166 SPECIAL HISTOLOGY. 



tinually more and more oblique position ; their posterior 

 extremities forming, on the surface of the nail, transverse streaks 

 one behind another at short intervals. After dividing the 

 nervus ischiadicus, Steinriick ( f De nervorum regeneratione/ 

 pp. 45 — 49) observed, in the Rabbit, that the nails and hair 

 fell off, which is a result of the influence of the nerves upon 

 the vessels. Finally, the shape of the bed of the nail also in- 

 fluences its formation. It is thus explained, how (see Henle, 

 I. c), after 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 surrounding 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 epider- 

 mis (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— 00016'"). In 

 the fourth month there arises, between the stratum Malpighii 

 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 nucle- 

 ated cells, 0-009'" in diameter, which are closely united toge- 

 ther, and must be regarded as the first indication of the proper 

 substance of the nail ; at the same time also, the stratum Mal- 

 pighii under these cells becomes thickened so that it is certainly 

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

 tvholly 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 

 024.'", in the sixth 001", of which in the latter 0025'" must 



