SOURCE OF CARBONIC ACID EXHALED. 263 



acid thus disengaged, bears a very regular proportion to the 

 amount of exertion which is made during the same time. 

 Hence it is much greater in tribes whose habits are active, 

 than in those which are inert ; and it is also greater in any 

 individual, during a day spent in active exercise, than it is in 

 the same person during a day passed in repose. This obviously 

 results from the fact, now established beyond all doubt, that 

 every muscular contraction or production of muscular-force, 

 and every production of nerve-force by which muscular contrac- 

 tion is usually called forth, involve, as their essential condition, 

 the death and disintegration of proportionate amounts of 

 muscular and nervous substances, which pass from the state of 

 living tissues to that of dead matter ; and for this operation, 

 the presence of the oxygen in arterial blood is required. This 

 oxygen combines with part of the materials thus set free as 

 waste ( 55), and converts them into the products that are 

 thrown off by the various excretions. One of the chief of 

 these products is carbonic acid, which is carried off by the 

 lungs in the manner already described. Thus the presence 

 of oxygen in the blood is essential to the development of 

 nervo-muscular force ; while the elements of the blood 

 itself are required to re-form the tissues which have been thus 

 destroyed. 



308. It is among Birds and Insects that we find the greatest 

 quantity of carbonic acid produced, in proportion to the size 

 of the animals ; and in both these classes we find extraordi- 

 nary provisions for the energetic performance of this function 

 ( 321 and 326). The greater energy of the respiration of 

 Birds than that of Mammals, when compared with the greater 

 number of the red corpuscles in their blood, gives an increased 

 probability to the idea, that the red corpuscles are the chief 

 carriers of oxygen from the lungs to the capillaries of the 

 system, and of carbonic acid from the capillaries of the system 

 to those of the lungs ( 235). The energetic respiration of 

 Insects, though these corpuscles are absent, is fully accounted 

 for by the peculiar manner in which the air enters every part 

 of their bodies (321). In no case do we see the influence 

 of muscular activity, on the amount of carbonic acid thrown 

 off, more strongly manifested than in Insects. A humble-bee, 

 while in a state of great excitement after its capture, made 

 from 110 to 120 respiratory movements in a minute; after 



