gS FROM FISH TO PHILOSOPHER 



arterioles, thus reducing the jfiltration pressure. When 

 excess water is available (as when a frog is partially im- 

 mersed in water), the filtration rate is increased and wa- 

 ter reabsorption is decreased, thus increasing the excre- 

 tion of water; when the animal is out of water and is 

 suffering some dehydration, the filtration rate is reduced 

 and water reabsorption is increased, thus conserving wa- 

 ter. In the latter circumstance, tubular function is sus- 

 tained, despite the reduction in glomerular circulation, 

 by the blood coming to the kidney by way of the renal- 

 portal circulation. 



In the mammals (the only forms that have been care- 

 fully studied in this respect), the secretion of ADH into 

 the blood is controlled through osmotically sensitive cells 

 located in the midbrain (hypothalamus) and connected 

 by nerve fibers with the neural lobe of the pituitary; the 

 effective stimulus that excites these 'osmoreceptors' is the 

 osmotic pressure of the plasma (which is, of course, de- 

 termined primarily by the sodium chloride concentra- 

 tion), and so sensitive are they that an increase in con- 

 centration so small that it is scarcely detectable by 

 chemical means suflBces to induce maximal ADH secre- 

 tion. It is assumed that increased secretion of the effec- 

 tive pituitary factor in the Amphibia is similarly ehcited 

 by increased concentration of the blood when the ani- 

 mal, sitting too long on a lily pad, has lost water by 

 evaporation from the lungs and skin and in the excretion 

 of urine; and that decreased secretion of the pituitary 

 factor is conversely related to excess hydration, as when 

 the animal has been too long in the water, with only its 

 nose exposed to air. But these are areas that remain for 

 future investigation. 



When the Amphibia abandoned the scales of their pis- 

 cine ancestors in favor of a naked skin they encountered 

 a double hazard— excessive hydration of the body when 

 they were immersed in water, and excessive dehydration 



