THE CIRCULATORY SYSTEM 137 



with the exception that the soluble fibrinogen has been changed 

 by the action of the enzyme thrombin to the insoluble fibrin. 

 The fibrin forms a fine meshwork, which holds the corpuscles 

 firmly together. There is evidence that the blood-plates dis- 

 integrate almost immediately after coming in contact with a 

 foreign surface and release a substance which renders the enzyme 

 thrombin effective. When fresh blood is drawn into a beaker 

 and defibrinated, by whipping with a stiff wire or shaking with 

 glass beads, the fibrin separates out as an elastic, stringy mass. 

 The corpuscles in defibrinated blood being heavier than 

 the serum, gravitate to the bottom of the fluid. As the red 

 blood-corpuscles are heavier than the white blood-corpuscles, 

 the latter form a thin layer on the surface of the former. 



The functions of the blood may be summarized as follows: 

 Blood carries oxygen from the lungs and food materials from the 

 alimentary canal to the tissues; it receives carbon dioxid and 

 other waste products of activity from the tissues and transports 

 them to the organs of excretion through which they are elimi- 

 nated; it distributes heat and assists in equalizing the body tem- 

 perature by carrying heat to the surfaces of the body where it 

 can be dissipated; it carries hormones from organ to organ; it 

 neutralizes toxins and destroys bacterial invaders. It may be 

 considered as an extension to the external media (air and water) 

 for the transportation of gases between the lungs and tissues, and 

 water-soluble substances between the digestive canal and the 

 tissues. 



Lymph is not secreted by the lymph-glands in the sense that 

 saliva is by the salivary glands, but comes from the fluid os- 

 mosed through the blood-vessel and cell walls. A sample taken 

 from a small lymph-channel, or a "water blister," is a straw- 

 colored fluid which has less tendency to coagulate than blood. 

 That coming from the lacteals or receptaculum chyli is thicker 

 and milky in appearance, due to the presence of the fats absorbed 

 from the alimentary canal, and is termed chyle. Lymph con- 

 tains numerous lymph corpuscles, or lymphocytes. 



The functions of the lymph are to carry nutritive substances 

 from the blood to the tissues and convey away waste materials, 

 and to act as a lubricant. The latter function is seen when an 

 examination is made of the synovia of the joints and the peri- 

 cardial, pleural, and peritoneal fluids, all of which contain consid- 



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