368 



ORGAN SYSTEMS OF MAN 



regulating the water content of the body, 

 functioning differently in aquatic and ter- 

 restrial forms. Heat regulation is also con- 

 trolled by the skin indirectly through water 

 loss, as well as by various insulators such as 

 hair and feathers. The penetrating effect 

 of light is regulated by pigmentation of the 

 skin. Among some vertebrates the skin takes 

 part in respiration (for example, the frog). 

 These many functions of the skin signify its 

 importance. 



It is necessary to examine the skin micro- 

 scopically if one is to understand how it 



sloughed off in mammals and many of the 

 lower vertebrates. Everyone is familiar with 

 the loss of these cells in unexposed parts of 

 the body such as back of the ears and be- 

 tween the toes. They are particularly notice- 

 able in the hair, where they resemble flaky 

 "scales" and do not have an opportunity to 

 escape readily. These dead cells are spoken 

 of as dandruff and often erroneously as- 

 signed a pathological condition, particularly 

 by certain business establishments whose 

 chief concern is to sell a product that will 

 clear up this "malady." The corneum is per- 



F g. 14-2. Homologous digital tips, claws, nails, and hoof. 



performs its many and sundry jobs, and we 

 will take human skin as our example. It is 

 usually divided into two parts, the outer, 

 thinner epidermis and the inner, thicker 

 dermis (Fig. 14-1). The epidermis is com- 

 posed of an outermost non-living covering, 

 the corneum, which is the part in immediate 

 contact with the outside world; lying be- 

 neath it is a layer of epithelial tissue which 

 is composed of actively growing cells. As 

 the cells grow they move towards the out- 

 side, die, and eventually become the cor- 

 neum. These dead cells are constantly being 



forated by many tiny holes through which 

 sweat passes from glands that lie deep be- 

 low. The corneum becomes very thick on 

 the soles of the feet and the palms of 

 the hands (calloused), especially in people 

 who perform heavy labor requiring the use 

 of these appendages. Another interesting 

 characteristic of the corneum of these areas 

 is the formation of friction ridges. It is the 

 presence of these in tlie hand and foot which 

 causes fingerprints and footprints. These 

 friction ridges apparently have come 

 through a long evolutionary history, being 



