FIBROUS TISSUE 



171 



surface is compressed while the lower is stretched. There- 

 fore midway between the upper and lower surface lies a neutral 

 zone or line of no stress and in its neighbourhood the material 

 needs to have little strength. The girder maker can therefore 

 quite safely cut away the centre of his beam, leaving only the upper 

 and the lower surfaces, and of course some connection between 

 them which may be almost as thin as he likes without destroying 



BEAM UNLOADED 



BEAM LOADED 



FIG. 29. To show lines of compression (dark) and lines of tension (dotted) in a 

 loaded rectangular beam. The clear space between the strut-lines and the tie lines 

 indicates the neutral zone. 



the strength. In other words, if the engineer can map out the 

 lines of stress or directions of compression and tension in the 

 loaded structure, all the manufacturer has to do is to see that these 

 lines lie in his material ; all the rest may be cut away. 



By means of the truss (Fig. 30) the simple girder becomes a tie 

 between two struts. The horizontal member of the truss 

 undergoes tension only, while the sloping beams are compressed. 



LOAD 



FIG. 30. A simple triangular roof truss. 



Such a structure permits of the use of two kinds of material 

 matter with a high tensile strength for the tie and matter able to 

 bear up under compression for the struts. 



The two principal connective tissues are fibrous tissue and 

 cartilage and their modifications. Fibrous tissue is the main 

 binding medium of the body. It is derived from the mesoblast 

 of the embryo. The cells of the mesoblast, which are typical 

 spherical bodies lying close together, are gradually pushed apart 

 by a clear transparent jelly-like exudant from the cells. They 



