PHYSIOLOGICAL ANATOMY OF THE NAILS. 



347 



the use of reagents. If the different strata of this portion of the nail be 

 studied after boiling in a dilute solution of sodium or potassium hydrate, 

 it becomes evident that here, as 

 in the horny layer of the epider- 

 mis, the lower cells are rounded, 

 while those nearer the surface 

 are flattened. These cells are 

 nearly all nucleated. The thick- 

 ness of this layer varies in differ- 

 ent portions of the nail, while 

 that of the Malpighian layer is 

 nearly uniform. This layer is 

 constantly growing, and it con- 

 stitutes the entire substance of 

 the free borders of the nails. 



The connections of the nails 

 with the true skin resemble those 

 of the epidermis; but the rela- 

 tions of these structures to the 

 epidermis itself are somewhat 

 peculiar. Before the fourth 

 month of foetal life, the epider- 

 mis covering the dorsal surfaces 

 of the last phalanges of the fin- 

 gers and toes does not present 

 any marked peculiarities ; but at 

 about the fourth month, the pe- . 



A, section of the nail : 1, 1, superficial layer ; 2, deep layer ; 



Cllliar hard Cells of the homy 3 . 3, 4, 4, section of the grooves on the attached sur- 



face : 5, 5, union of the superficial with the deep layer ; 

 laver of the nails make their ap- 6 > <>< dark line between the two layers. 



* B. cells of the superficial layer, lateral view. 



pearance between the Malplglli- C, cells of the superficial layer, flat view. 



D, cells of the deep layer. 



an and the horny layer 01 the 



epidermis, and at the same time the Malpighian layer beneath this plate, 

 which is destined to become the Malpighian layer of the nails, is thickened 

 and the cells assume a more elongated form. The horny layer of the nails 

 constantly thickens from this time ; but until the end of the fifth month, 

 it is covered by the horny layer of the epidermis. After the fifth month, 

 the epidermis breaks away and disappears from the surface; and at the 

 seventh month, the nails begin to increase in length. Thus, at one time, 

 the nails are actually included between the two layers of the epidermis ; but 

 after they have become developed, they are simply covered at their roots by a 

 narrow border of the horny layer. The nails are therefore to be regarded as 

 modifications of the horny layer of the epidermis, possessing certain anatom- 

 ical and chemical peculiarities. The Malpighian layer of the nails is con- 

 tinuous with the same layer of the epidermis, but the horny layers are dis- 

 tinct. 



One of the most striking peculiarities of the nails is their mode of 



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