XIII 



PHYLUM CHORDATA 



461 



jSc 



8M- 



Co 



rod, and is composed of two parts, a central part or pith (M) con- 

 taining air, and an outer more solid part or cortex (R) in which 

 air does not occur. Com- 

 monly the cortical part 

 presents transverse ridges 

 so as to appear scaly. In 

 one case only, viz., Sloths, 

 is the hair fluted longitu- 

 dinally. The presence of 

 processes on the surface, 

 by which the hairs when 

 twisted together interlock 

 firmly, gives a special 

 quality to certain kinds 

 of hair wool used for __ 

 clothing, the felting qual- 

 ity as-.it is termed. A hair 

 is usually cylindrical ; but 

 there are many excep- 

 tions : in some it is com- 

 pressed at the extremity, 

 in others it is compressed 

 throughout ; the latter 

 condition is observable in 

 the hair of negroid races 

 of men. The fur is usually 

 composed entirely of one 

 kind of hair ; but in some 

 cases there are two kinds, 

 the hairs of the one sort 

 very numerous and form- 

 ing the soft fur, and those 

 of the other consisting of 

 longer and coarser hairs 

 scattered over the surface. 

 An example of a hairy 

 covering of this kind is 

 seen in the case of the 

 Platypus and the Fur 

 Seals. 



A hair, like a feather, 

 is formed from the epi- 

 dermis. The first rudi- 

 ment of a developing 



hair (Fig. 1039) usually takes the form of a slight downwardly 

 projecting outgrowth, the hair ge**m (grm.), from the lower mucous 

 layer of the epidermis, beneath which there is soon discernible 



FIG. 1038. Longitudinal section through a hair (dia- 

 grammatic). Ap, band of muscular fibres inserted 

 into the hair follicle ; Co. corium (dermis) ; F. ex- 

 ternal longitudinal ; F'. internal circular fibrous layer 

 of follicle ; Ft, fatty tissue in the dermis ; GH, hyaline 

 membrane between the root-sheath and the follicle; 

 HBD, sebaceous gland ; HP. hair papilla with vessels 

 in its interior ; M. medullary substance (pith) of the 

 hair ; R, cortical layer ; Sc, homy layer of epidermis ; 

 SM, Malpighian'-layer of epidermis ; WS, WS', outer 

 and inner layers of root-sheath. (From Wiedersheim's 

 Comparative Anatomy.) 



