evidence, both anatomical and physiological, favors the idea 

 of a major absorption from the cranial portion of the subarachnoid space. 



With this conception of the pathway of the cerebro-spinal fluid, it becomes 

 evident that the obstruction to outflow of fluid may be either intraventricular or 

 extraventricular. Pathologically, the differentiation of hydrocephalus into two 

 classes with reference to the point of obstruction has repeatedly been made. In 

 the one, the block to outflow occurs usually at the points of constriction of the 

 ventricular system, as at the foramina of Munro, within the aqueduct of Sylvius, 

 or (in rarer cases) within the fourth ventricle. In the second variety of cases the 

 obstruction to flow of the fluid occurs within the subarachnoid space. The mechan- 

 ism of this intrameningeal block is not well understood, except in the cases of obvious 

 filling of the cisterna cerebello-medullaris with fibrinous exudate, occasioning a 

 macroscopic closure of the exit. 



The occurrence of a typical internal hydrocephalus due to block within the 

 narrow portions of the ventricular systems seems of easy explanation. With con- 

 tinued elaboration of cerebro-spinal fluid by the choroid plexuses, and practically 

 no intraventricular absorption, it seems inevitable that dilatation of the ventricles 

 should take place. Such enlargement reaches its fullest expression in the lateral 

 ventricles with compression and thinning of the cerebral cortex. W T ith the increased 

 pressure of the fluid there is ultimately brought about a balance between the 

 production of fluid by the choroid plexuses and the minimal intraventricular 

 absorption, plus other potential agencies of escape. 



