HAIR IO29 



A like structure is found to exist in the ' spiny rays ' of the dorsal fin, 

 which, also, are parts of the dermal skeleton ; and these rays usually 

 have a central cavity filled with medulla, from which the tubuli 

 radiate towards the circumference. This structure is very well seen 

 in thin sections of the fossil * spiny, rays.' which, with the teeth and 

 scales, are often the sole relics of the vast multitudes of sharks that 

 must have swarmed in the ancient seas, their cartilaginous internal 

 skeletons having entirely decayed aAvay. In making sections of bony 

 scales, spiny rays, &c. the method must be followed which has been 

 already detailed under the head of bone. 1 



The scales of reptiles, the feathers of birds, and the hairs, hoofs, 

 jiails, claws, and horns (when not bony) of 'mammals are all epi- 

 dermic appendages ; that is, they are produced upon the surface, not 

 within the substance of the true skin, and are allied in structure to 

 the epidermis, being essentially composed of aggregations of cells 

 filled with horny matter and frequently much altered in form. This 

 structure may generally be made out in horns, nails, &c. with little 

 difficulty by treating thin sections of them with a dilute solution of 

 soda, which after a short time causes the cells that had been 

 flattened into scales to resume their globular form. The most 

 interesting modifications of this structure are presented to us in 

 hairs and in feathers ; which forms of clothing are very similar to 

 each other in their essential nature, and are developed in the same 

 manner viz. by an increased production of epidermic cells at the 

 bottom of a flask-shaped follicle, which is formed in the substance 

 of the true skin, and which is supplied with abundance of blood 

 by a special distribution of vessels to its walls. "When a hair is 

 pulled out ' by its root,' its base exhibits a bulbous enlargement, 

 of which the exterior is tolerably firm, whilst its interior is occu- 

 pied by a softer substance, which is known as the i pulp ; ' and it 

 is to the continual augmentation of this pulp in the deeper part 

 of the follicle, and to its conversion into the peculiar substance of 

 the hair when it has been pushed upwards to its narrow neck, that 

 the growth of the hair is due. The same is true of feathers, the stems 

 of which are but hairs on a larger scale ; for the l quill ' is the part 

 contained within the follicle answering to the * bulb ' of the hair ; 

 and whilst the outer part of this is converted into the peculiarly solid 

 horny substance forming the ' barrel ' of the quill, its interior is 

 occupied, during the whole period of the growth of the feather, with 

 the soft pulp, only the shrivelled remains of which, however, are 

 found within it after the quill has ceased to grow. 



Although the hairs of different mammals differ greatly in the 

 appearances they present, we may generally distinguish in them 

 two elementary parts viz. a cortical or investing substance, of a 

 dense horny texture, and a medullary or pith -like substance, usually 

 of a much softer texture, occupying the interior. The former can 



1 For further information regarding the scales of fishes, see the papers by O 

 Hertwig in vol. viii. of the Jenaische Zeitschrift, and vols. ii. and v. of the 

 Morpholog. Jahrbuch. A condensed summary of our knowledge, from the more 

 recent standpoint, will be found in Dean's Fishes, Living and Fossil (New York, 

 1895), pp. 23-6. 



