146 MANUAL OF HISTOLOGY. 



With those mammals in which the choroidea forms a tapetum, the 

 epithelial cells of the same undergo an interesting modification, being 

 here destitute of the pigment molecules of the contents. On the bound- 

 aries may be found certain intermediate forms, with very scanty colouring 

 matter (fig. 139, cd}', besides which some isolated black cells are encoun- 

 tered among the colourless ones of the tapetum. In albinos, where the 

 pigment fails completely in the eye, all these cells with which we are now 

 engaged are completely bleached, appearing in the form of a very delicate 

 pavement epithelium. This interesting fact may be verified on any white 

 rabbit. The more markedly laminated epithelia have no pigment cells in 

 man, but such may make their appearance in other mammals, as, for 

 instance, in the conjunctivas of the horse (Bruch). 



KEMARKS. 1. This layer of cells belongs, however, as we learn from the history 

 of development, not to the Uvea, but to the Retina. 



90. 



The region in which pavement epithelium is most strongly laminated, 

 though indeed with certain modifications, is the external surface of the 

 body. 



The surface of the cutis, which appears quite smooth to the unaided 

 eye, is covered, nevertheless, by a number of minute prominences known 

 as the papillae tactic (fig. 140, a, a, a). These, together with the depres- 

 sions between them, are covered with very numerous layers of cells lying 

 one over another (b c d). Of course, the latter naturally possess a far 

 greater depth in the intervals between the papillae than on the apices of 

 the latter, in that the surface of all the strata collectively, or the epidermis, 

 is tolerably even. 



But apart from, these inequalities, produced by the ridges of the cutis, 

 the thickness of the whole clothing of cells is very different .in the various 

 parts of the body. It may range from 0*04-3 mm. and upwards, the 

 more superficial layers of flattened cells being subject to the greatest 

 change, the deeper, smaller, and rounder, to least of all (C. Krause). 

 The unequal pressure which the various portions of the skin experience, 

 differences of occupation, and consequent use of certain parts of the body, 

 especially of the hands and feet, account, at least in a great measure, 

 for this. And yet it has long been known that the epidermis on the 

 sole of the foot,, even in .the foetus, is much thicker than that of any 

 other region of the body. 



The cuticle of human beings and other mammalia may be divided into 

 two groups of strata, into a superficial and a deep, which are continuous 

 with one another, at one time gradually, at another with a tolerably sharp 

 line of demarcation. The first (d) is usually termed the epidermis in the 

 more precise meaning of the word, while the second has received the 

 name of the Malpigliian layer, or rete mucosum (b, c). By a certain 

 amount of maceration, these may be separated from one another. 

 From the fact that the deeper strata fill up the intervals between the 

 papillas, they must naturally possess here quite a different depth from that 

 the points of the latter, as already mentioned. Hence their appearance 

 is rendered more or less sieve-like or reticulated, which has given rise to 

 the name generally employed by the older anatomists. 



In these deepest layers we encounter not free nuclei, but small cells of 

 about 0-0075-0-0090 mm. in size, of roundish or oval form, in which case 



