58 DEVELOPMENT OF CEREBKO-SPINAL SPACES IN PIG AND IN MAN. 



Consideration must next be given to the factors of diffusion, filtration, and 

 osmosis in the passage of fluid through the roof of the fourth ventricle. The third 

 factor, however, may be largely excluded, owing to the fact that the solutions of 

 potassium ferrocyanide and iron-ammonium citrate employed were for the most 

 part practically isotonic with the body-fluids. Furthermore, the use of hypertonic 

 solutions apparently gave no different results (except in the increased density of the 

 resultant precipitate) from those obtained by the employment of the isotonic solu- 

 tions. Finally, it was found to be of service to use hypotonic replacement solutions 

 in order to obtain very slight precipitates; in these experiments also the spread of 

 the replaced ferrocyanide solution was similar to the standard result afforded by the 

 isotonic solution. These observations with varying concentrations of the foreign 

 solutions replacing the cerebro-spinal fluid serve to indicate that osmosis plays but 

 little part in the passage of fluid through the roof structures of the fourth ventricle. 

 Undoubtedly the factor of osmosis can not be ignored in any consideration of the 

 passage of fluid through a cellular membrane, but it seems unlikely that with solu- 

 tions of practically the same salt-content it should be of great importance. 



The influence of diffusion in this passage of the solution of the ferrocyanide and 

 citrate from the cerebral ventricles into the extraventricular space is probably great. 

 The whole plan of the experiment concerns the introduction of salts foreign to the 

 body-fluids, even though in analogous concentrations. It seems not unlikely that 

 as soon as the replacement of the existent cerebro-spinal fluid is effected the ferro- 

 cyanide and citrate must immediately begin to diffuse out into the periaxial tissues 

 and the normal salts return to the ventricles. Probably, however, this same 

 phenomenon plays a normal role in the human body. JacobsonV 27 ) extensive and 

 important studies on the chemistry of cerebro-spinal fluid have shown that the ven- 

 tricular cerebro-spinal fluid is not identical with the subarachnoid fluid. The 

 differences in the two fluids are probably to be accounted for by the fact that the 

 ventricular fluid represents the pure elaboration of the chorioid plexuses, whereas the 

 lumbar subarachnoid fluid is composed not only of the products of the chorioid plex- 

 uses but also of the fluids from the perivascular system. In this transference of the 

 ventricular fluid to the subarachnoid space diffusion may play some part, the rela- 

 tive importance of which can hardby be estimated. 



But will diffusion alone account for the passage of the experimental fluid in the 

 ventricle through two well-defined areas into the periaxial tissues? Will diffusion 

 account for the varying extent of the injection in different stages of embryonic 

 development? There are several arguments against according diffusion a maximal 

 importance in the process. In the first place, an injection of the solution of the ferro- 

 cyanide under mild syringe-pressure will give a spread similar in every respect to 

 those obtained by the replacement method. This indicates that the course taken 

 by the two solutions is not necessarily the result of diffusion, but rather of the capa- 

 bilities of the tissues for fluid-spread; and similarly the passage of this true solution 

 through the roof areas need not be solely a diffusion process, but may be accounted 

 for by the true flow of the fluid in this direction. Again, in the stages represented 



