THE INTEGUMENT 



06' 



skin are formed, while the corresponding part of the ectoderm con- 

 tributes to such sensory structures as the nose and ear. 



The hair, nails, claws, feathers and other outgrowths of the cutis 

 come from the epidermis (Figs. 393, 394). Land animals usually have a 

 thicker epidermis than those which live in water. The latter keep the 

 outer portion of the body constantly moist and so show less of the hard- 

 ened or horny consistency Avhich is found in animals living in the air. 

 The corium lies directly beneath the epidermis, there being a loose layer 

 of connective tissue separating it from deeper structures. The corium 

 itself is a mass of fibrous connective tissue in which there is an inter- 

 mingling of elastic tissue, blood vessels, nerves, smooth muscle fibers, 

 etc. It is much thicker in mammals than in the lower vertebrates. It is 



the corium which is commonly 

 known as leather. "Pigment cells 

 may be found in both epidermis 

 and corium. These are mesen- 

 chyme cells loaded with pigment 

 which are frequently under the 

 control of the nervous (sympa- 

 thetic) system and can be altered 

 in shape (chromatophores), thus 

 producing color changes, which 

 as in the chameleons, may be very 

 marked." 



If the epidermis becomes 

 cornified, scales are produced. 

 This takes place by certain cells 

 in both corium and epidermis be- 

 ginning to multiply in certain 

 definite regions. These thicken- 

 ings become future scales by the 

 stratum corneum turning into a horny material. In snakes and 

 lizards these scales, together with all of the stratum corneum (even the 

 covering of the eye), are periodically moulted, the separation taking place 

 at the surface of the stratum Malpighii. In turtles and alligators there 

 is a gradual wearing away of the surface. 



Claws, hoofs and nails are closely allied in their manner of growth 

 to scales (Fig. 396). In fact, a claw is formed by two scales. The dorsal 

 one is called the unguis and the ventral the sub-unguis. The dorsal scale 

 grows continually from a root and in mammals is forced over its bed. 

 The unguis is curved both transversally and longitudinally, while the 

 sub-unguis forms its lower surface. 



In the human nail the unguis is nearly flat in both directions, and 

 the sub-unguis is reduced to a narrow plate just beneath the tip of the 

 nail. In the hoof the nnguis is rolled around the tip of the toe, while the 



Fig. 394. 



A diagram of a developing feather, highly 

 magnified. der., Dermis ; epL, epidermis ; fol., 

 follicle; fth., feather; Mp., Malpighian layer of 

 epidermis ; pap., papilla by the growth of whose 

 epidermis the feather is formed. (From Shipley 

 and MacBride.) 



