172 THE CIVIL ENGINEERS OF THE BODY 



retain connection with one another by elongated processes giving 

 the whole tissue the appearance of an attenuated sponge filled 

 with a gel. The cells apparently secrete a colloid in a non- 

 hydrated form which then swells up to form a gel by the imbibition 

 of water. A model of this process may be made by adding water 

 to a mixture of gum and oil (Part II.). 



As development advances the cells of this mucoid tissue become 

 longer and more spindle shaped (fibroblasts). The fibres are of 

 two classes, differing from one another in chemical constitution 

 as well as in physical properties. 



(a) The white fibres are delicate transparent non-elastic fibres 

 arranged in bundles which do not branch. 



(b) The yellow fibres are highly refractile elastic fibres which 

 branch and anastomose with one another. (Feebly but perfectly 

 elastic.) 



The difference in their physical properties may be explained 

 by their different chemical constitution. The former are composed 

 mainly of a non-elastic protein collagen which readily takes up 

 water to form gelatine. The latter have in place of collagen 

 another sclero-protein elastin. Though difference in chemical 

 constitution may explain difference in physical properties, it does 

 not make any clearer how such a difference is brought about. If 

 elastic and non-elastic fibres existed side by side in definite 

 proportions one could easily mimic the formation by the separation 

 of two colloids from a colloidal matrix. But there is no such 

 definite proportion. Some tissues, e.g. tendons, are almost 

 entirely composed of white fibres, while elastic fibres predominate 

 in ligaments. In short, white fibrous tissue is found where 

 binding power alone is required, and where elasticity as well as 

 strength is desirable, there one finds elastic fibrous tissue. The 

 function of the tissue governs its form. 



Just exactly how function governs form, one cannot at present 

 say. There is no doubt that external physical forces do affect 

 chemical actions and internal physical properties. Material 

 under strain acts quite differently from the same matter unacted 

 on by any force. An almost non-elastic block of rubber may be 

 endowed with considerable extensibility by being worked with. 

 The optical properties of glass can be altered by submitting it to 

 pressure. The electrical conductivity of selenium depends on 

 the amount of light falling on it. When more is known of the 

 laws governing matter in the colloidal state, then one may be 

 able to give a clear answer to this problem. 



