504 URINE. [CHAP, xxxiv. 



too slight to be recognised. This circumstance would account for the result 

 of several experiments we have ourselves made upon this point, in which we 

 have found no diminution in the quantity of phosphates. 



To sum up, the kidneys appear to be special organs for the removal of effete 

 material produced in the vital processes from the system, and they serve as 

 channels for the elimination of water and certain saline matter, as well as 

 excess of nitrogenous material which is not required for the maintenance of 

 the tissues. The chief constituents of the urine consist of compounds result- 

 ing from the action of oxygen upon the albuminous or allied substances ; and 

 in urea and uric acid we have probably examples of the highest state of oxida- 

 tion which the chemical elements of the tissues are capable of undergoing, and 

 urea may be looked upon as the last of a series of compounds resulting from 

 the successive action of oxygen upon those bodies which stand above it on this 

 scale. The fixed salts which occur in urine also exist in a highly oxidized 

 state. There can be little doubt, that the highly complex substances entering 

 into the formation of the tissues, by chemical action taking place in the 

 organism, become resolved into bodies of a more simple composition, until 

 they are eliminated in the form of urea or some allied compound, the elements of 

 which are so loosely combined, that by external circumstances alone new sub- 

 stances are formed of a still simpler composition, such as carbonic acid and 

 ammonia. In these, however, the elements are united with such force, that 

 it is only by most powerful chemical action, or by the still more powerful 

 influence of the vital properties of plants, that they can be separated from 

 each other, and again applied to the building up of those highly complicated 

 sut stances of which the tissues of animals consist, and which, by their vital 

 processes are again reduced in complexity as before. 



The more actively the vital phenomena are performed, or, in other words, 

 the greater the rapidity with which the disintegration and repair of the 

 tissues takes place, the larger is the quantity of urea excreted from the system. 

 With this increase of the urea, there is certainly a corresponding increase of 

 the sulphates, and perhaps also of the phosphates. If, however, the activity 

 of these changes be interfered with, as from impairment of the respiratory 

 apparatus, or from other causes, as might be expected, we find an increase of 

 that constituent which stands next above urea in the descending series of 

 compound resulting from oxidation, namely, lithic acid. This finds its way 

 out of the system in the form of lithate of soda, forming the amorphous 

 sediment generally known as lithate of ammonia, but which really consists 

 almost entirely of lithate of soda. Under similar circumstances, we often 

 meet with a deposit of oxalate of lime. The urine of the active carm'vora 

 contains, like that of man, a large quantity of urea ; on the other hand, the 

 urine of serpents and many other reptiles consists almost entirely of uric acid 

 in combination with ammonia. The urine of birds much resembles in 

 character that of serpents, which appears somewhat to be opposed to the 

 doctrine we have been endeavouring to inculcate, as the vital changes are 

 carried on with greater activity in this than in any other class of animals ; 

 but, on the other hand, it may be argued, that the demand for oxygen is so 

 great in birds, and their vital functions so actively carried on, that the exten- 

 sive respiratory apparatus necessary for the supply of sufficient oxygen to 

 convert all the uric acid into urea would be incompatible with the lightness 

 of their bodies, which is so necessary for flight ; while the removal of the 



