OF THE NAILS. 169 



inflammation and closure of the fold of the nail, the formation of new 

 nail at the posterior edge ceases, the nail no longer growing forwards, 

 but at all its edges exactly covering its bed. 



52. The development of the nail begins in the third month with the 

 formation of the bed and fold, which are marked off from the surround- 

 ing parts by the gradual growth of the skin into the wall of the nail. 

 At first the bed of the nail is lined by the same cells as those which form 

 the other parts of the epidermis (see 47), only that even in the third 

 month the cells of the stratum Malpighii are distinguished by their 

 elongated and polygonal form (length 0-004, breadth 0-001-0-0016 of 

 a line). In the fourth month there arises between the stratum Mal- 

 pighii and the horny layer of the bed of the nail, which latter is formed 

 by a simple layer of polygonal clearly nucleated cells, a simple lamina 

 of pale, flat, but also quadrangular and nucleated cells, 0-009 of a line 

 in diameter, which are closely united together, and must be regarded as 

 the first indication of the proper substance of the nail ; at the same time 

 also, the stratum Malpighii under these cells become thickened so that 

 it is certainly composed of, at least, two layers. The nail is therefore 

 at first wholly included within the epidermis ; it is formed over the whole 

 surface of the bed of the nail as a quadrangular plate, and arises between 

 the embryonic mucous layer and the horny layer, without doubt by a 

 metamorphosis of the cells of the mucous layer, as is probable, especially 

 from the minute size of the original cells of the nails. In the course of 

 its further development, the nail is thickened by the addition of new 

 cells from below (in the fifth month its thickness is about 0-024, in 

 the sixth 0-04 of a line, of which in the latter 0-025 must be reckoned 

 as proper nail-substance); it increases by the extension of its elements, 

 and by the addition of new ones at its edges ; but it remains, even to 

 the end of the fifth month, hidden under the horny layer of the epider- 

 mis, until finally it becomes free, and in the seventh month, even begins 

 to grow longitudinally, so that at this period, except in its greater 

 softness and its smaller dimensions, it presents no essential difference 

 from the perfect nail. With regard to the bed of the nail, its ridges 

 are already indicated at the end of the fourth month, and in the fifth 

 they are well-marked, 0-02-0-024 of a line deep, 0-004-0-005 of a 

 line broad, and 0-008-0-014 of a line distant; these measurements also 

 indicate the breadth of the laminae of the stratum Malpighii. At the 

 sixth month they are somewhat larger and further apart. 



In the new-born infant, the whole nail is 0-3-0-34 of a line thick ; 

 0-16 of a line being proper nail-substance, 0-14-0-18 of a line stratum 

 Malpighii. Its elements are still almost identical with those at the sixth 

 month, and they appear with tolerable distinctness in the nail proper 

 without any reagents, as elongated polygonal nucleated plates 0-002-0-28 

 of a line, as Schwann has already partly remarked. The free edge, 



