GENERAL SUMMARY. 105 



occurs a breaking-down of some of the syncytial strands; these ruptured mesenchymal 

 processes then retract and adhere to the persisting trabeculae. The process con- 

 tinues with the formation of larger channels in this mesodermal tissue, with also 

 the formation of the permanent arachnoidal trabeculae. Throughout these larger 

 spaces, hi the smaller fetuses, the coagula of protein material are everywhere found, 

 the remains apparently of the albuminous portion of the circumambient fluid. 



In the formation of the various cisternse, particularly the great cisterna cerebello- 

 medullaris, the process of the dilatation and confluence of the original mesenchymal 

 spaces reaches its maximum. Here the breaking-down of the original syncytial 

 strands proceeds to such an extent that very few of the strands remain to persist 

 through life. 



Such a process of the enlargement of mesenchymal spaces to form the larger 

 subarachnoid spaces, as described in some measure by His for the spinal meninges, 

 is apparently intimately connected with the circulation through these spaces of the 

 embryonic cerebro-spinal fluid. The fluid flows everywhere through the spaces, as 

 evidenced by the replacement experiments and by the increased content in albumen, 

 before the process of enlargement of the mesenchymal spaces begins. It seems most 

 likely that this circulation of the fluid acts as the causative agent in initiating and 

 probably also in completing the enlargement of the "tissue spaces." The great 

 content of albumen in the embryonic cerebro-spinal fluid has greatly facilitated the 

 investigation, as the presence of the coagula from this protein has permitted the 

 absolute exclusion of artifacts in the process of the tissue-dilatation. 



This mechanism of enlargement of the tissue spaces finds its analogue in the 

 formation of the anterior chamber of the eye and in the perilymphatic spaces of 

 the ear (Streeter). In both these situations, as in the meningeal spaces, there are 

 special body-fluids, more or less characteristic in their physical and chemical char- 

 acters, obviously subserving specialized functions. In both the eye and cranium, 

 the absorption of the fluids is by way of special organs, directly into venous sinuses; 

 in both, the origin of the specialized fluid is from epidermal organs; this fluid is at 

 first poured into epidermal spaces and then subsequently into mesodermal spaces 

 (subarachnoid space and anterior chamber of the eye). Thus, in these situations, 

 the characteristic fluids have certain definite channels through rather larger spaces, 

 connected finally with the venous system, and only indirectly with the lymphatic 

 system. 



In no sense must the cerebro-spinal circulation be taken as a portion of the 

 lymphatic system. Increasing knowledge of the cerebro-spinal fluid, of its physi- 

 ology and chemistry, and of its pathway, have separated it permanently from any 

 connection with the lymph of the lymphatic system, variable though that be. No 

 longer may the meningeal spaces be compared to serous cavities, except possibly 

 in the case of the subdural space, and this space is really a space apart from the 

 true cerebro-spinal or subarachnoid spaces. Quite similarly, hi place of the many 

 varying channels in the dura and to a lesser extent in the leptomeninges, which 

 older writers considered lymphatic in nature, our increasing knowledge has caused 



