KERATOSES. 36a 



less accentuated curves. It is clear that the process was origin- 

 ally confined to a smaller group of cutaneous papillae, and 

 spread, at first gradually, then with progressively increasing 

 rapidity, to suiTounding parts. 



If we examine the under surface of one of these horns after 

 its removal, we not unfrequently observe a number of small 

 conical projections ; these were recognised by Virchoio to be- 

 homy casts of hair-follicles. So that here also, as in ichthyosis, 

 the morbid process may extend to the hair-sacs. The horny 

 matter produced by the hair-sacs can never indeed contribute to 

 raise the level of the surface ; so that it would be absurd to talk 

 of the cornu humanum originating from the hair- follicles. Their 

 implication in the process, however, induces a thickening of the 

 horn at its base ; and as this is lodged in tlie very substance of 

 the cutis, it raises the notion that the horn springs from a 

 saccular depression — a dilated follicle — of the skin. This ex- 

 plains much of what has been put forth concernnig the follicular 

 origin of cutaneous horns. The necessary basis for the growtli 

 of a horn is always a group of papilla3 ; and when we find horns 

 springing from the fundus of a sebaceous cyst we are tempted to 

 inquire, first, whether the cyst was really prior in point of time, 

 and secondly, whether these horns also do not spring from a 

 basis of proliferated papilla?. I have often -cQqw vast numbers of 

 small, pointed papilla? on the inner surface of sebaceous cysts, 

 whose grovrth must obviously have been secondary to the forma- 

 tion of the cyst itself. 



§ 306. Appendix. — The appendages of the horny layer of 

 the cuticle — the hair and the nails — are, under ordinary circum- 

 stances, the largest accumulations of horny matter in a compact 

 form, to be met with in the body. After they have once 

 assumed their proper shape, they are not liable to any im- 

 portant change. At the utmost they can only undergo atrophy, 

 i.e, they may split up and fall off prematurely : and even this 

 is a result, not of any disease proper to the hair or the nails, but 

 of some morbid condition of their matrix. The same may bo 

 said of the opposite condition of hypertroplu' of the hair and 

 the nails. Whatever points of interest these conditions may 

 present to the histologist, v>'ill be considered, as regards the hair, 

 in the chapter on the Hair-Sacs and Sebaceous Follicles ; — and 

 as regards the nails, at once. 



