210 THE CIVIL ENGINEERS OF THE BODY 



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 exudate from the cells. They 

 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 

 which do not branch arranged in bundles which do branch. 



[b) The yellow fibres are highl}^ retractile elastic fibres which 

 branch and anastomose v/ith one another. The fibres are formed 

 by the coalescence of granules which appear in rows in mucoid 

 tissue subjected to intermittent stretching. They have no obvious 

 connection with any cell and are 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 eaplain difference in physical properties, it does 

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

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

 portions 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 goNcrns 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. 



