VARIABILITY OF THE MUSCULAR SYSTEM. 99 



of movement is concerned. The articulations of the limbs, and 

 those of the vertebral column, undergo normally a sort of 

 incomplete anchylosis, which continues to lessen more and 

 more the flexibility of the trunk. 



Look at a young child tossing about at his ease : one of his 

 movements is to play with his foot ; to take it in his hands 

 and carry it to his mouth appears to him very natural, and as 

 easy as possible. In the adult, the muscular force attains its 

 maximum ; but the movements are not so extensive as in 

 infancy ; man has no longer, as is well known, the same 

 flexibility in his limbs. 



The old man can neither stoop readily nor completely 

 draw himself up ; his vertebral column has lost its supple- 

 ness ; he takes only short steps ; to sit down on the ground, 

 with the knees raised, is to him extremely difficult ; and if 

 we examine the extent of flexion and extension in his foot, 

 we find that it has become very limited. 



The function of the muscles, therefore, changes with the 

 different periods of life, and becoming more and more restricted, 

 employs continually less contractile fibre. It is thus that the 

 muscular modification of which we have been speaking is 

 naturally explicable. This modification, which consists in the 

 increase of the tendinous element at the expense of red fibre, 

 may be prevented by keeping up the extent of muscular 

 movements, by means of suitable exercise. 



Let us now return to comparative anatomy. Since it 

 shows us perfect harmony between the form of the muscles in 

 different species of animals and the characters of muscular 

 function in the same species, the most natural conclusion seems 

 to be that the organ has been subjected to the influence of 

 function. 



If the race-horse is modified in its form by the special exer- 

 cise which is called training, is it not an evident proof of the 

 influence of function on the anatomical characters of the 

 organism? And if a species, thus modified artificially, 

 returns to the primitive type when replaced under the con- 

 ditions from which it had been taken, is it not the counter- 

 proof of the theory which assigns to function the office of 

 a modifier of the organ ? 



