THE FROG AND VERTEBRATES IN GENERAL 199 



creted from the cells is transported in the plasma, mostly in the 

 form of sodium bicarbonate, to the lungs, gills, or skin. 



Excretion. It is believed that all the excretory products of the 

 cells are carried in solution in the plasma. These consist of the 

 carbon dioxide, already considered in connection with respiration, 

 nitrogenous wastes, and various inorganic salts in solution. These 

 nitrogenous and inorganic wastes are for the most part given off 

 into lymph by the contiguous cells, and reach the blood plasma 

 indirectly. The nitrogenous wastes are removed from the blood 

 stream by the selective action of the liver cells which change them 

 into urea and then return them again to the blood stream. The 

 urea, as well as the inorganic salts and excess water, are removed 

 from the blood stream by the specialized cells of the kidney tubules 

 and then passed to the exterior. The sweat glands in the skin 

 also remove considerable quantities of excess water, containing 

 small quantities of urea and salts, from the blood stream. 



Secretion. The blood plasma carries the secretions, or hor- 

 mones, formed by the ductless glands, or endocrine organs, 

 examples of which will be noted in a later section. The hormones 

 are of vital importance in the economy of the organism and must 

 be present at all times in the blood stream, so that the tissue cells 

 can secure the necessary ones. For example, the absence of the 

 thyroid hormone will bring on a gradual deterioration and general 

 breakdown of nerve tissue, while the absence of an almost infini- 

 tesimal amount of the parathyroid hormone will almost imme- 

 diately throw the organism into violent paroxysms ending very 

 quickly in death. 



Infection. The control of infection, which is due to the invasion 

 of some disease-producing organism, is largely a function of the 

 vascular system. This may be said to be accomplished in two 

 ways. In the first place there are certain chemical substances in 

 the plasma of the blood and lymph known as antibodies. Their 

 exact nature is unknown, but apparently they are able to render 

 the environment unsuitable to the invading organism. If the 

 parasitic organism does get a foothold, then more and more anti- 

 bodies are produced, presumably by the cells of the host, and 

 given off into the blood stream, until the disease is overcome. 



Another method of disease control is through certain of the 

 white corpuscles, or leucocytes, which, as noted above, are amoeba- 

 like in structure and habits. At the time of an infection they are 



