272 ANNALS NEW YORK ACADEMY OF SCIENCES 



Propulsion and the center of gravity. — "Perfect quadrupedal locomo- 

 tion," says Stillman, 17 "requires uniform support to the center of gravity 

 (of the whole animal) and continuous propulsion by each extremity in 

 turn." In walking and running, by the straightening or extension of the 

 limbs, the center of gravity is raised and thrown in advance of the centers 

 of support. The body thus falls forward, the center of gravity describing 

 a curve of greater or less convexity, the forward motion being accelerated 

 by the thrust of the propelling limb. 



The first shock of the downward fall in the running horse is taken up 

 by the forwardly stretched and slightly bent hind limb (Fig. 1) placed 

 beneath or in advance of the center of gravity ; the gradually stiffening 

 muscles of the thigh and back checking the downward momentum ( Still- 

 man, p. 91). The rearing muscles thus come into play and serve to let 

 the fore part of the body down gently. 



Dislocation of the fully extended forelimb in landing is prevented 

 partly by (a) the crutch-like action of the limb itself (which is slung 

 from the converging fibers of the serratus magnus attached to the top of 

 the scapula) and by (5) the contraction of certain muscles of the shoul- 

 der, neck and back (Stillman, pp. 61, 62, et seq.). To these arrange- 

 ments and conditions, Stillman attributes the absence of the clavicles in 

 the horse. 



The center of gravity in the smoothly trotting horse describes a rela- 

 tively flat trajectory, whereas in the "bounding" movement, or gallop, the 

 center of gravity ricochets and, the trajectory consists of a series of 

 cycloids of marked convexity. This mode of locomotion, while very 

 rapid for short distances, is too wasteful for heavy-bodied animals, which 

 require a relatively flat trajectory and a maximum saving of inertia. 



Sinuous movement of the body in running. — By the bending backward 

 of the pelvis, first on one side and then on the other, the thrusts of the 

 femora are brought more nearly into line with the anteroposterior axis, 

 while wrenching of the pelvis is prevented by the contraction of the 

 longissimus dorsi of the opposite side (Stillman, p. 36). By this means 

 also, the length of the stride is directly increased. The same sinuous 

 motion of the body is associated with the "figure-of-8" movement of the 

 limbs noticed by Pettigrew. 18 . 



In this connection may be noted also the devices for avoiding "interfer- 

 ence" of the limbs (e. g., "stifle action" of the iliacus, preventing the knee 

 from striking the abdomen; oblique trochlea of the astragalus carrying 

 the advancing foot around its fellow of the opposite side). Dislocation 



» Op. cit., p. 87. 



1S Animal Locomotion, p. 39. 12mo. New York, 1874. 



