74 DEVELOPMENT OF CEREBRO-SPINAL SPACES IN PIG AND IN MAN. 



of any penetration of the pia mater by the fluid. This is well brought out in figures 

 14 and 18. In every respect (as demonstrated by numerous experiments of this 

 type in pig embryos of varying lengths) the pia mater is wholly impenetrable to true 

 solutions of foreign salts when injected so that the normal tension is not altered. 

 The whole subarachnoid space may, in such an experiment, be filled with the prussian- 

 blue, but none of these granules are found within the cells of the pia mater or in 

 any layer between these cells and the nervous system. Evidence that the fluid has 

 bathed the outer pial cells is afforded by the adhesion of granules of prussian-blue 

 to the outer cytoplasmic borders of the cells. 



Likewise the cells comprising the embryonic pia have been found to be impene- 

 trable to true solutions (ferrocyanide) when injected under varying pressures from 

 a syringe. In these cases, rupture of the roof of the fourth ventricle or of the infun- 

 dibulum may be produced by great pressure, without causing any of the fluid to 

 penetrate the intact layer of the pia mater. The same result is obtained when 

 india ink is substituted for the true solution. 



The pia mater, then, even in its embryonic form, serves as an efficient fluid- 

 barrier. This is demonstrated, in regard to the adult pia mater, in the report ( 55 ) 

 of the observations made on adult cats, dogs, and monkeys. But the barrier which 

 the pia offers to the entrance of fluid from without exists also for fluid coming in 

 the reverse direction. This is shown by the well-known phenomenon of the so-called 

 subpial extravasation, which occurs in blood vascular injections when the injections 

 are continued for too long a time at too high a pressure. The perforating vessels 

 in such cases rupture as they enter the nervous system, and the injection mass 

 spreads extensively beneath the pia, stripping it away from the nervous tissue. Of 

 interest in this discussion is the fact that the injection mass in these extravasations 

 does not rupture the pia, which seemingly is an equally efficient fluid barrier to 

 pressure exerted on it from within. Similar subpial spreads of the injection fluids 

 have been observed in the course of this work. These extravasations resulted from 

 the rupture of the whole nervous tissue from within, particularly in the region of 

 the infundibulum, when the injection was made into the ventricular system under 

 excessive pressure. In this respect, too, the pia seems to be wholly efficient as a 

 retainer for true solutions or for granular suspensions. It is realized that the 

 embryonic pia mater will not resist the passage of fluids through it under the highest 

 pressures afforded by the syringe, but the membrane serves as an efficient barrier 

 for all pressures such as are employed in careful anatomic injections. 



With this conception of the impenetrability of the pia mater to fluids under 

 ordinary pressures, it does not seem strange that there is a variation in the process 

 of formation of the pia mater in the region of the roof of the fourth ventricle. It 

 has been shown in the foregoing paragraphs that the phenomenon of mesenchymal 

 condensation which results in the formation of pia elsewhere does not occur in the 

 region of the two arese membranaceae. In view of the passage of cerebro-spmal 

 fluid through these two membranous areas, the pia mater must necessarily be altered 

 in these places. For were it not adapted to the purpose of affording fluid passage 



