THE BLOOD 201 



dioxide and its excretion are the result of a struggle between 

 that gas and phosphoric acid for the soda of the plasma. 



Nitrogen. The amount of nitrogen in the blood is not in 

 excess of what can be held in solution ; and we may there- 

 fore infer that it is simply dissolved in the blood plasma. 



Y. Source of the Blood Constituents. 



A. Of the Plasma. The water of the blood is derived 

 almost entirely from the water ingested. 



The source of the proteids has not been fully investigated. 

 Undoubtedly they are partly derived, somewhat indirectly as 

 we shall afterwards see, from the proteids of the food. Very 

 probably, too, they are in part derived from the tissues. But 

 the significance of the two proteids, albumin and globulin, 

 and their variations has not yet been elucidated. 



The glucose is derived from the carbohydrates and possibly 

 from the proteids of the food, and during starvation it is 

 constantly produced in the liver and poured into the blood. 



The fats are derived from the fats and carbohydrates and 

 possibly from the proteids of the food. 



The urea and other waste constituents are derived from 

 the various tissues. 



B. Of the Cells I. Leucocytes. These are formed in the 

 lymph tissue and in the red marrow of bone. 



1. Lymph Tissue (see p. 30) is very widely distributed in 

 the body, occurring either in patches of varying shape and size, 

 or as regular organs, the lymphatic glands (Fig. 102). These 

 are placed on the course of a lymphatic vessel, and consist of 

 a sponge-work of fibrous tissue, in the interstices of which are 

 set the patches of lymph tissue or germ centres, each sur- 

 rounded by a more open network, the sinus, through which 

 the lymph flows, carrying away the lymphocytes from the 

 germ centres. Round some of the lymphatic glands of 

 certain animals large blood spaces or sinuses are seen, and 

 these glands are called hsemolymph glands. They are inter- 

 mediate between lymphatic glands and the spleen. While 

 these glands produce lymphocytes, they also play an im- 

 portant part in disposing of disintegrating erythrocytes and 

 in storing iron. * 



