AND THE LOWER ANIMALS. 43 



than that at which the capability of procreation ex- 

 hibits itself. This period is indicated in most animals 

 by well-marked phenomena. Mammals, Birds, Reptiles, 

 and Fishes throw off, as it were, the garb of youth, and 

 assume the costume of adult age. Not only are super- 

 ficial characters affected, and a few special organs 

 completed, but new functions present themselves, and 

 control all those which heretofore held the entire or- 

 ganism beneath their sway. In fact, the whole animal 

 is altered in regard to those functions which most 

 directly affect its general life. Here, again, man 

 affords us a striking illustration. 



Respiration is, as we know, a sort of combustion, 

 and at every expiration we give off a certain quantity 

 of carbonic acid. We can estimate the activity of this 

 function by knowing the quantity of the latter gas 

 formed by the combination of the oxygen of the air 

 with the carbon derived from the decay of the tissues. 

 Now, the researches of Messrs. Andral and Gravarret 

 prove that respiration is a little more energetic in 

 youth of both sexes than in adults.* Girls and boys 

 of eight years old consume from five to six grammes 

 of carbon per hour. This amount increases slowly, in 

 proportion to each, up to the period of puberty. Till 

 this period they are neither males nor females, but 

 neuters. But as soon as the sexes are marked, respira- 

 tion becomes twice as energetic in the man, although 

 it remains the same as before in the woman. At about 

 the age of thirty the former consumes from eleven to 

 twelve grammes of carbon per hour, whilst the female 



* " Kecherches sur la quantite d'acide carbonique exhale par le 

 poumon dans l'espece humaine." — Annales des Sciences naturelles, 

 1843. 



