1G8 SPECIAL HISTOLOGY. 



selves. These seem to occur, however, almost solely in the lower and 

 posterior parts of the nail, for if, as Schwann states (Fig. 91), two points 

 be marked upon the posterior portion of the free surface of the nail, 

 one behind the other, by piercing it with a needle and coloring with 

 nitrate of silver, they in no wise alter their relative position in the course 

 of the two or three months, during which they are moving towards the 

 point of the nail. 



As to the patJiological conditions of the nails, they are readily regene- 

 rated when they have been detached, in consequence of crushing, burn- 

 ing, freezing, cutaneous disorders (scarlet fever, &c.), injBammations, 

 exudations, suppurations, and effusions of blood in the bed of the nail; 

 in fact, as Pechlin (" Obs. Phys. Med.," p. 315) narrates, this regene- 

 ration may take place periodically ; in a boy, the nails, every autumn 

 became bluish-black and desquamated, together with the epidermis (the 

 horny layer ?), and were subsequently regenerated. In such a case, accord- 

 ing to Lauth (" M^m. sur divers points d'Anatomie," in the " Annales de 

 la Societe d'Histoire Naturelle de Strasbourg," t. i. ISSi), and Hyrtl 

 (" Anatomic," p. 882), the whole bed of the nail becomes covered by soft 

 horny plates, which harden by degrees, grow into a regular nail, and 

 eventually project with their free edges beyond the end of the finger. 

 When the last joint of the finger has been lost, rudimentary nails fre- 

 quently appear upon the back of the second and even of the first phalanx. 

 The older cases are quoted in Pauli ("De vulneribus sanandis," Got- 

 tingjie, 1825, p. 98), more recently Hyrtl (1. c.) saw such a nail 2 lines 

 long and 3 lines broad on the first phalanx of the thumb. As the for- 

 mation of nail-substance depends upon the vessels of the bed of the nail, 

 "we may, with Ilenle, assume that varying conditions of the latter 

 may frequently produce local thickening, thinning, or even detachment 

 of the nail, and that their deformities in cyanosis and phthisis depend 

 on these causes. The thickening and abnormal development of the 

 nails, however, arise very frequently, as I have observed, from a partial 

 obstruction of the capillaries of their bed. Thus in the lamellated nails 

 of old people, greatly thickened and curved downwards in front, I find 

 all the capillaries of the anterior segment of the bed of the nail closely 

 filled with fat granules of different sizes, and wholly impermeable to the 

 blood; in such a case the development of nail-substance can take place 

 only in small lamellre in the fold, which then, as may be readily under- 

 stood, are raised up by those which are growing behind into a continu- 

 ally 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, Steinrlick (" 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, 1. c), after 



