THE METABOLISM OP THE BODY. 429 



of groups of animals including some invertebrates. 3. Among 

 mammals it is most abundant when the animal feeds largely 

 on carbohydrates. 3. It is found in the liver of the camivora, 

 and in those of omnivora, when feeding exclusively on flesh. 

 4. When an animal starves (does not feed), the glycogen grad- 

 ually disappears. 5. A fat-diet does not give rise to glycogen. 

 6. During early foetal life glycogen is found in all the tissues, 

 but later it is restricted more and more to the liver, though 

 even in adults it is to be found in various tissues, especially the 

 muscles, from which it is almost never absent. 



From the facts the inference is plain that glycogen is formed 

 from carbohydrate materials ; or, to be rather more cautious, 

 that the formation of this substance is dependent on the pres- 

 ence of such material in the food. 



The Uses of Glycogen. — No positive statement can be made on 

 this subject. It is generally believed to be transformed intp sugar. 



What is the fate of the transformed glycogen ? What be- 

 comes of the sugar ? We can answer, negatively, that it is not 

 used up in the blood, it is not oxidized there ; but by what 

 tissues it is used or how it is made available in the economy is 

 a subject on which we are profoundly ignorant. The presence 

 of so much glycogen in the partially developed tissues of the 

 foetus points to its importance, and suggests its being a crude 

 material which is laid up to be further elaborated, as in vege- 

 tables, by the growing protoplasm. 



metaboiiISm; of the spleen. 



The physiological significance of the peculiar structure of 

 this organ, though not yet fully understood, is much plainer 

 than it was till recently. The student is recommended to look 

 carefully into the histology of the spleen, especially the dis- 

 tribution of its muscular tissue and the peculiarities of its 

 blood-vascular system. It has already been pointed out that 

 there is little doubt that leucocytes are manufactured here even 

 in the adult, possibly also red cells ■ and that the latter are dis- 

 integrated, and the resulting substances worked over, possibly 

 by this organ itself. This view is rendered probable, not only 

 "by microscopic study of the organ, but by a chemical examina- 

 tion of the splenic pulp ; for a ferruginous proteid, and numer- 

 ous pigments, of a character such as harmonizes with this con- 

 ception, are found. 



