274 



MAMMALIA. 



and when still stronger and thicker they constitute spines (Hedgehog, 

 Porcupine.) To the stronger hairs are attached smooth muscles of 

 the dermis, by means of which each one of them can be moved singly, 

 while the striped muscular system of the dermis causes the bristlinr 

 of the hairy covering and the erection of the spines over larger 

 extents of surface. 



The epidermis may also give rise to smaller horny scales as well as 

 to large overlapping scales ; the former on the tails of Rodents and 

 Marsupials, the latter upon the whole dorsal and lateral surfaces of 



the Pangolins (Manis), 



T tnr ,\ ^P v u, which thus possess a 



horny epidermal exo- 

 skeleton. Another form 

 of exoskeleton is found 

 in the Armadillos; it 

 arises by ossification of 

 the dermis, and con- 

 sists of suturally united 

 plates, and in the middle 

 of the body of broad, 

 movable, bony girdles. 

 Amongst the dermal 

 ossifications must also 

 be reckoned the antlers 

 of the Deer which are 

 periodically renewed. 

 The horny sheaths of 

 the Cavicornia, the 

 horns of the Ehinoce- 

 ridce and the various 

 horny coverings of the 

 extremities of the digits 

 are epidermal structures. The latter may be distinguished into 

 nails (unguis lamnaris, unguis tegularis), claws (fulcula), and hoofs 

 (ungula.} 



Cutaneous glands. Sweat glands and sebaceous glands (fig. 666) 

 are widely distributed. Sebaceous glands are invariable accompani- 

 ments of the hair follicles, but they are also found on naked parts of 

 the skin ; they secrete a fatty grease, which keeps the surface of the 

 skin soft. The sweat glands have the form of coiled glandular tubes 

 with sinuous ducts, and are only seldom absent (Cetacea, Mus, 



Fiu. 666. Section through the human scalp. JEp, Epi- 

 dermis ; Ul, transverse bands of the connective-tissue 

 of the cutis ; Uq, longitudinal bands of the same ; H, 

 hair; Hz, root of hair; P, hair papilla; HI, Hair 

 follicle ; Ma, musculus erector pili ; T, sebaceous 

 gland ; SD, sweat gland ; F, fat body. 





