460 Animal Biology 



sists of bands of yellow elastic and white fibrous connective tissues which 

 contain adipose tissue ("fat") and sweat glands. 



In some parts of the human body the skin is very tightly attached 

 to the deeper tissues, while in other parts it is loosely attached to permit 

 free movements. On the inner surface of the hands and fingers and on 

 the soles of the feet there are many minute ridges which increase fric- 

 tion and form the individually distinctive fingerprint and footprint 

 patterns which remain constant throughout life. The dermis contains 

 thousands of sensory nerve endings for the reception of heat, cold, pain, 

 pressure, and touch (tactile) stimuli. Receptors for each of these sen- 

 sations are present in all parts of the skin but some are much more 

 concentrated in certain regions than in others. For example, tactile 

 receptors (touch) are much more numerous on the finger tips than on 

 the back of the hand. 



Sweat- Duct 



Sebaceous Glancl> 



Bomy layer 

 Pigment layer 



Tactile Organs" 

 Nerve — 

 Blood Vessels ' 



Sweat Gland^zirS-^ 



Fat 



Epidermis 



)> Dermis 



Fig. 228. — Human skin shown in cross section. The horny layer of the epi- 

 dermis is composed of three parts: the outer part, stratum corneum, made of 

 layers of flat cells containing keratin to prevent water loss; a middle part, stratum 

 lucidum, composed of transparent cells; and a lower part, stratum granulosum, 

 whose cells contain granules. The pigment layer of the epidermis is also known 

 as the Malpighian layer or stratum mucosum. The dermis or corium has an upper 

 or papillary layer containing papillae (elevations), nerves, tactile organs (sense 

 of touch), blood vessels, sebaceous glands, etc.; and a lower or reticular layer 

 containing sweat glands, fat (adipose tissue), etc. Contrast human skin with 

 that of the frog (Fig. 208). (From Guyer: Animal Biology, Harper & Brothers.) 



Accessory structures of the skin include hairs, oil glands, nails, teeth, 

 and sweat glands (Fig. 228). The Malpighian layer of the epidermis 

 is extended (invaginated) downward into the dermis to form the tube- 

 like hair follicles. The cells at the base of a follicle produce the hair 



