304 THE HUMAN BODY 



are nearly the same size as those of man. In no mammals do the 

 fully-developed red corpuscles possess a nucleus. In all other 

 vertebrate classes the red corpuscles possess a central nucleus, and 

 are oval slightly biconvex disks, except in a few fishes in which they 

 are circular. They are largest of all in the amphibia. Those of the 

 frog are 0.022 mm. ( T AD inch) long and 0.015 mm. ( i ^ inch) broad. 



The blood of certain crustaceans contains instead of hemo- 

 globin a substance of similar physiological action, hemocyanin, 

 which is blue instead of red, and contains copper in place of iron. 



Histology and Chemistry of Lymph. Pure lymph is a color- 

 less watery-looking liquid; examined with a microscope it is seen 

 to contain numerous white corpuscles closely resembling those of 

 the blood, and no doubt many are leucocytes which have mi- 

 grated. For the most part, however, these lymph-corpuscles or 

 lymphocytes have another more important origin. In many 

 parts of the Body there are collections of a peculiar lymphoid or 

 adenoid tissue (p. 383), sometimes in nodular masses (lymphatic 

 glands). This tissue consists essentially of a fine network, the 

 meshes of which are occupied with lymphocytes which frequently 

 shows signs of division. The meshes of the network communi- 

 cate with lymphatic vessels and the lymph flowing through picks 

 up and carries off the new-formed lymphocytes. The function of 

 the lymphocytes is not clear. They are believed not to share 

 in the phagocytic function of the leucocytes. 



The lymph flowing from the intestines during digestion is, as 

 already mentioned, not colorless, but white and milky. It will be 

 considered with the process of digestion. During fasting the 

 lymph from the intestines is colorless, like that from other parts 

 of the Body. 



Lymph is feebly alkaline, and has a specific gravity of about 

 1,045. The chief chemical difference between lymph and blood- 

 plasma is that the former contains somewhat more waste ma- 

 terials and less food stuffs than the latter. This is because the 

 consumption of food by the cells and their production of waste 

 keep slightly ahead of the interchange of these substances between 

 blood and lymph by the processes of filtration and dialysis. Lymph 

 contains carbon dioxid in solution but no uncombined oxygen, the 

 latter substance being taken up by the living cells as fast as it enters 

 the lymph from the blood. 



