616 ANATOMY OF VERTEBRATES. 



existing in the islands of the Malayan Archipelago are the oldest 

 inhabitants of such — those most entitled to be termed aborigines. 

 The Hindoos, by the same pigmental test, would be deemed older 

 than the Parsee or Mahometan natives of Hindostan, as history, 

 indeed, testifies. In extra-tropical latitudes, human generations 

 may have succeeded each other for the same duration of time as 

 in tropical ones, without further deepening or development of 

 pigment than such diminishing influence of the sun may effect. 

 Such peoples, migrating to tropical countries, may long maintain 

 their inherited complexions ; just as the black races migrating to 

 extra-tropical latitudes long retain the tint inherited from fore- 

 fathers in whom it has been established primarily by the requisite 

 continuance of exposure to extreme solar heat and light. 



§ 359. Callosities. — The epiderm, besides forming the firm and 

 more or less insensible outer protection of the derm, acquires un- 

 usual thickness at certain parts in different mammals. It forms 

 callosities over the sternum of the Camel and Dromedary, and 

 upon the parts of the joints (carpal and rotular) on which these 

 useful beasts of burden kneel. It defends the broad back of the 

 penultimate phalanges of the fingers of the knuckle- walking 

 Apes, the ischial tuberosities of most lower Catarhines, and the 

 prehensile surface of the tail in many Platyrhines. 



In the Equidce there are callosities on the inner surface of the 

 limbs, which, however, are more dermal than epidermal. In the 

 Horse, on the inner side of the fore-leg, a little above the carpus 

 ('fore-knee' Hippotomy), and on the inner side of the hind-leg, 

 a little below the ' tarsus ' (hock-joint, Hippotomy), is a naked 

 protuberance of a soft horny consistence, about the size of a 

 chestnut, and called ' chataigne ' by the French veterinarians. 

 Behind the metacarpophalangeal joint is a similar but smaller 

 horny tubercle, called the ' ergot,' or spur. The Ass has not the 

 ( chataigne ' on the hind-leg ; but there is the vestige of one on 

 the fore-leg, situated there as in the Horse; it consists of a 

 patch of black skin devoid of hair, but not horny. There is a 

 similar trace of the spur (ergot) behind the metacarpo- and 

 metatarso-phalangeal joints. The Zebra resembles the Ass in 

 these respects : the homologue of the fore-leg callosity is a 

 patch of black naked skin about 3J inches long and 3 inches 

 broad ; the callosities behind the metacarpo-tarso-phalangeal joints 

 are like those of the Ass. 



§ 360. Hair. — The cutaneous clothing characteristic of the 

 Mammalian class is 'hair.' It consists of unbranched filaments 

 of epidermal material, usually composed of ( pith ' and ' crust,' 



