446 Dr. Davy on the Temperature, $c., of Birds. [18G5. 



IV. Of the Kidneys and their Excretion. 



Another element in the problem of the temperature of birds is the kidneys, 

 with th excretion. , As is well known, these organs in birds are propor- 

 tionally large and active,; their secretion, not inconsiderable in quantity, 

 and formed chiefly of urate of ammonia, is voided in a state far removed 

 from the liquid, hardly semifluid from the little water it contains. Hence 

 in the performance of .the .function there is but little loss of heat. Moreover, 

 as it would appear from ultimate analysis that the urate contains less 

 oxygen than urea, there must be a less expenditure of oxygen in its forma- 

 tion, leaving more for a more profitable conversion into carbonic acid. 



What are the general conclusions which are admissible from the pre- 

 ceding results ? 



Do they not warrant the inference that the high temperature of birds is 

 owing to a combination of circumstances, some positive, some negative; the 

 one, the positive, acting through the air inspired and the conversion of oxy- 

 gen into carbonic acid gas, productive of heat ; the other, the negative con- 

 ditions, such as those mentioned, influential mainly by economizing the heat 

 when produced, or checking its escape ? 



Besides these negative conditions, it may be open to question, consider- 

 ing the proportional smallness of the lungs of birds, and the smallness of 

 the nerves with which they are supplied, whether there are not other cir- 

 cumstances concerned of an ancillary kind such, to enumerate some of 

 the most probable, as a powerful heart, especially a powerful left ventricle; 

 the quality of their blood, that but little viscid, as indicated by the little, 

 if any tendency of the red corpuscles to collect in piles * ; the large pro- 

 portion of these corpuscles, and their nucleated structure, a structure with 

 which may be connected an electrical influence. 



If the chief use of the peculiar pneumatic system of birds be to secure 

 a high temperature, it is probable, and is in part already admitted, that it 

 may subserve other uses inferior only in degree of importance in relation 

 to the habits and well-being of the class : for instance, as generally ad- 

 mitted, it may conduce to great power of flight in some, to running power 

 in others, to vocal power in a third ; and, in all, may not the thorough 

 aeration of the blood, as denoted by its more florid hue even in the veins, 

 be essential to the energy, to that intensity of action and endurance for 

 which the muscles of those birds in which the structure under considera- 

 tion is most developed, are so remarkable ? 



The subject in its entireness, it must be allowed, is full of interest. It 

 affords in the variety of structure exhibited by different birds, supple- 

 mentary to the lungs, ample scope for further research. Why some birds, 

 such as the woodcock, the snipe, the swallow, birds of rapid and long 

 flight, should be destitute of air in their bones ; why the small birds, with 

 few exceptions, the tits for instance, some of the smallest, should expe- 

 * In no instance have I seen the blood-corpuscles of any bird to cohere and form 

 rouleaux or piles, nor have I seen a buffy coat on the blood of birds. 



