of the Leg oj the Ostrich. 271 



From this it would appear that the moments of the muscles 

 flexing the joints of the limb during the spring are represented 

 by 33-75 oz. 



This is a small amount of force compared with that employed 

 in extension and in propelling the body forwards ; but the flexor 

 muscles are aided by inertia and by a special contrivance in the 

 heel-joint that remains to be described. 



The articulating surfaces of the bottom of the tibia and top of 

 the metatarsus or cannon bone are so formed that it is easier to 

 flex the joint forwards than backwards; and this natural dis- 

 position of the joint is further aided by two admirably con- 

 trived check ligaments, shown in figs. VI., VII., which render 

 the motion forwards much easier than that backwards. 



In fig. VI. I have shown the tendons of M. gastrocnemius and 

 M. solseus, and in fig. VII. the tendons of the flexores digitorum 

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

 cnemius. 



In fig. VI. are also shown the tendons of M. tibialis anticus and 

 M. extensor digitorum communis passing through a common 

 " dead-eye '^ bolt at A — a contrivance necessary 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 position of unstable 

 equilibrium, or "dead-point," as it is called by mechanical 

 engineers, and is slowly drawn backwards beyond that point by 

 the action of the powerful extensors of the joint when they are 

 employed in making a spring. 



Any observer who examines the gait of the Ostrich, Emu, 

 Cassowary, or Rhea, in confinement, cannot fail to be struck 

 with the ungainly efiect produced, on the appearance 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 admira- 

 tion if he could see the bird in rapid motion over rough ground, 

 springing from foot to foot, and bending with the rapidity of 

 lightning the foot as it left the ground, avoiding skilfully and 

 without an apparent effort the dangers of the rough soil. I 

 have been informed by eye-witnesses that, under such circum- 

 stances, the Ostrich in South Africa almost touches the ground 

 with its body alternately on each side, in every successive spring, 

 and leaps with ease over rocks and shrubs of moderate dimen- 

 sions that lie in its onward path. 



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

 force expended in propelling the body of the Ostrich forward is 



