THE MAMMALIA 285 



of a hair and its relation to the hair-follicle is shown in fig. 71. 

 The sebaceous glands near the upper end of the follicle should 

 be noted. They secrete a fatty substance which lubricates 

 the hair and prevents it from becoming dry and brittle. 



The sweat glands, the tiny mouths of which are scattered 

 over the skin, are as characteristic of mammals as the hairy 

 covering. They are formed as downgrowths of the Malpighian 

 layer of the epidermis which extend into the vascular corium 



>cor 



Fig. 71 



Diagrammatic section through the skin of a mammal, showing a 

 hair in its follicle, and two sweat glands, ca, cuticle ; cor, 

 corium ; ct, cortex of hair; d, duct of sweat gland ; Ep, epider- 

 mis ; f, fat lobules ; ihs, inner hair sheath ; md, medulla of hair ; 

 mo, mouth of sweat gland ; 0/15, outer hair sheath ; pp, papillae 

 of Malpighian layer ; sbg, sebaceous gland ; sc. stratum corneum 

 of epidermis; sm, stratum Malpighii of epidermis; sivg. sweat 

 gland. 



underlying the epidermis. Their inner ends are thrown into 

 many convolutions, and form a tightly-coiled knot ; the distal 

 ends retain their connection with the surface ; a lumen is 

 developed, and the epithelium lining the coiled end forms the 

 glandular part; the straight tube leading to the exterior, the 

 duct of the sweat gland. 



The mammary glands, from which the class Mammalia 



