PHYSICAL PROPERTIES OF TISSUES. 67 



Elasticity is a property of the tissues cf the animal body which is 

 of great importance in many physiological operations. It is a force 

 which acts either against constant forces, such as gravity, or temporary 

 forces, such as muscular action. Thus, the intervertebral disks, through 

 their elasticity, serve to deaden the shock given to the spinal column in 

 jumping; the elastic ligaments of the spinal column serve to preserve it 

 in its normal position without there being a constant strain on the muscles, 

 and in animals in whom the backbone is horizontal it serves to counteract 

 the weight of the abdominal viscera. In the herbivorous animals the 

 yellow elastic tissue of the ligamentum nucha' serves to assist the muscles 

 in supporting the head. 



In expiration the elasticity of the costal cartilages and ribs, together 

 with that of the lungs, — forces which have to be overcome in inspiration, 

 — tend to restore the thorax to its natural form, and thus drive the air out 

 of the lungs. 



The elastic tissue of the arteries tends to aid the intermittent pro- 

 pelling force of the heart in producing a constant forward motion of the 

 blood. "When the heart contracts it drives a definite quantity of blood 

 into the arterial system, already filled with blood, and thus still further 

 distends the arteries. During the pauses between the contractions of the 

 ventricles the elastic tissue recoils, from the removal of the distending 

 force, on the contents of the blood-vessels, and, backward motion being 

 prevented by the closure of the semi-lunar valves, drives the current of 

 blood forward in the vessels. This point will again be alluded to in more 

 detail under the subject of the Circulation. 



In addition to these properties most of the tissues of the animal 

 body are also flexible and extensible, the degree varying greatly accord- 

 ing to the structure of the parts. Flexibility and extensibility must not 

 be confounded. Flexibility means capability of being bent or twisted ; 

 extensibility means capability of being increased in length. Thus, the 

 tendons are flexible, but not extensible ; were they capable of being in- 

 creased in length it would be at the expense of the force developed by 

 muscles. Tendons are, however, very flexible; they adjust themselves 

 to the position the part may occupy, so that sometimes they transmit 

 muscular force at right angles to the line in which the muscle acts. Liga- 

 ments, again, are flexible, and also somewhat more extensible than tendons. 

 In joints they permit of the free play of one bony surface on the other, 

 and yet by their inextensibility serve to keep the articular surfaces in 

 apposition. In dislocations the articular ligaments are rent, and the bony 

 articular surfaces are no longer in contact : in sprains the limit of elas- 

 ticity of the ligaments has been passed; that is, they have been stretched 

 beyond the point at which their elasticity enables them to regain their 

 original form, and partial ruptures take place. 



