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MANUAL OF HISTOLOGY. 



sharply defined one against the other by a curved line, which is visible 

 through the nail as the edge of the so-called lunula. 



Now, as we have already remarked, the rete Malpigliii dips down with 

 jagged projections into the intervals between the ridges of the cutis; it 

 conducts itself consequently just as at any other part of the skin (fig. 

 154, d). The young cells of which it is composed correspond also in 

 their histological constitution with those of the external skin (fig. 156,/). 

 In size they range between 0*0090 and 0*0160 mm., and their nuclei 

 between 0*0065 and 0-0075 mrn. The only difference appears to be, that 

 in 'the deepest layers the cells of the younger lamime are apparently 

 more or less oval. According to C. Krause, the nuclei of such nail cells 

 contain, in negroes, the same dark brown pigment as the skin itself 



( 90) ; a fact of much interest. Cells 

 with a double nucleus are not unfre- 

 quently met with here also (g). That 

 the rete Malpigliii of the nail is conti- 

 nuous with the younger cells of the 

 epidermis in the furrow, and at the 

 point of the finger, hardly requires to 

 be mentioned, and may be seen in fig. 

 154, c, and 155, b. 



Now, while the cells of the deeper 

 layers have but little about them that 

 is striking, the reverse is the case with 

 those of the superficial laminse or true 

 horny substance of the nail. Generally 

 speaking, we have only to remember 

 that the under surface of the horny layer 

 clings, by means of slight indentations, 

 to the rete Malpicjliii (fig. 154, /), and that on the root of the nail it is con- 

 siderably thinner and softer than the free uncovered portion. Finally, 

 the epidermis of the skin passes forwards a certain distance on the surface 

 of the nail from the inferior fold (fig. 155, i\ while that of the tip of the 

 finger is lost under the free edge of the same (fig. 155, /). 



Sections of this horny substance give no clue to its texture with- 

 out further treatment ; for we have to deal with a brittle, hard, and 

 tolerably transparent mass, which appears, to a certain extent, split up 

 and torn by the edge of the knife. If we subject such a section, how- 

 ever, to the action of sulphuric acid, or, still better, to that of caustic 

 soda or potash, the whole of it swells up in a very remarkable manner 

 (especially when warmed) into the most beautiful epithelial tissue (fig. 

 156, a-e). At first the cells are marked off one against the other, as flat- 

 tened polyhedra (d) ; but eventually they separate from one another, 

 under the continued action of the reagent. Their size is usually 

 0-0375-0-0425 mm. 



But though they correspond so far with epidermis cells, the elements of 

 nail-tissue possess one distinguishing characteristic (if the chemical action 

 of the reagents have not gone too far), in the form of a rounded granular 

 nucleus, a delicate lenticular structure, seen in fig. 156, &, c, d, e, from 

 above, as compared with the side view at a. Its diameter lies between 

 0-007o and 0'0090 mm. 



Fig. 156. Tissues of the human nail, mostly 

 after treatment with caustic soda, a, cells 

 of the superficial layer in profile ; 6, one seen 

 from above ; c, halt profile ; d, a number of 

 cells, of polyhedral outline, in contact with 

 one another; e, a cell whose nucleus is about 

 to disappear; /, cells from the undermost 

 part of the rete Malpighii; g, one of the same 

 kind, with double nucleus. 



