164: Dynamic Theory. 



pedigree of every bone, muscle and nerve, and at the same time disclose 

 not only the general habits of our variously formed ancestors, but de- 

 termine the exact nature of the stimulus to which each part has been 

 subjected, and reduce it to its mechanical value, even as an engineer 

 constructs a strain sheet for a truss bridge. As yet, we can only catch 



FIG. 86. Diagram of Arm of Man. 



h Humerus. r u Radius and Ulna. t Triceps muscle. 3 Its tendon. 6 Biceps 



muscle. 1 Its attachment to the shoulder. 2 Its attachment to the ulna. 

 While the biceps is rigid the weight puts a tension upon it and a compression strain on 

 the humerus. When the biceps contracts the weight is raised. 



on to a few of the most obvious points, but still enough to give a very 

 certain clew to the principle of differentiation, which we may safely as- 

 sert to be habit. 



And habit may be defined to be a monotonous repetition of identical 

 reactions against the same constantly recurring stimulus. The stimulus 

 is the cause of the habit, and it must be constant over a considerable 



FIG. 87. Strain Diagram of a Bridge. 



The broken lines are the parts subjected to tension strains they are the muscles of the 

 bridge. The unbroken lines receive the thrusting or compression strains and are its 

 bones. The figures indicate the amount of strain in tons on each member. (Trautwine.) 



e i, 4 tons. 

 dj, 6.5 " 

 &;) 1.0 " 

 6 /, 16.5 " 



i d, 2.8 tons. 

 1 c t 8.39 " 

 k b, 13.98 " 

 I a, 19.01 " 



a b or I k, 8.5 tons. 

 be " .7,14.75 " 

 cd "ji, 18.60 " 

 dc 19.75 " 



period of time in order to produce permanent perceptible anatomical 

 effects. 



When there is a change in the stimulus, habits change rapidly to suit, 

 and we may be sure that the effect on anatomical structure likewise 

 begins at once. But much molecular change may take place before it 

 becomes apparent to our senses or even to scientific tests. 



