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



methods of approach it is not surprising that observations such as RefordV 47 ) fail 

 to coincide with these findings. The immutability of suspensions of granular 

 material in the investigation of the cerebro-spinal spaces has been many times 

 verified in this work. 



In the further study of the course of the spread with injections of india ink it was 

 found that, in pig embryos of approximately 22 mm. and over, a partial periaxial 

 injection could be secured by plunging the syringe-needle into the perispinal spaces. 

 The carbon granules could subsequently be seen filling the perispinal spaces and 

 also mounting upwards in partial pericerebral relationships, particularly around the 

 medulla. This result was obtained by the use of strong syringe-pressures. Appar- 

 ently the resistance to the spread of the ink in injections or replacements in the 

 medullary-canal system occurs in the passage of the fluid from the roof of the fourth 

 ventricle into the periaxial spaces. So far as is known, Reford (4? ) did not control 

 his injection pressures. These results with the injection of india ink under strong 

 pressures coincide with the idea of his observations afforded by the abstracts given 

 by Sabin^ 49 ) and Cushing (9) . Suspensions of india ink, then, injected under mild 

 syringe-pressure or by the replacement method, offer no evidence, in the pig embryo, 

 of a passage of the cerebro-spinal fluid into the periaxial spaces. Only by employing 

 pressures much above the normal tension can such evidence be obtained. 



V. UNDESCRIBED STRUCTURES IN ROOF OF THE FOURTH VENTRICLE. 



The results of the replacement of the existing cerebro-spinal fluid by a true solu- 

 tion of potassium ferrocyanide and iron-ammonium citrate in a living pig embryo 

 indicated, as detailed in the foregoing section, that the fluid passed from the ven- 

 tricular system into the periaxial tissues in the region of the roof of the fourth 

 ventricle. This important transit of the fluid, agreeing with the established con- 

 ception of the relationship in the adult, was first observed in an embryo pig of 

 14 mm. (fig. 3). At this stage the exudation of the replaced fluid occurred in one 

 defined area, seemingly corresponding to the dense oval in a smaller embryo shown 

 in figure 2. 



Such a passage of fluid from ventricle to periaxial tissue is necessarily a physio- 

 logical phenomenon, and it was in the hope of finding an anatomic basis for this 

 phenomenon that the roof of the fourth ventricle was studied histologically. It was 

 realized that failure to demonstrate anatomically differentiated structures would not 

 vitiate the physiological observations, but that a correspondence between function 

 and structure was most desirable. Hence observations were undertaken to deter- 

 mine, if possible, an area of histological differentiation in the roof of the fourth 

 ventricle which might be concerned in the primary passage of fluid from the cerebral 

 ventricles into the periaxial tissues. The investigation concerned first the exami- 

 nation of this region in pig embryos of 14 to 15 mm., at which stages the fluid passes 

 from a single area. Subsequently, similar studies were undertaken in regard to the 

 second, more inferior area (shown in figure 4). The results of these studies will be 

 given here. 



