THE INTERNAL MEDIUM. 49 



sociated with the removal of dead and broken-down tissues, 

 though it may be carried to excess as in the case of purulent 

 accumulations. 



The Blood Platelets or Plaques are a third kind of blood- 

 corpuscle, considerably smaller than the red, but somewhat 

 resembling them in form. They adhere together, break down 

 and form sticky clumps with great rapidity in drawn blood 

 unless special precautions are taken. 



Blood of Other Animals. In all animals with blood the 

 pale corpuscles are pretty much alike, but the red corpuscles, 

 which with rare exceptions are found only in Vertebrates, 

 vary considerably. In all the classes of the mammalia they 

 are circular biconcave disks, with the exception of the camel 

 tribe, in which they are oval. They vary in diameter from 0.02 

 mm. (ygVo inch) (musk deer) to .011 mm. (^^Vo inch) (ele- 

 phant). In the dog they 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 cor- 

 puscles possess a central nucleus, and are oval slightly bi- 

 convex disks, except in a few fishes in which they are cir- 

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

 the frog are 0.02 mm. ( r / ffQ inch) long and .007 mm. (^ 

 inch) broad. 



Histology of Lymph. Pure lymph is a colorless watery- 

 looking liquid; examined with a microscope it is seen to con- 

 tain numerous pale corpuscles closely resembling those Of the 

 blood, and no doubt many are pale blood-corpuscles which 

 have migrated. These lymph-corpuscles or leucocytes have, 

 however, another more important origin. In many parts of the 

 Body there are collections of a peculiar lymphoid or adenoid 

 tissue, sometimes in nodular masses (lymphatic glands). 

 This tissue consists essentially of a fine network, the meshes 

 of wnich are occupied with leucocytes which frequently show 

 signs of division. The meshes of the network communicate 

 with lymphatic vessels and the lymph flowing through picks 

 up and-cairies off the new-formed leucocytes. The lymph 

 being ultimately poured into the blood, the leucocytes be- 

 come the colorless corpuscles of the latter; and the migrating 

 cells of the blood are therefore but lymph-corpuscles restored 

 to the lymph, perhaps somewhat changed during their life in 

 the blood-plasma. 



The lymph flowing from the intestines during digestion 



