244 ELEMENTARY ANATOMY. [LESS. 



A feather is formed by means of the peculiar structure of 

 the dermal papilla which gives rise to it. That papilla 

 (instead of being smooth, like the papilla of a hair) has on 

 one side a deep vertical groove, broadest at the base and 

 vanishing towards the apex of the papilla. Other less deep 

 grooves go from each side of this vertical groove at right 

 angles to it, and at very short distances from one another. 

 They extend all but round the papilla, vanishing at the 

 middle of the opposite side to that which bears the vertical 

 groove. Grooves smaller still and much shorter are given 

 off again at right angles to the second set of grooves, parallel 

 therefore as far as they go to the main and vertical groove. 



Now, as horny matter is deposited on the papilla, it be- 

 comes thickest where the grooves are deepest, and of course 

 thinnest where there are no grooves at all, i.e. in the inter- 

 spaces of the grooves. With the progress of growth, this 

 whole horny investment splits up along these interspaces 

 of thinnest deposit. The part which was in the main ver- 

 tical groove is thickest of all, and becomes the shaft of the 

 feather, the parts in the secondary grooves become the 

 " barbs," and the still smaller portions at right angles to the 

 latter the " barbules." When these last are long and hang 

 freely, they form the sort of structure we see in an ostrich 

 feather. Occasionally, as in the Cassowary, a feather will 

 have two shafts ; this is due to the papilla having borne a 

 vertical groove on each side. 



ii. The only form of epidermal appendage, besides hair, 

 which is found in man is the NAIL. 



The structure of this organ is also explained in the Twelfth 

 Lesson of " Elementary Physiology." It is not formed in a 



c 



FIG. 210. TRANSVERSE SECTION OF A NAIL. 



rt, small lateral folds of the integument ; b, nail ; c, bed of the nail, with its 

 ridges. 



bag (like a hair is), but only in a fold of skin (the root of the 

 nail), where horny matter is deposited upon a number of 

 minute, parallel, raised ridges of the deep layer or dermis. 



