FOEMATION OF URINE 567 



9. Oxalic Acid 

 O 



II II 

 H_0— C— C— O— H 



is a substance in a stage of oxidation just above that of carbonic 

 acid. It is frequently present in the urine linked with lime, 

 and the lime salt tends to crystallise out in characteristic 

 octohedra, looking like small square envelopes under the micro- 

 scope. Under certain conditions these crystals assume other 

 shapes. The oxalic acid of the urine is chiefly derived from 

 oxalates in vegetable foods, but it has been detected in the 

 urine of animals on a purely flesh diet. 



The differences between the urines of dififerent herbivora 

 are not important. The urine of the ox and cow is more 

 abundant and more dilute than the urine of the horse, while 

 the urine of the sheep is considerably more concentrated and 

 contains a very high proportion of hippuric acid. 



II. FORMATION OF URINE. 



No problem of physiology has proved more difficult than 

 that of the mode of formation of urine in the kidneys. 



This is largely due to the fact that theories were made by 

 famous physiologists upon imperfect data, and that subsequent 

 workers have tended to view their results in the light of one or 

 other of these theories. 



The purpose of the formation of urine, as already explained, 

 is twofold — 



1. To get rid of waste matter from the body. 



2. To help to maintain the Ce of the blood plasma (p. 554). 

 The problem of how it is produced may best be approached 



by considering — 



1st. The differences between the urine formed and the blood 

 from which it is formed. 



2nd. The apparatus which has to bring about these changes 

 — the kidney. 



