30 life's beginning on the earth 



influence of various conditions. By simply stretching the 

 gelatin, the "cells" composing the network arrange them- 

 selves in a line, giving the whole the aspect seen in Figure 

 14, which shows the appearance of stripes in the direction 

 of the tension. 



Stripe formation is visible also in quite a few living struc- 

 tures such as fibers and tendons. As is well known, a ten- 

 don is a connecting band in the human body which is natur- 

 ally subjected to pull and stress. The stripe formation 

 which it exhibits when seen under the microscope is the 

 result of this stress, and by no means a purposeful arrange- 



Fig. 14. Microscopic View of a Stretched Thread of Gelatin 



This photograph shows that by stretching the network structure of the 

 gelatin, it has taken on a more striped appearance. (Magnified 700 times.) 



ment to increase its strength. Similarly in the bone, the 

 solid bony material is not evenly distributed, but collected 

 or condensed in a network of thin sheets. The arrange- 

 ment of this network coincides exactly with the lines of the 

 greatest compression which the bone suffers under the load 

 which it has to carry while supporting a part of the human 

 body. 



The story is told of an engineer who happened to see a 

 sketch showing how this bony material was arranged in (he 

 thigh bone (femur). He was at once impressed with the 

 resemblance of this arrangement to stress lines of which he 

 had made extensive studies in his mechanical devices. He 

 made a sketch of the landmarks of the thigh bone, and 



