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SCIENCE 



[N. S. Vol. XLIV. No. 1127 



of lymphatic capillaries as definite vessels 

 completely lined by endothelium, and re- 

 lated to tissue-spaces just as blood-capil- 

 laries are, it will be necessary to emphasize 

 first the importance of tissue-spaces. In- 

 deed, the general subject of tissue-spaces, 

 as important systems in the body, related 

 to blood-capillaries and to lymphatic capil- 

 laries in function, is, I believe, nowhere 

 sufficiently emphasized in the literature. 



It is well known that the plasma of the 

 blood is constantly exuded from the blood- 

 vessels into the tissue-spaces, so that all the 

 cells of the supporting tissues, as well as 

 the special cells of each organ, are bathed 

 in fluid. Moreover, it is obvious that with 

 all the varying activities of the cells of the 

 body, the fluid becomes laden with different 

 nutritive and with different stimulative 

 substances and with different waste prod- 

 ucts, so that it varies widely in its compo- 

 sition. The subject of tissue-spaces — mean- 

 ing not empty spaces, but spaces which al- 

 ways contain fluid — is by no means a simple 

 one. There are primarily the general, 

 small spaces to which I have just referred, 

 between all of the fibers and cells of the 

 connective tissues and between the pa- 

 renchyma of each organ and its supporting 

 tissues: but there are also special systems 

 of great spaces, which arise from the small 

 spaces by a definite method, which have a 

 definite structure and contain a fluid which 

 is different from the other fluids in the 

 body — such, for example, as the suba- 

 rachnoid spaces which surround the central 

 nervous system. 



That the cerebro-spinal fluid is secreted 

 by a special organ and contains certain 

 products of internal secretion is now 

 known. The pia-arachnoid membrane has 

 been shown by Weed 2 to have an extremely 

 interesting structure and development. I 

 will mention here only the very important 



- Weed, L. H., Jour, of Med. Research, Vol. 31, 

 1914. 



arachnoidal villi which are lacelike projec- 

 tions of the arachnoid into the dura. They 

 lie along the dural veins and lead to the 

 dural sinuses. These villi, which he has 

 shown to be the main organs of absorption 

 for the cerebro-spinal fluid, are covered 

 with a layer of mesothelial cells, which 

 tend to become more abundant at the tips, 

 forming cell nests. 



Other great systems of spaces are found 

 in the internal ear and in the eye. The 

 scala tympani and scala vestibuli of the 

 cochlea have been called peri-lymphatic 

 spaces, though they have no relation to the 

 lymphatic system. These spaces of the ear 

 have just been shown by Streeter 3 to have 

 a most interesting development. The scala 

 tympani and scala vestibuli are formed 

 from spaces in the mesenchyme which at 

 first become slightly larger than the usual 

 spaces and then coalesce into still larger 

 spaces. Moreover, this process is not indefi- 

 nite, but has two distinct places of origin, 

 one between the sacule and the oval win- 

 dow and the other between the cochlea and 

 the round window. From these two areas 

 the formation of the two great spaces of the 

 cochlea proceeds in a definite and constant 

 direction, so that a model of their form 

 from one specimen is the same as that from 

 any other specimen of the same stage. 

 Moreover, when studied in sections this 

 process appears to be a gradual dilatation 

 of preexisting tissue-spaces, with a disap- 

 pearance of more and more of the original 

 connective tissue syncytium, rather than 

 being caused by a differentiation of the 

 mesenchyme cells forming the border of 

 these spaces. As the cavity thus formed 

 reaches its ultimate dimensions some of the 

 remaining mesenchyme cells do differen- 

 tiate to form a mesothelial lining. I em- 

 phasize this method of the formation of a 

 cavity out of mesenchymal spaces, for the 



3 Streeter, 6. L., to be published in the ' ' Proc. 

 of the Amer. Asso. of Anat., " 1916. 



