57 



Their total weight is 9 J oz. 



26. M. tibialis antieus. — This is a flexor of the metacarpus, and 

 arises from the two anterior tubercles of the tibia, and from the front of 

 the external condyle of the femur (Fig. 8). It is inserted below the 

 heel joint, by means of a split tendon, into the front of the metatarsal 

 bone, two inches below the joint (Fig. 10). Its weight is . ... 8joz. 



27. If. extensor digitorum communis (Fig. 10). — This muscle takes a 

 fleshy origin from the tubercles of the tibia, and from the crest in front 

 of the tibia. It is inserted into the toes, and into the annular ligament 

 of the phalangeal joints. It has no action in flexing the metatarsal 

 bone, except perhaps in some unusual positions of the foot. Its weight 

 is 5^ oz. 



28. M. poplitcBUS. — Well marked, and rotates the fibula on the 

 tibia. "Weight, f oz. 



Paet II. — Action op the Muscles. 



In the act of running, the leg of the Ostrich is to be regarded as a 

 jointed lever, having four joints, viz., the hip, the knee, the heel, and the 

 metatarsal joints. As the animal springs from foot to foot, the whole limb 

 on reaching the ground is bent as far as possible at each of the articu- 

 lations; and when the spring is made, the muscles proper to each joint 

 increase the angle made by the bones meeting at the joint, so that the 

 effect of the whole is to unbend the limb, and give it a maximum of 

 extension at the moment of leaving the ground. During the spring, 

 the antagonist muscles again bend the joints, so that on next touching 

 the ground it is at its maximum of flexion, again waiting to be unbent 

 by the muscles that open the angles of the joint ; and so on for ever, 

 as the animal runs, it is thrown alternately from each foot in contact 

 with the ground, as from a catapult, and advances by successive leaps or 

 springs from foot to foot. 



To take mathematical account of the moments of the muscular forces 

 round each joint, it would be necessary to make the following measure- 

 ments : — 



Let 0 and / be the origin and insertion of any muscle, and A 

 the centre of motion of the joint. The effect of the contraction of 

 the muscle is to develope a moment or couple round the point A, 

 acting in the plane OA I, and proportional to the weight of the given 

 muscle, as already shown in Note I. If it be required to find the total 

 effect of the muscles acting on a given joint with respect to any line, 

 A X, we should have to find the angle made by each plane of moment, 

 OAI, with the line AX ; and so calculate by the usual rule of compo- 

 sition of moments the total effect of all the forces acting, and tending 

 to produce rotation. Without going into minute detail, and taking 

 only the muscles that act perpendicularly, or nearly so, to the transverse 



E. I. a. peoc. — VOL. IX. I 



