CHAP, iv.] METABOLIC PROCESSES OF THE BODY. 573 



liver as glycogen. At those times, on the other hand, when 

 we may suppose that sugar ceases to pass into the blood from 

 the alimentary canal, the average percentage in the blood is 

 maintained by the glycogen previously stored up becoming 

 reconverted into sugar, and being slowly discharged into the 

 hepatic blood. 



Moreover, this view, that the glycogen of the liver is a 

 reserve fund of carbohydrate material, is strongly supported 

 by the analogy of the migration of starch in the vegetable 

 kingdom. We know that the starch of the leaves of a plant, 

 whether itself having previously passed through a glucose stage 

 or not, is normally converted into sugar, and carried down to 

 the roots or other parts, where it frequently becomes once more 

 changed back again into starch. 



367. Glycogen is found in other parts of the body than 

 the liver, and a study of the facts relating to the presence of 

 glycogen in other tissues will help us to a true conception of 

 the purpose of the hepatic glycogen. Next to the liver, the 

 skeletal muscles are perhaps the most conspicuous glycogen 

 holders. So frequently is glycogen found in muscle that it 

 may be regarded as an ordinary though not an invariable con- 

 stituent of that tissue ; indeed it may almost be considered as 

 a constituent of all contractile tissues. The quantity varies 

 very largely both in the different muscles of the same animal 

 and corresponding muscles of different animals. It disappears, 

 according to some observers, readily upon starvation, even 

 before the hepatic glycogen is exhausted ; but all observers are 

 not agreed on this point, and in some muscles, at least, it 

 appears to be retained for a very long time. It is said to be 

 increased in quantity when the nerve of the muscle is divided, 

 and the muscle thus brought into a state of quiescence. On 

 the other hand it diminishes or even disappears, being appar- 

 ently converted into dextrose, when the muscle enters into 

 rigor mortis. Some observers have found that it diminishes 

 during tetanus, and maintain that it, after conversion into dex- 

 trose, is used up in the act of contraction, forming through its 

 oxidation the immediate supply of the energy set free in the 

 contraction. But even granting that the glycogen in a muscle 

 may be diminished during prolonged labour, it cannot be 

 admitted that the oxidation or other chemical change of gly- 

 cogen is a necessary part of the ordinary metabolism of a mus- 

 cular contraction, since many muscles wholly free from glycogen 

 are perfectly well able to carry on long-continued contractions. 



What is probably the use of glycogen in muscle is sug- 

 gested by the fact that undeveloped embryonic muscles 

 are peculiarly rich in glycogen. In a young embryo, at the 

 time when the muscular substance, though undergoing stria- 

 tion, is still largely c protoplasmic ' in nature, the quantity of 



