PAET III. 



THE CHEMICAL CORRELATION OF THE 



TISSUES. 



CHAPTER XV. 



THE VEHICLES OF CHEMICAL CORRELATION; BLOOD 

 AND LYMPH. 



THE COMPOSITION OF THE BLOOD. 



The distributing agents which accomplish the transportation of 

 substances from one part of the body to another are the Blood and 

 Lymph. Through their intermediation oxygen and the products arising 

 from the digestion of the foodstuffs are carried to the tissues, the waste- 

 products which result from their activity are carried from the tissues 

 to the excretory organs, and an exchange of products between diverse 

 and widely separated tissues is also rendered possible. Among this 

 latter class of materials there are included a number of substances 

 which, arising in one tissue or group of tissues, stimulate other and 

 distant tissues to correlated activity. These substances are collectively 

 designated Hormones, or chemical messengers (from op/mco, I arouse, 

 or excite). 



The blood consists of a suspension of cellular elements, the red cor- 

 puscles or Erythrocytes and the white corpuscles or Leukocytes in a pale, 

 straw-colored or almost colorless fluid, the Plasma. Of the two types 

 of corpuscles the erythrocytes are much more abundant than the 

 leukocytes, the normal average number of erythrocytes in man lying 

 between five and six million per cubic millimeter of blood, while the 

 leukocytes vary in number between 7000 and 15,000 per cubic milli- 

 meter. In other species the number of formed elements per cubic 

 millimeter of the blood may te higher or lower than in man. Thus in 

 the mouse the normal erythrocyte-count lies between ten and twelve 

 million per cubic millimeter. 



When the blood is shed from the vessels it forms within a few minutes 

 a gelatinous clot, which is due to the separation from the plasma of an 

 insoluble protein Fibrin. On standing, the clot shrinks or undergoes 

 Syneresis, expressing a colorless or very pale yellowish fluid, rich in 



