188 INTERMEDIATE METABOLISM 



namely, how is carbohydrate metabohsm normally regu- 

 lated ? If the tissues, principally the muscles, require an 

 amount of sugar which varies with their activity, and if 

 the output of sugar from the Hver is subject to nervous 

 and chemical influences, there must be some mechanism 

 for adjusting the supply to the demand. The hyper- 

 glycsemia which is caused by emotional conditions may be 

 regarded as a mobihsation of sugar in anticipation of the 

 muscular efforts of offence or defence which will be demanded 

 of the animal by the cause of the emotion. But how is 

 the carbohydrate supply increased to meet a demand 

 unaccompanied by any emotional state, as in ordinary 

 exercise ? There are several ways in which the muscles 

 may influence the liver to satisfy their needs : — ■ 



1. The path may be nervous throughout, originating in 

 the afferent nerve-endings of the muscles and reaching the 

 hver by the sympathetic. The only evidence suggesting 

 such a mechanism is the reflex production of hyperglycsemia 

 above noted. 



2. Changes in the composition of the blood may affect 

 the central nervous system, and this in turn the hver. 



3. Changes in the composition of the blood may have 

 a direct chemical effect upon the hepatic cells. 



4. The effect upon the liver may occur only through 

 an increased output of adrenahn, which may be caused 

 either reflexly or by changes in the blood. 



Experiments, so far as they go, indicate that several of 

 these factors co-operate. Glycosuria, as we have seen, can 

 be produced reflexly by stimulation of afferent nerves. 

 As regards changes in the composition of the blood, these 

 may be of two kinds — a diminution in the amount of 

 sugar or an increased H. ion concentration. So far there 

 is no evidence that diminished sugar content has any 

 influence upon the hver. On the other hand, increased 

 sugar output has been observed to follow an increased 

 H. ion concentration, as after severe haemorrhage. Since 



