72 KERATIN AND KERATINIZATION 



peculiarities of the life of their possessors. Only certain broad features 

 distinguishing the main types can be mentioned here. In the classification 

 based on various sources given by Danforth (1932) hairs are primarily 

 divided on the basis of the presence or absence of erectile tissue surround- 

 ing their follicles. In effect this division amounts to typing hairs according 

 to their sensory role. The follicles containing erectile tissue are richly 

 enervated (vibrissae, tactile hairs, sinus hairs, whiskers, etc., are common 

 names) and their function is primarily sensory, the possibility of erecting 

 them stiffly adding to their sensitivity. This could represent a more 

 primitive function. Hair follicles without erectile tissue are usually also 

 enervated but their role becomes more purely defensive or protective. 

 Commonly the stiff, longer hairs (guard hairs) are distinguished from an 

 undercoat of finer, softer, and often more curly or crimpy hair, whose 

 function largely is that of heat insulation. Usually the coarser hairs 

 appear ontogenetically earlier than the finer and their follicles are said to 

 be primary; later-formed follicles are termed secondary (Fig. 35). 



The much-studied human hair and sheep's wool are each exceptional 

 cases. The long human hair often called " terminal," a variety of guard hair, 

 is of limited distribution on the body which elsewhere is covered by a short 

 fine hair. Wool is largely made up of fine, crimpy secondary hairs and even 

 the primary hairs, although distinguishable, have also become fine and 

 crimped or curly. 



The hair grows from a follicle which is an invagination of the epidermis 

 deep into the dermis (Figs. 40 and 43). Embryonically this forms 

 immediately as a downgrowth from the epidermis and is, in this sense, 

 in contrast with the early steps in the formation of a scale or a feather 

 and constitutes a reason for regarding hairs as having a different phylogeny 

 from these (see p. 73). 



The fibrous properties of a hair reside in the cortex, a bundle of 

 longitudinally-aligned closely-adhering spindle shaped ( ~ IOOju, X 5 — 7/x) 

 keratinized cells (Lehmann, 1943, for illustrations). The keratinized 

 residues of these cells, when liberated from the fibre by enzymatic digestion 

 (p. 271) appear fibrous and are birefringent. They are similar, with 

 small differences in length, in all species. Woods (1938) showed that 

 cortical cells paralleled closely, in elastic behaviour, optical and diffraction 

 properties, the properties of the whole fibre. 



The cuticle is of a contrasted construction. It consists of thin ( ~ 1/x) 

 sheet-like overlapping cells forming a protective sheath to the cortex. 

 Whereas the cortex is similar in most hairs, the cuticle is highly variable 

 and its features are much used in fibre identification. The thickness 

 varies owing to the degree of overlapping of the cells, or scales — one to 

 two in sheep, up to twenty or more in some fur hairs (Rudall, 1941 ; 

 Stoves, 1947). The degree of overlap of free margin, and the shape 



