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



accounted for in two ways. In the first place, the experimental replacement in the 

 embryo pigs lasted at most one hour (due to the fact that the embryo's heart 

 frequently ceased beating at the end of this tune), while in the adult cat or dog they 

 were continued for several hours; and it was only in the long-continued experiments 

 hi the adult that the extensive perineural injections were obtained. On this basis 

 it seems more than likely that the communications between peripheral perineural 

 spaces and the subarachnoid space are very small and that diffusion must account 

 for the slow filling of the peripheral system. The second explanation seems un- 

 doubtedly to concern the tune of development of these perineural spaces in the 

 embryo. It may be that the spaces are morphologically non-existent until late in 

 fetal life; in that case, of course, it is not strange that they have not been filled 

 with the injection fluid. 



From the observations recorded above it is quite apparent that in the typical 

 experiment in which the normal cerebro-spinal tension is not increased no evidence 

 of the perineural space, as injected by Miss Sabin, has been adduced. However, 

 the possibility of injecting these spinal spaces as was done by Miss Sabin is easily 

 demonstrated. The injections may be made with ease, either with . granular sus- 

 pensions or with true solutions. Success invariably attends such an injection into 

 the perispinal tissues. The injection solutions easily run out around each nerve, 

 more readily, apparently, in the younger embryo than in the older. It is not clear 

 whether this difference is due to the fact that in younger embryos the resistance is 

 greater to the perispinal flow and less peripherally, or merely to the fact that a 

 greater amount of fluid must be introduced in order to attain the same result. 

 Careful repetition of these observations has led to the conclusion that such a demon- 

 stration of the spinal perineural spaces results from excessive pressures of injection. 

 Whenever the pressure exerted by the injection is but slightly above the normal, or 

 does not exceed the normal (as in replacements), the perineural spaces are not 

 injected around the spinal nerves. Miss Sabin's conclusions from her results, that 

 no connection exists between the spaces and the lymphatic system, seem to be 

 wholly substantiated by these observations. 



The apparent perineural spaces around the embryonic nerves must be looked 

 upon as artifacts. In tissue carefully fixed, dehydrated, and embedded, there is 

 no real evidence of these spaces. Their size apparently varies with the care observed 

 in the histological technique. 



