PROTECTION, SUPPORT AND MOVEMENT 



503 



(Fig. 25.1). Intermediate layers can also be recognized where the epi- 

 dermis is especially thick, as on the palm of the hand and the sole of 

 the foot. As new cells are produced and differentiate, the outer cells 

 of the stratum corneum are lost. Groups of such cells are continually 

 being shed in mammals; dandruff is a familiar example. 



The dermis is composed of fibrous connective tissue; bone may 

 develop in it in certain regions. The dermis is richly supplied with blood 

 vessels, some of which lie close to the surface and enter papilla-like pro- 

 jections of the dermis that extend into the base of the epidermis. In 

 addition to their nutritive function, these vessels in mammals play an 

 important role in thermoregulation. Xerves and microscopic sense organs 

 that receive stimuli of touch, pressure and temperature are abundant 

 in the dermis, but only a few naked nerve endings, which are believed to 

 initiate pain impulses, penetrate the epidermis. Fat may accumulate in 

 the deeper parts of the dermis and in the subcutaneous tissue. The fat 

 serves as a reserve supply of food, as a thermal insulator, and as a cushion 

 against mechanical injury. The blubber of Avhales serves as a good insula- 

 tion in the aquatic environment. Hair is not an efficient insulator in 



Stratum 



corneum 



Epidermis 

 Stratum 



oermmativum 

 o J 



Sense or^an 



Bloodvessel 



ermis 



Hair follicle. 



Dermal papilla. 



Figure 25.1. Diagrammatic section through the skin of a mammal. 



