82 Journal of Comparative Neurology. 



fluid in one sitting by this method from a case of hydrocephalus 

 due to a tumor, an amount which could hardly have been 

 afforded by the spinal subarachnoid space with its unyielding 

 walls. 



The evidence in regard to the function of the communica- 

 tions is more or less negative in these cases, for a hydrocephalus 

 might occur with the openings present and be due to a fault in 

 the absorbents. 



But the more numerous the cases of hydrocephalus with 

 closure of the communications, with otherwise normal condition 

 of the structures, the stronger the evidence that they are func- 

 tional in allowing the passage of the cerebro-spinal fluid. 



The present contribution is based upon researches conducted 

 for the greater part microscopically upon both adult and embry- 

 onic brains, both of man and the lower mammals. All the ob- 

 jects were cut serially ; great difficulties were encountered from 

 the necessity of keeping the membranes intact. 



In so far as practicable the entire head was cut ; when this 

 was impracticable, either a portion of the head containing the 

 cerebellum and oblongata with the tentorium intact was decalci- 

 fied and sectioned, or the entire brain was hardened in situ, 

 generally by the intra-arterial injection of formalin, and the cer- 

 ebellum and oblongata with their surrounding membranes then 

 carefully isolated by cutting away the adjacent structures. 



The reagents used for fixation and hardening were gener- 

 ally formalin or a mixture of formalin and bichromate of pot- 

 ash ; other usual fixatives were also employed for the embryonic 

 material. 



Both celloidin and paraffin were used for embedding, chiefly 



the latter. 



The most valuable stain for adult forms was found to be 

 Van Gieson's, it giving a very clear differention between the 

 membranes and epithelium. The usual hsematoxylin and car- 

 mine stains were used for the embryonic material with combina- 

 tion stains. 



In the illustration of this paper I am indebted to Mr. E. 

 A. Spitzka who made all the drawings of sections. 



