INTEGUMENT 45 



Microscopic sections stained with haematoxylin (blue stain) 

 and eosin (pink stain) show two types of glands differing both in 

 structure and in the staining reactions of the cytoplasm of the 

 alveolar cells (Fig. 21). In one, the mucous type, the cytoplasm 

 is in the form of a network and takes the blue stain; in the 

 other, the poison type, the cytoplasm is granular and stains a 

 deep pink. The size and shape of the cells of both types vary 

 with the activity of the gland. Cells filled with secretion enlarge 

 until they practically fill the cavity of the alveolus. As the 

 secretion is discharged, the cells shrink to a low cubical form. 

 Each type of gland is surrounded by a muscular and connective 

 tissue tunic, the contractions of the muscles 

 aiding the discharge of the secretion. 



Respiration. — The skin of the frog plays 

 an important part in aerating the blood, 

 supplementing the respiratory functions of 

 the oral epithelium and the lungs. The 

 cutaneous blood vessels are separated from 

 the surface of the body by at least the 

 thickness of the epidermis, so that oxygen FlG 2 i.— Section of a 

 absorbed from air or water must diffuse small immature poison 

 through the epidermis and the walls of g an 

 blood capillaries in order to reach the blood. Carbon dioxide 

 given off from the blood passes from the blood in the reverse 

 direction. In this respiratory exchange, therefore, the gases pass 

 through several layers of cells, though the total distance is not 

 very great. 



Water. — Water passes in either direction, to or from the blood 

 stream, through the frog's skin. Experiments indicate that 

 the rate of diffusion of water under pressure through the skin is 

 much greater from without than from within. Water thus passes 

 in more readily than out. The limited control of water loss 

 through the skin makes it necessary for the frog to live in a moist 

 environment. In a dry, warm room a frog will die overnight as 

 a result of the loss of water. 



Human Skin. — Though built on the same general lines as frog's 

 skin, human skin differs in a number of structural and functional 

 features. Human skin is much thicker and tougher, gives rise to 

 different types of glands, and is richly supplied with a variety of 

 specialized nerve endings. It is impervious to water and does 



