140 



THE STRUCTURE OF THE HORSE 



together at the joints, in being of a yellow colour and 

 in having in a very marked degree the property of 

 elasticity. It will bear considerable stretching, and 

 then will return again to its normal length, which a 

 true ligament composed of ordinary white fibrous tissue 

 will not do. 



Elastic material is often made use of in the animal 

 economy to great advantage mechanically, restoring 

 without effort to its proper position a part which has 

 been temporarily disturbed from it, and thereby saving 

 a vast expenditure of muscular power. The valves of 

 an oyster or a cockle-shell are opened by an elastic hinge 

 and closed by contraction of a muscle placed between 

 them. Under the ordinary conditions of life it is neces- 

 sary that they should remain open in order that the 

 water containing air and nutriment may pass freely over 

 the gills and mouth of the animal. They only need to 

 be closed occasionally on the approach of some danger- 

 ous enemy. To close the valves and to maintain them 

 in this position requires an effort ; directly this effort is 

 relaxed they open again by the simple mechanical effect 

 of the elastic ligament. If they had to be opened and 

 maintained in the open position, by a muscular effort, a 

 far greater expenditure of power would be required on 

 the part of the animal. In the same way in our own 

 breathing, in which the air is alternately drawn in and 

 expelled from the lungs by the expansion and contrac- 



