METABOLISM 251 



Thus, mental agitation, emotions, and also slight irritations at the distal 

 end of the urethra cause frequently a desire to micturate. 



It remains to suggest the ways in which the katabolic waste is excreted 

 through the lungs, the skin, and the rectum. 



The Expired Air excretes waste matter next in importance to that of the 

 urine, this matter consisting almost wholly of carbon dioxide and of 

 water. The katabolism which produces these two substances has been 

 already more or less described in our chapter on respiration and above 

 in this chapter. A very large part of the carbon dioxide and about one- 

 sixth of the water passes off through the respiratory organs. The sources 

 of the water we have already seen (page 240). 



The carbon dioxide comes from the oxidation of all the products of 

 katabolism that contain carbon. Some of that arising from proteid 

 passes out through the kidneys in the form of urea. Most of the carbon 

 dioxide excreted, therefore, comes from the carbohydrates and fats of 

 the tissues (and the blood?). The carbon from these is oxidized to 

 carbon dioxide. There is evidence (obtained by Stoklasa) that the tissue- 

 cells generally produce an anaerobic enzyme which occasions the fer- 

 mentations of sugar to carbon dioxide and water; it is found also in milk 

 and in blood. This evidence Borrino has confirmed. The chief seats 

 of this oxidation are the muscles; then come the glands and the nervous 

 system, the other tissues being far behind these as sites of oxidation. 

 The muscles furnish about three-quarters of all the carbon dioxide 

 excreted by these means combined. 



Suppositions as to the exact mode of oxidation have been various ; the 

 real way or ways are unknown. Hoppe-Seyler considered it possible 

 that nascent hydrogen, set free by certain decompositions, caused the 

 diatomic molecule of oxygen to split, the atoms of oxygen thus becoming 

 very active, making possible a vigorous attack on substances already in 

 process of katabolism. Again, Spitzer supposes iron may play an 

 important part in causing the absorption of oxygen into the breaking- 

 down cell-protoplasm. This always contains iron in the nucleus, and 

 ferrous iron may act as a carrier between the oxygen and the molecule, 

 alternately taking on and losing an atom of oxygen (Herter). Probably 

 (Traube) there are enzymes which cause the primary katabolism of the 

 fats and the carbohydrates, or even the absorption of oxygen into the 

 products of this katabolism. The pancreas may furnish such an enzyme 

 to sugar a ferment called an oxidase. Again, as Herter suggests, the 

 hydroxyl (HO) ions of the alkaline salines of protoplasm may be the 

 ultimate cause of oxidation, perhaps by dissolving the katabolic carbon 

 dioxide and so removing it from the tissues, perhaps by liberating nascent 

 oxygen to attack the tissue-molecules, water being simultaneously 

 formed. 



Regardless of hypotheses, it is certain that oxidation is inherent in 

 protoplasm and that without it life cannot continue. The nucleus in 

 case of the tissues is apparently the agent or at least the director of the 

 process. There is no reason why there should be only one way in which 



