272 Mr. H. G. Seeley on the Origin 



of the bone, and the articular cartilage itself gradually be- 

 comes thinner. The action of the lungs moves the muscles 

 which are attached to the ribs, and in some cases in this way 

 greatly modifies the form of the bones. 



Another example of a mechanical influence is seen in the 

 blood. The weight of blood in the body is not great ; but it 

 is the amount of nutriment sufficient to maintain healthy dia- 

 lysis in all the tissues. The whole of the blood makes its way 

 into the lungs, where it apparently loses bulk and gains tem- 

 perature. Under the heated condition it is driven through the 

 body in the arterial vessels by the left ventricle, and therefore 

 exercises an intermittent (pulsating) pressure not only upon 

 the arteries themselves, but, in a quieter way, upon the tissues 

 adjacent to them. That this muscular power has a mechanical 

 effect upon growth is shown in the heart itself, by growth being 

 continuous throughout life. The return of the blood to the 

 heart is facilitated by its decreased temperature lessening its 

 bulk, by the material left in the tissues, as well as by the 

 pump-action which passes it into the lungs and enables the 

 lymphatics to pour in new material. Evidence of its mecha- 

 nical power in producing growth is well seen in the thickening 

 of arterial walls in the condition named aneurism. 



These are some of the chief mechanical engines of the body 

 which are capable of influencing the skeleton. That they 

 actually produce by their mechanical action the phenomena of 

 growth which theoretically they are sufficient to produce is 

 not capable of elaborate proof in the healthy individual, be- 

 cause, from the deep-seated position of the changes, they can- 

 not usually be observed. Yet, in the case of athletes and 

 gymnasts, it is observed that, with exercise, the whole body 

 becomes heavier, and the circumference of the chest perma- 

 nently greater ; and often special muscles are seen in a short 

 time to augment visibly. This may be observed in tlie legs 

 of women who dance and the thighs of men who ride. But to 

 see the effisct upon the bones, it is necessary to contrast the 

 skeleton of the wild animal, where the muscles are used with 

 great power, with the skeleton of the tame animal, where the 

 muscles have more limited action ; and then it will be seen 

 that powerful crests and processes on the bones are developed 

 in direct proportion to muscular activity. Moreover Professor 

 Humphry finds that bones are densest in those parts which 

 are subject to the greatest mechanical stress, and hardest in 

 those persons who are strongest and most active — and that 

 the bones are most curved in those persons whose muscular 

 strength is greatest, while weak persons, on the contrary, 

 have comparatively straight bones. But, important as this 



