INTRACRANIAL AND INTRAOCULAR FLUIDS 



1 777 



* These results were obtained on anesthetized animals. 



vein some 10 to 11 mm Hg (m, 146) so that the 

 physical condition for a continuous flow of fluid is 

 satisfied. Whether the main loss of pressure from the 

 anterior chamber to ciliary veins takes place across 

 the scleral meshwork and the endothelium of 

 Schlemm's canal, as argued by Goldmann (111), or 

 more distally is not certain. Measurement of the pres- 

 sure in Schlemm's canal (170) and the effects of re- 

 moval of the meshwork (113) would suggest that t he- 

 resistance occurs mainly along the collectors and 

 aqueous veins, and this seems reasonable when it is 

 appreciated that the holes in the trabecular meshwork, 

 and in Schlemm's canal, must be large enough to per- 

 mit the passage of the large serum globulin molecules 

 so that the frictional resistance to the flow of fluid may 

 not be high. In the cerebrospinal system, drainage is 

 apparently by way of the arachnoid villi projecting 

 into the dural sinuses; the cerebrospinal fluid pressure 

 is normally greater than the pressure in the dural 

 sinuses (26, 239) so that a flow of fluid out of the cere- 

 brospinal into the venous system is mechanically 

 feasible. It has been argued, however, that besides the 

 difference between cerebrospinal fluid and dural sinus 

 pressures, a further factor influencing drainage is the 

 colloid osmotic pressure of the plasma proteins (236). 

 In order that such an osmotic pressure, drawing fluid 

 from the cerebrospinal to the venous system, may be 

 operative, however, the membranes separating the 

 contents of the arachnoid villus from the blood in the 

 dural sinus must be impermeable to proteins; the facts 

 that the plasma proteins are normally drained away, 



and that the plasma proteins after injection into the 

 subarachnoid space appear rapidly in the blood (51, 

 218) make it very unlikely that the membranes sep- 

 arating the two fluids in the arachnoid villus are im- 

 permeable to proteins; so this factor must be ruled 

 out.' 8 



FATE OF MATERIAL INJECTED INTO 

 SUBARACHNOID SPACE 



Cerebrospinal Fluid-Brain B<u ■ 



The fluid within the subarachnoid space is enclosed 

 within mesothclial membranes, the layers of cells 

 lining the arachnoid and pia. To the extent that these 

 cellular layers constitute an impediment to free diffu- 

 sion, we 111.1% si\ that there is a barrier between the 

 cerebrospinal fluid and its underlying nervous tissue. 

 Moreover, beneath the pia there is a thick laver of 

 neuroglial cells which ma) also, to some extent, retard 

 diffusion; it is customary id describe the 'membrane' 

 separating the subarachnoid fluid from the nervous 

 tissue ,iv the 'pia-glia'. When a substance is injected 

 into the subarachnoid fluid, there are at least two 

 theoretical pathways of escape into the blood stream. 

 First, there is tin- passage through the arachnoid villi, 

 i.e. alons> the main drainage route. Since there is every 

 reason to believe that proteins may p.iss b\ this route, 

 we may expect all noncolloidal matter to pass out at 

 roughly the same rate by this channel. If the rate of 

 renewal of the fluid is some 0.5 per cent per min., then 

 the rate of loss of material by this route should be of 

 this order. 19 Secondly, there is a direct diffusion across 

 the pia-glia so that the substance reaches the capillary 



18 Weed 123(11 replaced the cerebrospinal fluid of a cat with 

 solutions containing varying concentrations of protein and 

 found that the rate of flow of fluid into the cisterna magna from 

 a reservoir maintained at constant height varied inversely with 

 the concentration of protein. This seemed to indicate that the 

 normal rate of outflow was determined by the colloid osmotic 

 pressure of the cerebrospinal fluid; unfortunately, Weed was 

 ignoring the osmotic influx from the nervous tissue into the 

 cerebrospinal fluid. Thus, replacing the fluid with a solution 

 containing a high concentration of protein will cause a flow of 

 extracellular fluid from the nervous tissue into the subarachnoid 

 space; this will raise the fluid pressure and reduce the influx 

 from the reservoir. 



1 9 From the rate of disappearance of inulin from the sub- 

 arachnoid fluid of the rabbit one may compute a turnover rate 

 of about 0.5 per cent per min. (58). Inulin has such a large 

 molecule that it is unlikely to diffuse across the mesothelial 

 linings of the subarachnoid space, so that it is probably lost to 

 the blood exclusively by flow through the arachnoid villi. 



