THE REPTILES AND THE BIRDS 12$ 



aca the greater part of the uric acid precipitates out of 

 solution and leaves practically all the water osmotically 

 free to be reabsorbed. This reabsorption is carried out 

 by regurgitation of the urine into a terminal, speciaUzed 

 segment of the intestinal tract, and the semisoHd residue 

 of uric acid is then defecated as a white paste with the 

 intestinal residues. 



At the typical osmotic pressure of amphibian or rep- 

 tilian blood (which is about the same as that of the bony 

 fishes and the mammals), the 320 mg. of urea formed 

 from one gram of protein would require 20 cc. of water 

 for its excretion in an isosmotic urine— that is, one having 

 the same osmotic concentration as the blood; in the rep- 

 tiles and birds, where protein nitrogen is metabolized to 

 uric acid, the final 'urine' as defecated from the cloaca 

 may take the form of an almost dry concretion, the nitro- 

 gen derived from one gram of protein requiring less than 

 1.0 cc. of water, and possibly as Httle as 0.5 cc, for its 

 excretion, a tremendous saving in water. Since the oxi- 

 dation of one gram of protein itself supplies 0.4 cc. of 

 water, the animal is placed on an ahnost self-sustaining 

 basis. 



One might expect the reptiles and birds to be aglo- 

 merular, and they are nearly so, but not quite. All that 

 have been studied have small glomeruH in which the 

 capillaries are reduced to but two or three short loops, 

 very difi^erent from the large, well-vascularized glomeruli 

 of the Amphibia. The reason that they have not forfeited 

 their glomeruli is perhaps that the tubules are unable 

 to excrete sodium chloride (and possibly other salts), 

 and they must rely solely on the filtering bed for salt 

 excretion. Moreover, in so far as the mic acid precipi- 

 tates out of the tubular urine it no longer automatically 

 draws water with it (as do the salts in the urine of the 

 marine fishes) and it is questionable whether water 

 could be made available by the tubules to wash the uric 

 acid suspension into the cloaca. Consequently, in reduc- 

 ing the glomerular filtrate to a feeble stream they have 



