58 MANUAL OF HISTOLOGY. 



take off a thin slice with a knife, immerse for a few seconds in 

 a dilute solution of acetic acid (J per cent.), and then mount in 

 glycerine and water ; or a similar portion of the skin may be 

 steeped in a weak, sherry-colored, watery solution of the bichro- 

 mate of potassium (gr. ij. iij. f. j.) for several days and then 

 hardened in alcohol, first of 80 per cent., then of 90 per cent., 

 finally of 95 per cent, strength ; this latter process taking several 

 days, and ending when the specimen is thoroughly hard. Sec- 

 tions may then be made in the usual way. By the use of acetic 

 acid the nuclei will readily be seen in the lower strata of the epi- 

 dermis, while the outermost layers have none, or, at least, none 

 that can be demonstrated by the usual histological methods. 



Three different strata can now be recognized : 1, the stra- 

 tum corneum, or corneous layer, in which the corpuscles are 

 flattened, and appear to have no nuclei ; 2, the rete mucosum, 

 or malpighian layer, immediately underlying the former, and 

 composed of cuboidal elements, armed with spines or prickles, 

 as they are often called ; lastly, 3, there is the pigmented layer, 

 which overlies the papillae. The bodies of the latter corpuscles 

 are infiltrated with particles of melanine, which is the cause of 

 the dark color in the skin of the negro and swarthy races. 



Maceration of the epidermis in liquor potassse is an excellent 

 method for exhibiting the individual elements ; after a few min- 

 utes they will swell up and detach themselves from one another. 



It was thought, until quite recently, that these prickle cells interdigitate 

 with one another, but Ranvier has claimed that they are continuous with those 

 of adjacent corpuscles (see chapter on the Skin). This point is difficult to set- 

 tle, as it requires a special method and lenses of high power. Ranvier injected 

 a one-fourth per cent, solution of osmic acid into the lower layers of the epider- 

 mis, using a hypodermic syringe, and driving the fluid right and left. 



There is a form of flattened and pigmented epithelium that 

 may be seen by examining the external surface of the choroid, 

 the ciliary processes, and the posterior surface of the iris. In 

 the choroid these bodies look like a mosaic of polyhedral cells. 

 Such specimens may be permanently preserved by simply dry- 

 ing them, and then mounting in dammar or Canada balsam. 



Ciliated epithelium. The movement of living cilia is readily 

 seen. All that is necessary is to take the common frog (Rana 

 temporaria), draw out his tongue, and then observing the teat- 

 like projections at the posterior part, snip one off. 



