THE FROG AND VERTEBRATES IN GENERAL 201 



as perspiration. It evaporates and so cools the skin as well as the 

 blood passing through it. If perspiration is reduced, the tempera- 

 ture of the skin will rise, the blood coming to the surface will not 

 be cooled sufficiently, and the temperature of the body will rise. 

 This may be serious as in the case of a 'sun-stroke.' 



Blood Clotting and Wound Healing. The conservation of the 

 blood when injuries occur is, of course, of vital importance if the 

 organism is to survive, and accordingly we find in the plasma of 

 the blood a mechanism which causes coagulation, or clotting. This 

 results in stopping the hemorrhage from the injured tissues, pro- 

 vided the wound is not too large. The solid clot which gradually 

 forms from the liquid plasma is the culmination of a complicated 

 series of reactions which are only imperfectly understood at 

 present. It appears clear, however, that an inciting agent (kinase) 

 is present in the injured tissues, and it, acting on the calcium salts 

 and prothrombin in the blood, forms thrombin. The latter 

 then reacts with fibrinogen, also in the blood, to form fine, insol- 

 uble, needle-like filaments of fibrin. A section of a blood clot 

 examined under the microscope reveals the fibrin elements in 

 great abundance throughout the clot, with the blood cells embedded 

 in them. Blood in a dish or other container will normally clot in 

 a few minutes. If it is stirred during clotting the filaments will 

 unite to form long fibers which, together with the enmeshed blood 

 cells, may be removed as a fibrous mass, leaving a blood fluid, the 

 serum, which will not clot. 



In a wound, the fibrin filaments form a temporary and possibly 

 a permanent union of the tissues. Then the uninjured cells from 

 the nearby tissues move into the area and begin to divide and 

 form new tissues. Leucocytes are also present in great numbers. 

 If bacteria are present they ingest them and thus endeavor to 

 control the infection. They also remove the cell debris resulting 

 from the injured cells. 



4. Excretory System 



The excretory organs of the Vertebrates consist of the lungs, 

 skin, and kidneys. The lungs excrete carbon dioxide and water 

 vapor; the skin excretes small amounts of nitrogenous wastes 

 and various inorganic salts, all of which are in solution ; the kidneys 

 excrete the nitrogenous waste, urea, dissolved in water to form 

 urine as noted below, and salts in solution. Consideration has 



