HOUSING-SKIN AND ITS DERIVATIVES 369 



originally digital pads of four-footed ani- ranged as to offer the least resistance to for- 



mals. When mammals took to the trees, ward motion. 



the pads developed into transverse ridges Feathers resemble scales in their over- 

 which functioned in increasing the friction lapping arrangement, although otherwise 

 between the hand and the branch, thus pre- the likeness is not so obvious. They are 

 venting slipping. In man the ridges are gen- much lighter in construction, possessing 

 erally arranged in whorls, although they are numerous tiny filaments that offer resist- 

 transverse to the long axis of the fingers for ance to the passage of air through them, 

 the most part. The designs appear to be in- The wing feathers of birds will allow air to 

 finite in numbers and never seem to be pass one way but not the other— a beauti- 

 repeated on the tip of any digit, either on ful example of adaptation to flight, 

 the hand of one individual or on any other Teeth. Teeth are also epidermal out- 

 individual. This has provided a convenient growths, having a common origin with 

 means of identification because it positively scales (Fig. 14-4), particularly those of the 

 distinguishes one person from another, and shark (placoid). Since the mouth is lined 

 its primary use today is in criminal investi- with ectoderm (the germ layer that gives 

 gations. rise to the epidermis ) , we might expect that 



The actively growing layer of the epi- it (the mouth) could be equipped with any 



dermis (stratum germinativum ) produces structure that could come from ectoderm, 



many structures which on the one side are The scales in sharks enlarge and grow over 



sunk deep into the dermis and on the other the edge of the jaw, producing teeth. Hu- 



side are an important part of the external man teeth come likewise from ectoderm 



covering. The scales of fish and reptiles, and fit into cups or sockets provided for 



the feathers of birds, and the hairs of mam- them in the jaw. 



mals have such relationships (Fig. 14-1). Hair. These tiny projections from the 

 Although these all have similar origins, in skin of mammals perform a protective func- 

 the final adult stage they are quite different tion against both physical injury and heat 

 both in structure and function. Digital tips loss. The numerous hairs tend to provide a 

 in various vertebrates, such as the carnivore dead air space just above the skin which 

 claw, the ungulate hoof, and the primate prevents heat loss much like insulation ma- 

 nail, are likwise produced from this region terials in a house. Feathers also act as heat 

 of the epidermis (Fig. 14-2). They are all insulators, a fact readily observed on cold 

 homologous, since each has the same origin days when birds ruffle up their feathers to 

 but performs a different function. improve the insulating properties of their 



integument. On a hot day, a bird keeps its 



Derivatives of the epidermis feathers close to its body to allow as much 



Scales and feathers. Both of these outer heat to escape as possible, 



coverings have a common origin ( Fig. 14-3 ) Mammals, with the exception of the whale 



and they are much alike both anatomically and man, are covered with a thick coat 



and functionally. Scales are found princi- of hair. Man has lost most of his hair, prob- 



pally among the fishes and reptiles, although ably because he evolved in a warm climate, 



birds have them on their legs and some evi- Today it is present only in the pubic regions, 



dence of scales appears among the mam- under the arms, and on the face and head, 



mals (for example, tail of rat and beaver). The facial adornment is a male secondary 



Structurally, they resemble overlapping or sexual characteristic because it is not found 



abutting plates that offer considerable re- in the female. The rest of the body is usu- 



sistance to outside mechanical injury. When ally covered with very tiny hairs which are 



overlapping like shingles, they are so ar- vestigial, for they perform no function in 



