62 PROCEEDINGS OF THE ACADEMY OF [1887. 



When a limb of a terrestrial quadruped rests UY>on the ground it 

 may be said be " on," and when not on the ground, to be " off." 



The term "sura" will be employed as a convenient word to 

 include the hind limb from the knee to the ankle. " Cms" has been 

 retained so generally as a synonym for the entire posterior extremity 

 as not to be available. 



" Stroke " is the period of impact. It is included in flexion, and 

 constitutes its first stage. "Recover" embraces the last stage of 

 flexion and the whole of the period of foreward movement. The 

 terms "stroke" and " recover " are by no means the same as flexion 

 and extension. They simply express certain phases of limb-function 

 which are seen during the acts of backward and foreward movements. 



THE POSITION OF LIMBS. 



In studying the motions of the limb of a vertebrate the position 

 which answers to that taken by the salamander, Avhen at rest, is 

 assumed to be the best adapted for comparison. In this position 

 the limb is horizontal to the plane of the longitudinal axis of the 

 body. The venter of the body and the ventral surface of the limb 

 are on the same plane nearly. The limb of a reptile varies scarcely 

 at all from that just named. When a terrestrial animal is erect the 

 limb instead of being on the same plane with that of the body is moved 

 a quarter of a circle downward. In the bird the posterior extremity 

 when at rest is in the same position as the terrestrial, but the ante- 

 rior extremity, in marked contrast to it, is flexed. When extended 

 the extremity is thrown upward to a position as far removed from 

 the horizontal position of the salamander in one direction as is that 

 of the terrestrial quadruped in the other. 



In the movement of all limbs the directions in the main are for- 

 ward and backward. Both the movements are oblique but between 

 them is a position which is straight. In the terrestrial animal 

 this position may be said to answer to a line in the anterior extremity 

 which lies immediately in advance of the withers and in the posterior 

 extremity to the centre of the acetabulum. 



THE MOVEMENTS OF LIMBS. 



If a limb can be conceived moving in vacuo it can he at once 



understood that propulsion is impossible. For propulsion can follow 



.only upon the initiation of an impetus and this in turn only by 



the resistance of the limb against the medium in which the animal 



