ORGIAN AND FUNCTION. 



69 



CHAPTER VIIL 



HAKMONY BETWEEN THE ORGAN AND THE FUNCTION.— 

 DEVELOPMENT HYPOTHESIS. 



Each muscle of the body presents, in its form, a perfect harmony with the 

 nature of the acts which it has to perform — A similar muscle, in 

 different species of animals, presents differences of form, if the 

 function which it has to fulfil in these different species is not the 

 same — Variety of pectoral muscles in birds, according to their manner 

 of flight— Variety of muscles of the thigh in mammals, according to 

 their mode of locomotion— "Was this harmony pre-established ? — 

 Development hypothesis— Lamarck and Darwin. 



The comparison between ordinary machines and animated 

 motive powers will not have been made in vain, if it has 

 shown that strict relations exist between the form of the 

 organs and the characters of their functions ,* that this cor- 

 respondence is regulated by the ordinary laws of mechanics, 

 BO that when we see the muscular and bony structure of an 

 animal, we may deduce from their form all the characters of 

 the functions which they possess. 



It is known that the transverse volume of a muscle corres- 

 ponds with the energy of its action ; * that the athlete, for 

 instance, is recognized by the remarkable l*elief in which each 

 of his muscles stands out under the skin. But less is known 

 concerning the physiological signification of the length of 

 the muscles, that is to say, the less or greater length of 

 their contractile fibres. And yet Borelli has already given 

 the true explanation. In his opinion, as we have seen, this 

 length of red fibre is proportioned to the extent of movement 

 which the muscle is fitted to produce. 



This distinction between the contractile or red fibre and 

 the inert fibre of the tendon is of the utmost importance. 

 Experiment has shown that the muscles when they contract 

 are shortened to an extent which represents a constant frac- 

 tion of their length. We may, without erring from the truth, 

 estimate at ^ of their length, the extent to which a muscle 



