THK HORSI-: IN MOTION. 37 



referred to in Plate V., a, a, a. The great mass of the muscle 

 which fills the angular spaces on each side the spines is called 

 into action in rearing, or supporting, the anterior half of the body 

 when not supported by one of the fore legs. The greater part 

 of the ilio spinalis is concealed in the plate by the great glu- 

 tcus, f, f, c. The centres of motion between the vertebra are in the 

 bodies of those bones which are most distant from the spines, and 

 which form the rounded ridge of the backbone as seen in the great 

 cavity of the trunk. In man they constitute the supporting column. 

 The ilio spinalis muscle lies wholly above this axis, and its action 

 abstractly would curve it downward ; it can have no influence, there- 

 fore, in aiding to support a back load. The mechanical action of 

 this long and powerful muscle is therefore, first, when they both act 

 in unison to support the anterior half of the body while the pelvis 

 is fixed by other muscles; and in the second place, when they act 

 alternately, to counteract the wrenching effect of the propulsion of 

 the heads of the thigh bones. 



Before we proceed any further with the consideration of the mus- 

 cles of locomotion, we must agree upon the signification of terms 

 necessary to be employed. 



The words " flexor " and " extensor " may be proper enough in some 

 of their applications and express fully the action, but not in all. Some 

 muscles act as flexors and extensors at the same time ; others are exten- 

 sors at one part of the stride and flexors at another ; and some of the 

 most powerful propellers in the whole machine are flexors, as we shall 

 show in the course of this treatise. It will be seen that the actions 

 of the muscular powers are sometimes quite too complicated to be 

 expressed in one word. 



The term " extensor " is commonly applied to all muscles whose 

 action is to enlarge the angles and by so doing elongate the limbs ; 

 but this extension may be forward when the foot is in the air, or back- 

 ward when the foot is on the ground. There is no word in use by 

 anatomists to express the fundamental idea, propulsion. The terms 

 " flexion " and " extension " will be used in the following pages to 

 express the action of a muscle upon its attachments, without reference 



