112 



Elementary Zoology. 



Integumental Structures. 



Skin structures are either purely epidermal, or partly 

 epidermal and partly dermal. 



Purely epidermal structures are feathers, hairs, claws, and 

 the scales of birds (on the feet) and reptiles. They are formed 

 by a modification of the cells of the epidermis only, the lower- 

 lying dermis not actually entering into their formation. 



The frog is totally devoid of any such structures, its smooth 

 skin producing no hairs, feathers, or claws. The rabbit, like 



Fig. 54. — Section of the skin of the head, with 

 two hair-follicles. Diagrammatic. (Koiliker. 

 From Quain.) 



a, epidermis ; b, corium ; c, muscles of 

 the hair-follicles. 



Fig. 55. — Hair-rudi.Tient from an embryo of 

 six weeks, magnified 350 diameters. (K51- 

 liker. From Quain's " Anatomy.") 



a, horny, and b, mucous or Malpighian layer 

 of cuticle ; 2', limitary membrane ; w/, cells 

 (some of which are assuming an oblong 

 figure) which chiefly form the future hair. 



all other mammals, is provided with hairs which completely 

 cover the body, and are even found in the mouth cavity; 

 since, however, the inside of the cheeks is formed in the 

 embryo by an ingrowth from the outer covering of the body, it 

 is not surprising to find that those cells which have thus grown 

 inwards have retained the power of becoming modified into 

 hairs. The first appearance of a hair is a slight thickening of 

 the lowest layer of the epidermis, the stratum malpighii ; this 

 growth projects downwards, and becomes larger. Ultimately 

 the central cells change their character and become horny, 

 thus forming the hair itself, while some of the peripheral cells 

 grow out into little sac-like structures, which are the sebaceous 

 glands, always attached to hairs, and secreting an oily fluid. A 

 slight papilla of the underlying dermis projects into the centre 



