114 CHORD ATE ANATOMY 



nearer the surface, replace their Uving protoplasm by keratin, and become 

 the horny scales of the outer epidermis. In man, as in most mammals, the 

 stratum corneum wears away as rapidly as it is formed and never becomes 

 greatly thickened on most parts of the body. Amphibia, however, shed 

 the stratum corneum in sheets, sometimes sloughing off the entire covering 

 of the body at once. Serpents do the same thing, scales and all. 



Sections of the thick epidermis of the palms and soles show between 

 the stratum germinativum and the stratxim corneum, two intermediate 

 layers, a stratum granulosum and a stratum lucidum. These, however, 

 are merely transitions between the inner growing layer and the outer 

 lifeless horn. See Fig. 107. 



Coriimi. The deeper layer of the skin, the corium, cutis, or dermis, is 

 connective tissue, with a much greater variety of cell elements than the 

 epidermis, and, unUke the epidermis, richly supplied with blood-vessels. 

 Where it touches the epidermis, especially on the palms and soles, the 

 corium is thrown up into many fine papillae, the capillaries of which feed 

 the cells of the stratum germinativum. In some of these papillae are 

 tactile corpuscles and other nerve terminations. Cutaneous glands and 

 the roots of hairs, both derived from the epidermis, become embedded in 

 the corium, and from it they are fed. Fat cells are numerous, especially 

 in the lower layers. 



The greater portion of the corium is made up of connective-tissue 

 libers, both elastic and non-elastic. Most of these lie parallel to the 

 surface, interwoven like the fibers of felt; but some bundles are perpendicu- 

 lar to the surface. The fibers are more compactly set in the outer parts 

 of the corium than in the inner. The deepest layer is the loose or areolar 

 connective tissue by which the entire skin is attached to the underlying 

 muscle or bone. Skin muscles are few, and are mostly connected with the 

 bases of the hairs. The elasticity of the skin decreases with age. 



Leather is made from the outer, compact layer of the corium of animals. 

 The epidermis is removed by maceration, and the connective-tissue 

 fibers are toughened by tanning. 



DEVELOPMENT OF THE SKIN 



Notwithstanding the close connexion between the two main layers 

 of the skin, their origin in the embryo is diverse, the epidermis developing 

 from the ectoderm, the corium from mesenchyma. Since the mesen- 

 chyma is derived chiefly from the mesoderm, this contrast in origin is 

 fundamental. 



Epidermis. The embryonic epidermis arises directly from the 

 ectoderm, and is at first a simple cuboidal epithelium. By the end of the 

 first month, as the result of cell divisions in a plane parallel to the surface, 



