46 MATERIALS FOR A MEMOIR OX 



the rabbit aud the kangaroo it is easily understood how by the 

 simple extension of the tarsus from an acutely flexed position, 

 followed by the flexion of the digits, the body is sent upward and 

 forward ; but the leg of the dog is already extended at the time 

 that the "spring" is initiated. If, as is asserted by practical men, 

 the horse is prepared for the " spring" by the same succession of 

 foot-falls as in the gallop or run, it is only necessary to look upon 

 the last limb which leaves the ground as though it were engaged in 

 preparing the leg for a recov^er from an average stroke. Since the 

 eversion of the foot presses the inner border against the ground, 

 the inner aspect of the limb at the knee is directed inward and 

 subjected to strain. But the force generated by this strain is un- 

 expended at the time that the foot is raised from the ground, so 

 that the limb might be compared to a coiled spring which has not 

 lost its power, and the stored -up energy is engaged to propel the 

 body forward the moment that the inner border of the foot leaves 

 the ground. While acknowledging that this is an hypothesis, it 

 remains the best of the surmises which occur to the observer. 



It has already been seen that when the limb is in the position 

 of arrest and the momentum carries the body beyond the perpen- 

 dicular line it is thrown into " backward strain." The instant 

 that the strain beg-ins the knee is seen to move outward and the 

 hock to move inward. The parts of the foot below the heel re- 

 main unchanged. The impact of the structures of the limb is 

 thus impaired in backward strain. It is well known that in the 

 pentadactyle forms the foot can be readily rotated at the medio- 

 tarsal joint, and it is a reasonable assumption that it is at this joint 

 that the distal part of the limb moves when the entire limb rotates 

 outward. The femur, the bones of the leg, and the astragalus act 

 as one factor, and the calcaneum and the remaining bones of the 

 foot as the other factor. The socket for the proximal motion 

 occurs at the hip, and that for the distal at the concavity of the 

 scaphoid bone. There is also considerable motion between the 

 calcaneum and the cuboid bone and between the calcaneum and 

 the lower end of the fibula, if this bone is present, or with the 

 outer end of the tibia if it is absent. Outward rotation of the 

 main portion of the limb carries the calcaneum slightly inward 

 by reason of the articulation between the calcaneum and the bones 



