PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 251 



tliat exhibited in the motion of birds, it must be admitted tliat the 

 chemical action which takes phice in the substance of the muscle, and 

 ■svhich generates heat and motion as in a machine, is produced more 

 rapidly in the muscles of birds than in tliose of any other class of 

 animals. I nuist be permitted here to insist on the consideration of the 

 molecular phenomena of which the nniscles are the seat, since we shall 

 thus obtain more light upon our subject. Exteudirig to organized beings 

 the principle of the conservation of forces as v.ell as the identity of 

 power and heat, modern physiologists admit that a combustion takes 

 place in the muscles as in the furnace of steam-engines. This combus- 

 tion or chemical reaction liberates the forces vrhieh the separate atoms 

 contain, and renders them evident under two forms, heat and mechanical 

 energy, which are in one sense the complements of each other. When- 

 ever a muscle contracts Avitliout raising a weight or otherwise expending 

 power, it becomes sensibly heated. If it raises a weight, or otherwise 

 expends ]w\ver, it exhibits less hctit, and this loss of heat, if it could be 

 measured, would correspond to the mechanical equivalent of the work it 

 has performed. It is true that we cannot estimate exactly the heat that 

 a living muscle disengages during contraction, for the circulation of the 

 blood carries off to ditferent ])arts of the bod}- more or less of the heat 

 generated. xVll the ex])eriments of Beclard, Heidenhain, Hien, and 

 others, tend to prove that the production of heat diminishes in propor- 

 tion to the increase of power expended in work. This is enough to 

 authorize the application of the principle of the conservation of forces to 

 physiology, a principle, moreover, thoroughly justified by human reason. 

 Two methods may be adopted in explaining the production of power 

 from the chemical action which takes place in the muscles. Either the 

 chemical action which we have called combustion liberates force sim- 

 ultaneously in the two forms of heat and power, or, as in the steam- 

 engine, heat is first produced to be afterward partially converted into 

 power. Certain facts render this last hypothesis extremely probable. 

 We can detect, in some cases, in a muscle, the transformation of heat 

 into power. The phenomena are manifested in the following manner : 

 When the muscle of a frog is detached and movements are excited in it 

 by electricity, submitting- it, meanwhile, to a gradual increase of tem- 

 perature, it is seen that the amplitude of the movements produced 

 decrease up to a certain point, and that an instant arrives when the 

 muscle will not react at all. This loss of muscular irritabibty is produced 

 at about 33=^ Centigrade. If the muscle is gradually cooled it is seen to 

 resume its irritability. What has taken place! If the motions of the 

 gradually heated nuiscle be carefully registered side by side with o- 

 number of shocks, it is seen that the decrease of their amplitude is due 

 to the fact that the muscle when heated does not return to its normal 

 length after contraction. The minima of the curves are gradually in- 

 creased, showing that the weight lifted by each shock does not descend 

 again completely. The power exhibited during the contraction of the 

 muscle is not entirely expended by the time the incomplete relaxation 

 follows, and a certain amount of reserved power remains, of which the 

 cause seems to be the permeation of the muscle by heat. xVnd M'heu the 

 muscle, heated above S'S^, becomes inert, it is because it has obtained 

 through the action of heat all the contraction of which it is susceptible, 

 that is, it has expended all the power of which it is capable. Figure 9 

 shows the different phases of this phenomenon. 



While the muscle is cooling a contrary result is produced, namely, 

 the subtraction of an amount of heat equivalent to an amount of power 

 expended, that is to say, a relaxation of the muscle and a descent of the 



