ON THE STRUCTURE OF HYALINE CARTILAGE. 15 



termediate portions are represented in Fig. 13, but the 

 transition between the two is more abrupt in the plate than 

 it is in the preparations. 



An attempt will now be made to construct a theory of the 

 structure of hyaline cartilage by which the above data will 

 receive a snfhcient explanation. 



In regard to the appearances described, as seen by treat- 

 ment with potash and by maceration in sealed aqueous 

 humour or serum, the facts being once admitted, there can 

 be little difference of opinion as to their interpretation. With 

 regard to the nitrate of silver appearances, there may not be 

 so much unanimity. It may, however, be taken for granted, 

 without further discussion, that the dark lines represented in 

 Fig. 6 indicate the dark lines of epithelial (endothelial) cells. 

 In regard to the uncoloured spaces rej)resented in Figs. 

 10, 12, and 14, there is room for discussion. Fortunately 

 the same appearances are found in silver preparations of 

 tissues, in regard to whose structure more definite informa- 

 tion is obtainable. For example, let the eye of a frog be 

 placed entire in a half-per-cent. solution of nitrate of silver, 

 after a minute or two let the anterior corneal epithelium be 

 removed, and the eye replaced in the solution for some 

 minutes longer, and then finally removed. The cornea is 

 then to be excised, placed for a second in half-per-cent. salt 

 solution, and then exposed to the light in glycerine. After 

 it has become brown let the upper layer of the cornea be 

 removed at some parts under a dissecting-lens. The following 

 fact is then frequently to be made out : — the silver has diffused 

 itself equally through the whole thickness of the more super- 

 ficial layer, but has not penetrated to the deeper layer. 



There has been evidently an obstacle to the penetration 

 of the solution from the upper to the subjacent layer. The 

 nature of the part or whole of the structure which constitutes 

 this boundary or obstacle may be learned by a rare chance in 

 the frog's cornea itself, but may be more easily shown in the 

 cornea of the mouse. The mouse^s cornea is peculiarly well 

 adapted for the demonstration of many points in regard to 

 the histology of that structure. If a mouse's cornea be ex- 

 cised by a single stroke of the scissors and placed in silver 

 solution, and be thence transferred to the light in glycerine, 

 the epithelium of Schweigger-Seidel, of the existence of 

 which it has taken so long to convince some histologists, may 

 be demonstrated with comparative ease. 



It is not meant that success in its demonstration is inva- 

 riable in the cornea of the mouse, but it succeeds in that animal 

 as frequently as many of the ordinary histological processes 



