190 ELEMENTS OF BIOLOGY 



of the substances excreted by the glomeruU is a dehcate one; it may 

 be disturbed by many conditions in addition to disease. Violent 

 poisons may alter the permeability of the glomerulus to excretory 

 substances; other agents, for example narcotics, accelerate the activ- 

 ity of the excretory processes. And since the glomerulus acts pri- 

 marily as a filter, changes in the blood pressure bring about 

 corresponding changes in the excretory activity of the kidney. 



Excretion by Skin. A second organ of excretion important 

 for the human body is the skin. Both the sweat glands (Fig. 94) 

 and the oil glands excrete substances from the blood but the former 

 are much more prominent excretory organs. The control of the ac- 

 tivity of the sweat glands is a function of the nervous system. Many 

 drugs and some diseases by their effect on the nervous system cause 

 the glands to excrete rapidly. The substances excreted by the skin in- 

 clude NaCl, the chief salt constituent of perspiration, water, and 

 traces of other salts and fatty acids. The sudiferous glands are con- 

 stantly active, so that there is a constant loss of weight even when 

 the body is at rest. With increase in the temperature of the sur- 

 roundings or with exercise the glands excrete more or less profusely. 



Fate of Heat in the Animal Body. Constant evaporation 

 of watery perspiration on the body surface means that heat is being 

 constantly absorbed. Skin excretion is therefore an important factor 

 in regulating the temperature of the human body. This leads us to a 

 consideration of body temperature in general throughout the 

 animal kingdom. 



All oxidations are exothermic reactions, that is, energy is released. 

 In the protoplasm some of this energy is utilized as work, some in 

 the synthesis of compounds, the formation of which requires 

 energy; some is degraded into heat. In the simple animals this heat 

 is almost immediately dissipated and even in forms as complex as 

 the reptiles no provision is made for conserving it. In birds and 

 mammals it is retained and devices provide for controlling the rate 

 of heat loss. Thus animals according to whether or not they retain 



