Biology of Man 459 



Nervous and Sensory System. — Receive stimuli; transmit and interpret 

 impulses for purposes of correlation^ secretion, movement, locomotion, 

 behavior, etc.; centers of sight, hearing, taste, smell, equilibrium, etc.; 

 memory; imagination (Figs. 246 to 251). 



Endocrine (Ductless) Gland System. — Production of ductless gland 

 secretions for the correlation and regulation of various body processes 

 (Figs. 252 and 253). 



Reproductive System. — Production of sex cells by the growth and 

 development of which the species as well as the race will continue (Figs. 

 223to225, 254, and255). 



II. INTEGUMENT (SKIN) AND SKELETON 



There are two distinct layers of the human skin: (1) the external 

 epidermis (cuticle) and (2) the deeper dermis (corium) (Fig. 228). 

 The epidermis is stratified squamous and columnar epithelium, contains 

 no blood vessels, but has fine nerve fibrils. The hair, nails, and numerous 

 glands are all modified epidermis. When we "peel" after a sunburn, the 

 epidermis comes oflF in sheets or strips. 



The human epidermis is composed of the following four layers : ( 1 ) 

 The outer, thin stratum corneum is made of layers of cells, the lower 

 layers of which are living and which replace the upper dead layers. The 

 protoplasm of these cells contains a protein material, keratin, to prevent 

 the excess loss of water. Many bacteria are probably harmed by the acid 

 of this layer. (2) The next layer, the semitransparent stratum lucidum, 

 is made of cells which are practically dead and which are renewed from 

 below. (3) The next layer, the thick, granular stratum granulosum, con- 

 tains some dead cells which are also replaced from below. (4) The 

 lowest layer of the epidermis, the stratum mucosum or Malpighian layer, 

 is made of several layers of columnar <:ells and contains the pigments ol 

 the skin. This layer gives rise to the upper layers of the epidermis. 



The dermis or corium is well developed, thicker than the epidermis, 

 and contains blood vessels, lymph vessels, nerves, and sense organs as well 

 as hair follicles, glands, and papillae. The dermis is attached to the 

 deeper tissues by a type of connective tissue known as subcutaneous 

 tissue. The dermis is characteristic of vertebrates and is used when 

 leather is "tanned." The dermis is composed of the following: (1) 

 The superficial papillary layer contains numerous slight elevations called 

 papillae in order to increase the surface for nerves, blood vessels, lymph 

 vessels, sense organs, and glands. (2) The deeper reticular layer con- 



