OTHER MODES OF FOKOE. 175 



expressions are intelligible to beings who are capable of ex- 

 periencing similar sensations ; but the physical changes 

 accompanying these sensations are not thereby explained. 

 Without pretending to know what probably we shall never 

 know, the actual modus agendi of the brain, nerres, muscles, 

 &c., we may study vital as we do inorganic phenomena, both by 

 observation and experiment. Thus, Sir Benjamin Brodie has 

 examined the effect of respiration on animal heat by inducing 

 artificial respiration after the spinal cord has been severed ; 

 in which case he finds the animal heat declines, notwith- 

 standing the continuance of the chemical action of respiration, 

 carbonic acid being formed as usual ; but he also finds that 

 under such circumstances the struggles or muscular actions 

 of the animal are very great, and sufficient probably to ac- 

 count for the force eliminated by the chemical action in 

 digestion and respiration ; and Liebig, by measuring the 

 amount of chemical action in digestion and respiration, and 

 comparing it with the labour performed, has to some extent 

 established their equivalent relations. 



Mr. Helmholtz has found that the chemical changes which 

 take place in muscles are greater when these are made to 

 undergo contractions than when they are in repose ; and that, 

 as would be expected, the consumption of the matter of the 

 muscles, or, in other terms, the waste or excrementitious 

 matter thrown off, is greater in the former than in the latter 

 case. 



M. Matteucci has ascertained that the muscles of recently 

 killed frogs absorb oxygen and exhale carbonic acid, and that 

 when they are thrown into a state of contraction, and still 

 more when they perform mechanical work, the absorption is 

 increased ; and he even calculates the equivalents of work so 

 performed. 



M. Beclard finds that the quantity of heat produced by 

 voluntary muscular contraction in man is greater when that 

 contraction is what he terms static, that is, when it 



