92 BIOLOGICAL PHYSICS 



At the stage of embryonic development, when the neuro- 

 enteric canal has just been formed, we may infer that its 

 structural continuity warrants the assumption that there is 

 also functional continuity, and that there are, therefore, 

 common inlets and common outlets to and from the 

 common canal. That being so, we are warranted further 

 in inferring that with the developmental differentiation 

 which afterwards takes place at the posterior inferior aspect 

 of the canal, the common structural and functional attri- 

 butes are, to some extent, maintained, or, at least, that the 

 separation of the canal into two parts is not structurally 

 absolute, and therefore that there still continues a sympathy 

 between the two and a physical union sufficient to maintain 

 a modified circulation : in fact, just such a connection as can 

 keep open a channel, or channels, of escape for the super- 

 abundant fluid contents of the cerebro-spinal cavity at its 

 posterior extremity. In the process of developmental 

 separation of the neurenteric canal into two divisions and 

 their reunion anteriorly, the differentiating and uniting or 

 metamorphic structures laid down between them are left 

 perforated and patent patent, that is to say, on the prin- 

 ciple of the safety-valve. 



These perforated areas, or sections, of the cerebro-spinal, 

 meningeal, and ventral structures must, therefore, be 

 looked for where the neural and ventral canals were origi- 

 nally united, and where they were likewise originally 

 separated, and there we find them anteriorly in the nasal 

 fossae, the pharynx, and the hypophysial mechanism, with 

 its connected glosso-pharyngeal excretory structures, the 

 tonsils, lateral and pharyngeal, the uvula, and the tongue ; 

 and posteriorly in the peri- and endo-anal textures. 



Besides these three special outlets, we must here express 

 it as our deliberate opinion that every nerve, cephalic and 

 spinal, which leaves the cerebro-spinal cavity and of course 

 they all do takes with it at least two surrounding open 

 spaces, the continuations of the sub-arachnoid and sub-dural 

 spaces (Fig. 24), which afford an additional and, in fact, 

 continuous chain of openings by which, as occasion requires, 

 the cerebro-spinal fluid can be run off. We wish it, thus, 

 to be understood that we believe in the continuity of the 

 cerebro-spinal meninges and the inner layers of the neuri- 



