A REPRESENTATIVE VERTEBRATE. FROG 



329 



also carry on respiration, secretion, and ex- 

 cretion. Secretion and excretion take place 

 by means of glands, which may be simple as 

 the mucous glands of amphibians and fishes, 



or complex as the sweat, oil, and mammary 

 glands. The skin often produces outgrowths 

 such as hair, feathers, nails, hoofs, claws, 

 scales, teeth, and bony plates. 



Epidermis 



Dermis 



Sweat gland 

 (in section) 



Hair bulb 

 Hair papilla 



Subcutaneous 

 tissue 



Hair 



Duct of 

 sweat gland 



Duct of 



sebaceous 



gland 



ebaceous gland 



Hair muscle 

 Hair root 



Tubule of 

 sweat gland 



Blood vessel 

 Fat cells 



Figure 211. Skin of man showing cell layers, fibers, and other parts. Vertical section. 



In man the skin (Fig. 211) protects the 

 deeper tissues from drying, from injury, and 

 from the invasion of bacteria and other 

 organisms. It contains the end organs of 

 many sensory nerves. The balance between 

 heat production and heat dissipation is ef- 

 fected largely by the skin, since skin offers 

 a large surface for radiation and evaporation 

 of sweat, and it contains a large amount of 

 blood. About 87.5 per cent of the body heat 

 passes out through the skin as compared 

 with 10.7 per cent through the lungs, and 

 1.8 per cent in the urine and feces. Sebace- 

 ous glands occur everywhere except on the 

 palms of the hands and soles of the feet. 

 They are compound alveolar glands with 

 ducts that usually open into a hair follicle. 

 Their secretion, sebum, is a fatty, oily sub- 

 stance that keeps the skin and hair flexible 



and covers the skin with a layer that pre- 

 vents too rapid evaporation of water. Sweat 

 glands are distributed over the surface of the 

 skin, with the exception of the margins of 

 the lips, the skin under the nails, and the 

 glans penis. They are tubular, with the in- 

 ner portion coiled into a ball. The average 

 amount of sweat secreted in 24 hours is 

 about 16 to 20 fluid ounces. Sweat contains 

 some waste substances, but it is particularly 

 important because of the heat that is 

 necessary to evaporate it. 



General internal anatomy 



If the body wall of the frog is split open 

 in the ventral midline, from the posterior 

 end of the body to the angle of the jaw, the 

 organs in the body cavity or coelom will be 



