6 STRUCTURE AND FUNCTION 



flexion and extension of which locomotion is accomplished ; the 

 whole procedure being under the control of the nervous system. 

 Muscular Action. — In accomplishing locomotion, the part 

 played by the muscles is fundamental. The chief characteristic 

 of muscular tissue is its contractility. Contraction takes place 

 on receiving a stimulus through the voluntary nerve, which ter- 

 minates between the two extremities of the muscle. One end of 



Fig. 4. — Situation of the centre of gravity in the horse. 



the muscle has a fixed insertion ; the other is attached to a mov- 

 able bone lever. The two bones are apposed end to end, by means 

 of a joint, the surfaces of which permit of motion of one or 

 both bones, the degree or amount of motion depending upon the 

 kind of articulation. When, therefore, the contracting muscle 

 draws the movable bone to which it is attached toward the bone 

 to which it has it§ fixed insertion, either flexion or extension of 

 the joint takes place. 



