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 
--jungs 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- 
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 
