87 



webs altogether. We thus reach the principle of 

 the hollow column, and the hollow column, as every 

 one knows, is one of the commonest structural devices 

 adopted in engineering, 

 in shipbuilding and 

 architecture. If we 

 examine the long bones 

 of the body, we find 

 that they are all hollow 

 columns, combining 

 the maximum of 

 strength with the 

 minimum weight of 

 material (Fig. 39). 

 The long bones are, it 

 is true, in the em- 

 bryonic state, solid, 

 but are hollowed out 

 by certain cells which 

 have this special duty 

 to perform. 



In the case of the 

 plant, as an examina- 

 tion of erect, and, at 

 the same time, slender, 

 stems shows, the sup- 

 porting or skeletal 

 tissue is laid down on 

 the same principle. 

 For example, the stem 

 of such a plant as 

 wheat is a hollow column, and in other plants, 

 the special skeleton tissue is peripherally placed 

 in flanges, kept apart and yet held together 

 by more delicate central tissue. The varieties in 



FIG. 39. Longitudinal section of 

 human thigh bone. ( Natural 

 size.) 



