ABSOEPTION 353 



muscle. The amount varies with the diet, and in a dog 

 which is not fasting, it may be anything from 5 to 30 grms. 

 per kilo of body weight. This small store is rapidly used up 

 in fasting and is drawn upon in muscular exercise. Glycogen 

 may be compared to money at current account ; glucose, like 

 money in the pocket, may be used at once. 



B. The Liver as a Regulator of the Supply to Muscles. 



The liver develops as two diverticula from the embry- 

 onic gut, and is thus primarily a digestive gland. In 

 invertebrates it remains as a part of the intestine both 

 structurally and functionally. But in mammals, early in 

 foetal life, it comes to have important relationships with the 

 blood going to nourish the body from the placenta (see 

 p. 628). The vein, bringing the blood from the mother, 

 breaks up into a series of capillaries in the young liver. 



(1) Blood Formation. — The development of the cells of the 

 blood goes on for a considerable time in these capillaries. 



(2) Bile Secretion. — Soon the liver begins to secrete bile. 



(3) Glycogenic Function. — Animal starch and fat begin to 

 accumulate in its cells. 



Gradually, the formation of blood cells stops, and the 

 mass of hver cells becomes larger in proportion to the 

 capillaries. As the foetal intestine develops, the vein 

 bringing blood from it — the portal vein — opens into the 

 capillary network of the liver, so that, when at birth the 

 supply of nourishment from the placenta is stopped, the 

 liver is still associated with the blood which brings nutrient 

 material to the body, and it performs a7i important function 

 in regulating the supply of nourishment to the tissues, and 

 more especially to the great energy-liberating tissue, muscle. 



1 . Regulation of the Supply of Sugar. — It has been already 

 shown that sugar is an essential source of energy in muscle. 



(1) Production of Sugar. — The relationship of the liver 

 to the metabolism of sugar was discovered by Claude 

 Bernard in the middle of last century. Even in the most 

 prolonged fast, the liver continues to supply to the blood 

 enough dextrose to maintain the normal proportion of about 

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