ITS HISTOLOGICAL RELATIONS. 55 



have already observed, a closer insight into the histological than 

 the chemical constituents of the horny tissues. 



The horny tissues become gradually loosened when treated with 

 cold or warm water. The epidermis is rendered so soft after pro- 

 longed soaking, as to admit of being easily broken down, and 

 separated into individual cells, or smaller accumulations of them. 

 The cells themselves are rendered more distinct; the extremely 

 thin, irregularly formed epidermic plates appear somewhat swollen 

 and faintly granulated. The nucleus, if one be present, becomes 

 more distinct. The cylindrical or round cells of the rete muco- 

 sum, which contain a nucleus, and resemble vesicles expanded 

 with fluid contents, are but slightly altered by the action of water. 



The nails are on the whole very similar to the epidermis ; how- 

 ever they only swell in water and become softer, without admitting 

 of being triturated. 



Horns and hoofs soften in water, especially on the application 

 of heat, and then commonly develope a little sulphuretted hydro- 

 gen. The cellular structure cannot easily be recognised under the 

 microscope, even after the tissue has lain for a long period in water ; 

 scarcely anything beyond fibres, which often appear to be torn, can 

 be detected. 



Water produces no visible alteration on whalebone or tortoise- 

 shell, whether it be applied hot or cold, and however long it may 

 be suffered to act upon either. 



The best reagents for exhibiting the cellular structure of all 

 these tissues are highly concentrated solutions of the caustic 

 alkalies ; in many cases caustic soda, as recommended by Kolliker, 

 is preferable to caustic potash. A dilute solution of potash or soda, 

 especially on the application of heat, acts, however, more rapidly 

 upon the epidermis, and exhibits its cellular structure far better 

 than the concentrated solutions. The concentrated solutions 

 render the epidermic plates roundish, pale, and smaller ; and it is 

 only after a prolonged action that they swell, and distinctly exhibit 

 their cellular form. Dilute alkalies convert the epidermic plates, 

 in a short time, into oval or spherical clear vesicles, without a 

 nucleus or granular contents. The cells of the rete mucosum 

 show still more distinctly that the nuclei of the epidermic cells are 

 sooner and more rapidly dissolved than their cell-membranes ; for 

 if the epidermic cells are exposed to the prolonged action of hot 

 solutions of caustic alkalies, the cell-walls become dissolved, so 

 that there only remains a small gritty and partially granular mass. 



A solution of potash acts upon the substance of the nails iiv 



