THE KERATINIZED TISSUES 73 



of the free margin, affect the external appearance and are features relied 

 on in identification. The free edges, projecting in a direction away from 

 the skin, make the fibre feel rougher when rubbed towards the skin. This 

 " directional friction " assists in keeping clean and in the grooming of the 

 hairy covering and in freeing it from tangles. However, when the hair is 

 cut from the skin the same property promotes tangling since each individual 

 hair then tends to creep persistently in a rootward direction when the 

 fibre mass is disturbed (Speakman and Stott, 1931). This tendency is 

 made use of in manufacturing felts from wool and fur, but is also the cause 

 of the shrinkage of woollens during washing. 



The bulk of the keratinized feather also consists of long, spindle-shaped 

 cells very similar to the cortical cells of hairs. The surface layers are 

 covered with flattened, polygonal cells (Auber, 1955) perhaps analogous to 

 the flattened, cuticular scales of hairs but, unlike the hair cuticle cells, con- 

 taining within them a lattice-like network of fibrils. 



The medulla is remarkably developed in the hair of certain animals, e.g. 

 rodents, where there seems to have been a considerable pressure in the 

 direction of producing a lighter, more bulky and stiffer hair for a given 

 weight of material. Medullary cells are often large and their fibrous 

 contents are concentrated peripherally against the cell wall producing a 

 cavity largely air-filled when the hair is dry. The mechanical problem 

 here (as with feathers too) is similar to that met with in constructional 

 engineering: to obtain maximum stiffness for a given expenditure of 

 material, and in fact many medullated hairs are reminiscent of girders. 

 In such cases, the pattern is genetically determined and is often of use in 

 identifying the hair. 



In other types of hair only a feebler disposition towards medullary 

 formation is inherited; its actual manifestation depends upon the sizes 

 of the papilla and follicle and the nutrition of the growing hair. This is the 

 case among sheep where it assumes some economic importance. Rudall's 

 (1956) extensive survey of sheep follicles showed that the papillary 

 dimensions and shape control the appearance of the medulla (p. 150). 



The Phylogeny of Hair 



During the early heroic days of the application of the theory of evolution 

 to comparative anatomy, the phylogeny of so distinctive a mammalian 

 character as hair naturally attracted much attention and several theories 

 were advanced. That hairs are homologous at least remotely to feathers 

 and scales is obvious enough. Whereas the likeness between the feather 

 and the scale is close and is supported by their embryology, hairs are 

 sufficiently different to have led paleontologists to suppose that the actual 

 forerunner of the hair may have been some other organ of epidermal 

 origin. Some have found the precursor in teeth, others in cutaneous 



