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



the perivascular channels into the subarachnoid spaces. Probably this system 

 represents a mechanism for accessory tissue drainage comparable physiologically 

 to the lymphatic channels of the other parts of the body. 



In view of these findings in the adult mammal it seemed desirable to ascertain 

 at what period of intra-uterine life such function was acquired. It also seemed not 

 unlikely that information of interest might be acquired from the embryonic intra- 

 medullary circulation which would amplify our knowledge of this system in the adult. 

 It was thought that there might be a correlation between the production of the 

 perivascular fluid and the enlargement of the subarachnoid channels, similar to 

 the evident connection between the chorioidal invagination and the extraventricular 

 spread of the fluid. 



Experiments to demonstrate possible perivascular and perineuronal spaces were 

 first attempted on rather large fetuses (pig), as follows: The spinal meninges were 

 exposed in a fetus in which the heart was still beating vigorously. Into the spinal 

 subarachnoid space was introduced a needle connected with a small reservoir, con- 

 taining the injection solution (potassium ferrocyanide, 0.5 gm. ; iron-ammonium 

 citrate, 0.5 gm. ; water, 100 c.c.). The reservoir was then adjusted so that a pressure 

 of 160 mm. of water was maintained in the subarachnoid space. The arteries and 

 veins in the neck of the fetus were then severed, and the subarachnoid pressure 

 maintained at its former level. At the end of 20 minutes the head was placed in a 

 fixative containing 1 per cent hydrochloric acid. 



This procedure, as outlined above, in the adult laboratory mammal, usually 

 resulted in a complete injection of the perivascular system. In the embryo, how- 

 ever, the procedure was uniformly unsuccessful. The injection solution, as shown 

 subsequently by the precipitated prussian-blue, rarely ascended over a centi- 

 meter above the point of injection. This indicated that the existent cerebro-spinal 

 fluid was not replaced by the injection solution, and that the failure to demonstrate 

 the perivascular system was to be explained on this basis, if the system were func- 

 tional at this stage. Attempts were then made to replace the subarachnoid fluid 

 with the injection solution before the cerebral anemia occurred. These attempts 

 likewise met with failure, because of the impossibility of keeping the heart beating 

 for any length of time in the larger pig fetuses. Other attempts were also made to 

 demonstrate these channels, in larger pig embryos, by means of a procedure which 

 in the adult gave at times good injections of these intracortical canals. This 

 method differed from the method first employed only in the maintenance of a high 

 pressure (100 mm. Hg) in the spinal subarachnoid spaces. It likewise met with 

 failure, due apparently to the same causes which occasioned its failure in the adult : 

 the high subarachnoid pressure operated chiefly to compress the cerebral and spinal 

 tissues, rendering the injection of the perivascular spaces impossible. 



The same procedures were attempted in smaller pig embryos (15 to 60 mm.). 

 The method usually successful in demonstrating the spaces (subarachnoid pressure 

 slightly above normal, with subsequent cerebral anemia) failed, apparently because 

 the cranial cavity at these stages is in no sense a rigid closed box, as in the adult. 



