150 THE STEUCTUEE OF THE HOESE. 



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 contraction of the chest-walls, nearly half the 

 labor, with corresponding expenditure of energy and 

 waste of muscular tissue, is saved by the application 

 of elasticity as the principal cause of the contraction 

 which follows each muscular effort by which the act 

 of expansion is performed. 



As already mentioned, the horse's head, owing 

 chiefly to the immense apparatus required to grind 

 its necessary supply of food, is of great weight, and 

 if it had to be supported at the end of the long neck 

 entirely by a muscular effort on the part of the ani- 

 mal, great expenditure of force, requiring a still 

 larger supply of food to keep it up, would take place. 

 But, thanks to the structure, attachments and phys- 

 ical properties of the cervical ligament, it is no effort 

 whatever to the horse to keep its head in the proper 

 position. In fact, this ligament is so disposed, and 

 of such strength and elasticity, as to allow the head 

 to be moved up or down or from side to side, as re- 

 quired, by a very slight exertion of muscular action, 



