90 COMPARATIVE ANATOMY. 



Feathers are appendages to dermal papillae. The feather- 

 sac or follicle is an involution of the integument, having 

 at its base a modified tactile papilla. The derm constitutes 

 the fibrous wall of the follicle, the elements of the epidermis 

 comprising the root sheaths. These sheaths are two in 

 number, an outer and inner. The former is placed in con- 

 tact with the fibrous wall of follicle, the latter (first 'striated' 

 membrane) with the developing feather. A second 'striated' 

 membrane lines the shaft of the quill. The bulb is a collec- 

 tion of cells surrounding the papilla, from which are devel- 

 oped the constituent parts of the feather. 



Every feather is composed of an outer cortical and an 

 inner medullary portion. It is divided into regions, as fol- 

 lows: vane (shaft, 'quill'), primary branches (barbs), which 

 give off secondary branches (barbules) these in turn origin- 

 ating tertiary branches (barbulinae). In the majority of 

 feathers the proximal part of the vane has its barbs and bar- 

 bules long, loose, and floating, so as to form a compact downy 

 mantle next the skin of the bird, to retain the high tempera- 

 ture of the body. In the rest of the vane the barbs are more 

 firm, straight, regular, and united, to assist in flight, or to 

 protect the body. (Grant.) 



Mammalia. Organs of touch highly developed. The tac- 

 tile (Wagnerian) corpuscles upon terminal branches of nerves 

 to the hand of man furnish marked examples. 



Hairs are appendages to dermal papillae. The hair sac or 

 follicle closely resembles the feather follicle, but lacks the 

 lining membrane to shaft; the latter possesses an addi- 

 tional covering the cuticle. The cuticle on that portion 

 of the hair which lies within the sac is composed of two 

 layers. In man the outer layer ceases at the level of the 

 sebaceous glands; and the edge of the plates of the inner 

 layer lie very closely appressed to the shaft. In many of the 

 lower animals, however, as in some bats, the plates are at a 

 greater angle to the axis of the hair, and their projecting 

 edges give rise to the most elegant sculpturing of its surface. 

 (Huxley.) Although, wherever developed, hairs are consid- 

 ered as organs of special sense, it is particularly about the 



