SPECIAL PART. 



A. INTEGUMENT. 



THE skin consists of a superficial ectodermal and a deeper 

 mesodermal layer. The former is called the epidermis (scarf- 

 skin) and the latter the derma (corium, cut is). The subcu- 

 taneous connective tissue is usually not sharply marked off 

 from the derma, but either passes gradually into the other. The 

 epidermis always consists of cells only, while the derma is made 

 up principally of connective-tissue fibres, as well as those of an 

 elastic and contractile nature. Nerves, glands, pigment-cells 

 (chromatophores), bony-structures, and vessels occur principally 

 in the derma, the last two being found in this part of the 

 integument only. 



Thedermaisalso permeated throughout by leucocytes (white blood-corpuscles), 

 and this is especially the case in Fishes. These leucocytes penetrate to the 

 superficial layer of the epidermis, and carry pigment granules. 1 Here they 

 take on amoeboid movements, and break up into numerous small pigment- 

 containing particles, which are taken up by the epithelial cells. 



From what has been said above, it is clear that the skin pre- 

 sents much variety both in form and function, and this cannot 

 be wondered at when one considers how accessible its outer surface 

 is to external modifying influences. 



In the epidermis two layers may always be distinguished : an outer, 

 composed of horny cells (stratum corneum, horny layer), and a deeper, 

 made up of soft protoplasmic cells (stratum Malpighii, mucous layer). 

 The latter serves as a matrix for the regeneration of the horny layer, 

 the superficial part of which is continually scaling off. From the epidermis 

 all the glands of the skin, and all other parts spoken of as epidermic 

 structures take their origin. Such are hairs, bristles, feathers, nails, claws, 

 hoofs, &c. The peripheral sensory end-organs of the skin are to be con- 

 sidered as arising by a differentiation of epidermic cells : the definite relation 

 which many of these organs have with the derma must be looked upon as a 

 secondary acquirement. 



1 Pigment is never formed in the epithelial cells themselves, but always originates 

 in the derma. 



