60 



Fit?. 12- 



gitorum communes, passing under and through the tendon of M. gastro- 

 cnemius. 



In Fig. 1 1 are also shown the tendons 

 of If. tibialis anticus, and M. extensor di- 

 gitorum communis, passing through a com- 

 mon "dead eye" bolt — a contrivance ne- 

 cessary to keep them in their place on the 

 flexure of the heel joint, and corresponding 

 with the annular ligament of the human 

 foot. In consequence of the arrangements 

 just described, the foot of the Ostrich is 

 jerked forward suddenly, after passing a po- 

 sition of unstable equilibrium, or " dead 

 point," as it is called by mechanical engi- 

 neers ; and is slowly drawn backwards be- 

 yond that point by the action of the powerful 

 extensors of the joint, when they are em- 

 ployed in making a spring. 



Any observer who examines the gait of 

 the Ostrich, Emu, Cassiowary, or Ehea, in 

 confinement, cannot fail to be struck with 

 the ungainly effect produced on the appear- 

 ance of the animal, when walking slowly, 

 by the abrupt and odd-looking lifting of the 

 foot caused by the structure of the heel joint, 

 giving an effect like that of string halt in 

 horses ; but this unpleasing impression 

 would be rapidly converted into admiration, 

 if he could see the bird in rapid motion over 

 rough ground, springing from foot to foot, 

 and bending the foot as it left the ground 

 with the rapidity of lightning, avoiding skilfully, without an apparent 

 effort, the dangers of the rough soil. I have been informed by eye- 

 witnesses, that under such circumstances the Ostrich in South Africa 

 almost touches the ground alternately on each side with its body, in 

 every successive spring, and leaps with ease over rocks and shrubs of 

 considerable dimensions that lie in its onward path. 



I have shown in the preceding part of this paper, that the force ex- 

 pended in propelling the body of the ostrich forward is ten times the 

 force employed in restoring the legs of the animal, preparatory to its 

 next spring; more exactly, as 335*5 to 33*75. This enormous force 

 acts upon bony supports, which are required by the necessities of a 

 bird's existence to be as delicate and light as possible ; and it is, 

 moreover, exerted almost instantaneously, so as to give the fullest 

 effect to the bird's spring. The question naturally arises, how is this 

 force to be suddenly applied, without breaking or dislocating the ani- 

 mal's legs ? 



The answer to this question is to be found in the peculiar structure 



OUTER SIDE OF LEFT HEEL OF 

 OSTRICH. 



a, Tendon of flexor digitorum 



communis. 



b, gastrocnemido- 



solceus. 



