230 THE INDUCTIONS OF BIOLOGY. 



nary.* The increase of power which habitual exertion gives 

 to mental faculties needs no illustration: every person of 

 education has personal experience of it. Even from 



the osseous structures evidence may be drawn. The bones of 

 men accustomed to great muscular action are more massive, 

 and have more strongly marked processes for the attachment 

 of muscles, than the bones of men who lead sedentary live*s; 

 and a like contrast holds between the bones of wild and tame 

 animals of the same species. Adaptations of another order, 

 in which there is a qualitative rather than a quantitative 

 modification, arise after certain accidents to which the 

 skeleton is liable. When the hip-joint has been dislocated, 

 and long delay has made it impossible to restore the parts to 

 their proper places, the head of the thigh-bone, imbedded in 

 the surrounding muscles, becomes fixed in its new position 

 by attachments of fibrous tissue, which afford support enough 

 to permit a halting walk. But the most remarkable modifi 

 cation of this order occurs in united ends of fractured 

 bones. &quot; False joints &quot; are often formed joints which rudely 

 simulate the hinge structure or the ball-and-socket structure, 

 according as the muscles tend to produce a motion of flexion 

 and extension or a motion of rotation. In the one case, 

 according to Rokitansky, the two ends of the broken bone 

 become smooth and covered with periosteum and fibrous 

 tissue, and are attached by ligaments that allow a certain 

 backward and forward motion; and in the other case the 

 ends, similarly clothed with the appropriate membranes, 

 become the one convex and the other concave, are inclosed 

 in a capsule, and are even occasionally supplied with syno- 

 vial fluid ! 



The general truth that extra function is followed by extra 

 growth, must be supplemented by the equally general truth, 



* In the account of James Mitchell, a boy born blind and deaf, given by 

 James Wardrop, F. R. S. (Edin. 1813), it is said that he acquired a &quot;preter 

 natural acutencss of touch and smell.&quot; The deaf Dr. Kitto described him 

 self as having an extremely strong visual memory: he retained &quot;a clear 

 impression or image of everything at which he ever looked.&quot; 



