98 ANIMAL MECHANISM. 



sequently, under sucli conditions, the twin muscles lose only 

 a part of the length of their fibres ; they undergo what sur- 

 geons call partial fibrous transformation, a modification which 

 is only a change of proportion between the red fibre and the 

 tendon. 



Those who are accustomed to regard pathology as a com- 

 plete infraction of physical laws, will perhaps be astonished 

 to see us search among these cases of dislocation and anchy- 

 losis for the proofs of a law which regulates the form of the 

 muscular system in its normal state. It would be easy to 

 show that these scruples have no foundation ; but it will be 

 better still to bring forward other examples which may not lie 

 open to the objections so often urged against the applications 

 of medicine to physiology. 



It is again from J. Guerin, that we must quote the facts 

 of which \ve are about to speak. 



When we examine the muscular system at different periods 

 of life, we find that it varies greatly in its aspects. It seems 

 that the muscles have di.-tinct ages, and that, formed at first of 

 contractile substance, they lose by degrees, as they grow 

 older, their red fibres, which are replaced by the white and 

 glistening fibres of the tendon. 



Thus, the diaphragm of a child is principally muscular, 

 while in the old man the aponeurotic centre, the true tendon 

 of the diaphragm, is extended at the expense of the contrac- 

 tile fibre. The substitution of tendon for muscular fibre is 

 still more marked in the muscles of the leg in infancy; they 

 are relatively much more rich in contractile substance than 

 during adult age. In the old man, in fact, the tendon seems 

 to invade the nm de, so that the portion of the calf of the leg 

 which remains is placed very hijji, and is very reduced in 

 length. The muscles of the lumbar and dorsal regions present 

 the same character ; in old age they are poorer in red lilire, 

 but richer in tendon. 



What then, is ih ' change which takes place in the muscular 

 f'liicti-in during the diil'eient periods of life ? Kvery one knows 

 that, except in the very rare cases in which the man keeps up 

 the habit of gMimastic exercises, the muscular function be- 

 comes UK re and more restricted at least, as far as the extent 



