CHAP. IL] RESPIRATION. 613 



stream. The behaviour of sugar is in this respect interesting. 

 Undoubtedly sugar is largely oxidized somewhere in the body ; a 

 considerable quantity of it, introduced into the body, rapidly 

 disappears, is temporarily disposed of in some way or another- and 

 ultimately wholly oxidized. Now a small amount of sugar is 

 normally present in the blood, and so long as that normal amount 

 is not exceeded no sugar appears in the urine. If however, by 

 some reason or other, the sugar in the blood is increased beyond 

 that normal amount, the excess passes unchanged into the urine. 

 And the excess in the blood which thus leads to the excretion of 

 unchanged sugar, is so small that we might expect it to be 

 oxidized in the blood, did the blood possess any considerable 

 oxidizing powers. Again, when sugar is added to blood outside 

 the body, there is no evidence that the sugar is oxidized, even 

 when the blood is kept at the temperature of the body for some 

 time. It is true that some of the sugar is changed, disappears, 

 cannot be recovered as sugar from the blood ; and this power of 

 the blood to produce some change in sugar present in it has 

 been spoken of as its " glycolytic " property. But there is no 

 evidence that the sugar is oxidized ; indeed the change effected in 

 the sugar is probably akin to that brought about by the action of 

 a ferment. And, in general, the oxidative power which the blood 

 removed from the body is able to exert on oxidizable substances 

 and on substances undoubtedly oxidized in the body, is exceedingly 

 small. 



We have seen that in muscle the production of carbonic acid 

 is not directly dependent on the consumption of oxygen. The 

 muscle produces carbonic acid in an atmosphere of hydrogen. What 

 is true of muscle is true also of other tissues and of the body 

 at large. It was shewn long ago that animals might continue 

 to breathe out carbonic acid in an atmosphere of nitrogen or 

 hydrogen; and this is further illustrated by the remarkable 

 experiment, that a frog kept at a low temperature will live 

 for several hours, and continue to produce carbonic acid, in 

 an atmosphere absolutely free from oxygen. The carbonic acid 

 produced during this period was made by help of the oxygen 

 inspired in the hours anterior to the commencement of the ex- 

 periment. The oxygen then absorbed was stowed away from the 

 haemoglobin into the tissues, it was made use of to build up the 

 explosive compounds, whose explosions later on gave rise to the 

 carbonic acid. Or, to adopt a simile which has been suggested, 

 the oxygen helps to wind up the vital clock ; but once wound up 

 the clock will go on for a period without further winding. The frog 

 will continue to live, to move, to produce carbonic acid for a while 

 without any fresh oxygen, as we know of old it will without any 

 fresh food ; it will continue to do so till the explosive compounds 

 which the oxygen built up are exhausted; it will go on till the 

 vital clock has run down. 



