THE METABOLIC MACHINERY OF ANIMALS 319 



the rate of inspiratory discharge from the center down the cord to the 

 various levels where the relay apparatus or sympathetic system causes 

 inspiration. As the inspiration occurs the expiratory fibers of the 

 vagus are stimulated by the expansion of the lungs and the inspiration 

 is partially inhibited. Experiments clearly indicate that the gases in 

 the blood have a direct effect upon the activity of the center since, 

 for example, an increase of carbon dioxide in the blood results in an 

 increase in the force or rate of the respirations. This however does 

 not tell the whole story. Recently accumulated data furnish evidence 

 for the belief that the activity of the respiratory center is controlled 

 by the hydrogen-ion concentration of the blood passing through it, 

 which in turn is affected by the pressure of carbon dioxide in the 

 blood. 



SECTION D. EXCRETORY MECHANISMS 



Excretion 



This term is used to cover the separation, collection, and elimi- 

 nation of the waste products of metabolism from the body. These 

 waste products naturally vary within the organism itself from time to 

 time, and show even greater variation between different species of 

 animals. Fundamentally such devices center about mechanisms 

 which are adapted in different ways for the elimination of one funda- 

 mental by-product — nitrogenous wastes. In addition liquids in the 

 form of water, dissolved inorganic salts, and gases, as, for example, 

 carbon dioxide, are likewise eliminated by excretory devices. Like- 

 wise the digestive tract furnishes the avenue through which solid 

 wastes may be eliminated, although this latter method should not 

 be regarded as true excretion. Furthermore it should be realized 

 that, in the vertebrates at least, there is a constant elimination or 

 sloughing off of the exposed cells on various epithelial surfaces, as 

 well as from the linings of various tubes and ducts which connect 

 more or less directly with the outside. This section, however, is 

 primarily concerned with the various urinary devices for the disposal 

 of liquid wastes. 



In highly specialized forms such as mammals a number of devices are 

 adapted in one way or another for the elimination of waste products. 

 Before studying these mechanisms in any detail, we shall consider 

 briefly the various types of excretory systems found throughout the 

 animal kingdom. 



