THE RETURN OF CEREBRO-SPINAL FLUID TO THE VENOUS SYSTEM. 89 



premised the drainage of fluid into the great sinuses through a valve-like mechanism. 

 Dandy and Blackfan^ 10 ) suggested its absorption by the capillaries of the pia-arach- 

 noid an untenable hypothesis in view of the work of Kadyi< 28 ), Shroeder van der 

 KohV 51 ), Ekker^ 11 ), AdamkiewiczW, and others. Still another conception of the 

 process has been advanced by Mott( 41 ), namely, that the absorption of cerebro-spinal 

 fluid is one of the functions of the cerebral capillaries. In a previous investigation^ 55 ), 

 making use of a method similar to the one here employed in the replacement experi- 

 ments, evidence was presented indicating the drainage of cerebro-spinal directly 

 into the great dural sinuses through arachnoid villi. These structures represent an 

 invasion of arachnoid tissue through the lateral wall of the sinuses. 



In view of the findings in adult laboratory animals, interest naturally turned, 

 during the course of this work, to the process of drainage of the embryonic cerebro- 

 spinal fluid. The evidence afforded by the replacement experiments with the ferro- 

 cyanide solution indicated that in pig embryos of over 20 mm. cerebro-spinal fluid 

 circulated throughout most of the periaxial tissue, and that in embryos of about 

 26 mm. the periaxial distribution was complete, the relations of the fluid at this 

 stage becoming adult. With this evidence before us, the question of the drainage 

 of the fluid became important. Was the absorption process similar to the normal 

 adult procedure, or was it entirely lacking, the production of the fluid being balanced 

 by the growth of the nervous system and its meningeal spaces? 



The question of the absorption of cerebro-spinal fluid was approached in the 

 embryo in a similar manner to that employed in the adult animal. The problems 

 incurred by the use of abnormal intracranial pressures were eliminated by the method 

 of replacing, without disturbing the normal tension, the embryonic cerebro-spinal 

 fluid with the ferrocyanide solution. The embryo was then kept alive and was 

 finally fixed in a preservative which would precipitate the replaced fluid as prussiari- 

 blue. This procedure was carried out in many embryos of varying lengths and the 

 specimens were subsequently stained in serial sections. 



The smallest embryo in which any evidence of absorption of the fluid from the 

 periaxial tissue was obtained was a pig embryo, 23 mm. in length. In this specimen 

 granules of prussian-blue could be traced through the mesenchymal spaces (arach- 

 noidal) to the inner wall of the sinus transversus. The sinus is well differentiated at 

 this stage in the human embryo of 21 mm., as demonstrated by Streeter^ 54 ). The 

 wall of the sinus in this pig embryo was quite thin, the mesenchyme lending the 

 endothelium but little support. The prussian-blue granules could be traced directly 

 through the endothelial wall of the sinus, and a few were identified lying free in the 

 lumen. The conditions of the observations, permitting a flow of venous blood 

 through the sinus, undoubtedly accounted for the fact that but few of the granules 

 were found lying free in the sinus. This passage of the replaced fluid into the lateral 

 sinus is portrayed in figure 21, taken from the pig embryo of 27 mm. 



The same process of drainage of cerebro-spinal fluid may be observed in pig 

 embryos more than 23 mm. in length. In all but one particular it corresponds 

 exactly to the process observed in adult laboratory animals. There is the same lack 



