168 THE CIVIL ENGINEERS OF THE BODY 



II. Connective Tissues. 



To appreciate the significance of the structure of the vegetative 

 tissues, due attention must be directed to their function. These 

 tissues are cell communities with an important but little studied 

 industry. They are the civil engineers of the body. The structures 

 they build are designed to stand stress and strain. Before 

 critically examining their handiwork let us study some elementary 

 engineering problems, so that we may the better understand the 

 phenomena of cell structures. 



All the tissues of the body are more or less elastic. This 

 property includes (a) change of form under the action of some 

 force and (b) the return of the body to its original form when the 

 deforming force ceases to act. 



The elasticity of connective tissues plays an important part in 

 the body. (1) It is a permanent resistance to permanent distorting 

 forces such as muscular tension and gravity. The elasticity of 

 the intervertebral discs, and of the ligamenta subflava, assists in 

 maintaining the erect posture of the body. (2) The form of 

 tissues is preserved against distortion by temporary forces 

 (Buffer action) and by intermittent forces. The elasticity of the 

 costal cartilages and of the ribs restores the chest wall to its 

 original position when the inspiratory muscles relax. (3) Inter- 

 mittent movement is transformed into a continuous movement 

 by transmission through an elastic medium (see circulation). 

 (4) Elasticity economises muscular work by coming into play 

 in the intervals between successive shocks (Marey). 



The amount of elasticity is determined by ascertaining the force 

 necessary to change the form of the object to which the force 

 is applied, and the perfection of the elastic action, by the quickness 

 and accuracy with which the object returns to its former condition 

 on release from the mechanical force. Thus, when a strong force 

 is required to produce deformation of a body, that body is said 

 to be highly elastic. On the other hand feeble elasticity is over- 

 come by the application of a feeble force. 



The force which is applied to a body is termed by engineers 

 the strain. The resistance to strain by a body necessitates the 

 postulation of a stress within the body. Hooke (1660) stated 

 that stress was directly proportional to strain. As a formula 

 this is known as Young's Modulus, 



stress _T7 / .-, load on specimen 4- area of cross-section 



strain alteration in length 4- original length 



