vii.] THE EXTERNAL SKELETON. 243 



general characteristic of Vertebrates ; and even such aquatic 

 beasts as are destitute of hair (the Whales and Porpoises) 

 are yet endowed with a fatty layer beneath the skin which is 

 wanting in us. In Batrachians, however, we find a naked- 

 ness of skin greater even than in man. 



In that man is provided with hairy epidermal appendages 

 he agrees with all the members of his class except the 

 Whales and Porpoises ; but in the small amount and the 

 distribution of the hairy investment he stands alone. In 

 the forms nearest to him in structure, the Apes, hair is 

 always abundant on the back, though sometimes more or 

 less deficient on the ventral surface the very reverse con- 

 dition to that of man, in whom the back is always naked. 



Long hair on the head, as also whiskers and beard, are 

 characters variable in man and not peculiar to him, as some 

 Apes (e.g. Pithecia Satanas] have a luxuriant beard. 



The structure of hair, and how it is formed in a small bag 

 (the hair sac) by the deposition of horny matter upon a little 

 prominence (or papilla) of the dermis, which prominence rises 

 from the bottom of the bag, have been described at length in 

 the Twelfth Lesson of " Elementary Physiology." 



The difference in structure between the hairs of the differ- 

 ent parts of the body (as the beard, eyelashes, eyebrows, c.) 

 in man is but trifling. The contrast between the mane and 

 tail of a Horse and the rest of his coat is far greater ; as also 

 between the long whiskers or feelers of many animals (as the 

 Cat) and the rest of their fur. But the maximum of develop- 

 ment (as it exists in man's own class) is shown in such 

 creatures as the Hedgehog and the Porcupine, where hairs 

 become dense and solid spines. 



lo. Such structures as hairs are peculiar to warm-blooded 

 Vertebrates, but the warmest blooded class (that is, Birds) pre- 



FIG. 209. DIAGRAM OF A FEATHER PAPILLA, SEEN ON TWO OPPOSITE SIDES. 



i. Side on which is placed the deep groove in which the "shaft" is deposited 

 and from which the secondary grooves diverge at right angles to it. 



2. The opposite side, showing the median tract where the encircling grooves 

 vanish. 



sents a peculiar kind of greatly enlarged and complex hair 

 called FEATHERS found nowhere else in the animal kingdom. 



R 2 



