PASSAGE OF FLUID THROUGH ROOF OF THE FOURTH VENTRICLE. 61 



to the silver (unless the injection-pressure is extreme). When the superior area is 

 examined after such an injection under high pressure the silver is found deposited 

 throughout the cells of the area, extending only a short distance into the adjacent 

 tissue. This feature of the injection is pictured in figure 113. In these injections 

 the high pressure undoubtedly suffices to force the silver through the coagulated 

 area membranacea. Its coagulating effect on the ependyma is almost equally 

 marked, but the point of least resistance is apparently in the membranous area, allow- 

 ing the fluid to pass through it. 



Replacements of the cerebro-spinal fluid with diluted solutions of india ink 

 within the medullary-canal system of small pig embryos never result in any exten- 

 sion of the granules into the periaxial tissues, for under the normal tension in the 

 ventricles of the pig the arete membranacea? are impermeable to the passage of 

 granular material. After such a replacement the carbon masses may be found 

 everywhere throughout the ventricles, but not in the periaxial tissues. However, 

 india ink may be forced into the periaxial tissues by the use of high pressures of 

 injection, as shown in figure 10. In this specimen of a pig embryo (21 mm. in length) 

 the periaxial spread occurred solely from the superior membranous area. This is 

 analogous to the results obtained with silver nitrate, shown in figure 12. Without 

 doubt in the earlier stages the superior area is much more permeable than the inferior. 

 Histological examination of these specimens after an injection of india ink under 

 high pressure reveals that the carbon granules gain the extraventricular space only 

 through the area membranacea superior; some cells in this area are crowded with 

 the granules, but for the most part extensive intercellular stomata have been made. 

 The whole process must be viewed as a result of the excessive pressure of injection. 



In the more advanced stages of the pig embryo (30 mm. and upwards) the 

 pressure necessary to occasion an extraventricular spread of the india ink after 

 intraspinous injection decreases somewhat, so that with mild syringe-pressure a 

 local periaxial spread from the fourth ventricle may be obtained from an injection 

 into the central canal of the spinal cord. This is in accordance with the observation 

 of MalK 36 ), who found that the injection flowed "through the medial opening of the 

 fourth ventricle." The opening in these cases is in the area membranacea inferior, 

 and in many instances subsequent examination showed rupture of the membrane 

 with escape of the ink, even though the injection-pressure was moderate. 



Taken as a whole, then, the findings are against the passage of solutions of 

 silver nitrate or suspensions of india ink from the ventricles into the periaxial tissues, 

 except when injected under pressures far above the normal intraventricular tension. 



RELATION OF THE EPENDYMAL DIFFERENTIATION TO THE PASSAGE OF FLUID. 



Under this heading it is proposed to discuss the relationship, if any, existing 

 between the stages of differentiation of the ependyma of the roof of the fourth ven- 

 tricle and the passage of fluid through the two membranous areas. The discussion 

 must necessarily be of a temporal character, with an attempt to consider possible 

 factors in the process. 



