51 



the moment of leaving it ; and also, that the minimum possible amount 

 of force shall be expended in flexing each leg, preparatory to its next 

 spring, which must take place on touching the ground. 



I shall show that the leg of the Ostrich fulfils these two conditions 

 of perfect mechanism ; but, before doing so, I shall describe in detail 

 the muscles employed in the act of running by the Ostrich. 



Paut I. — Description op the Muscles. 



1 . A muscle, which must be regarded as a combined tensor vagina 

 femoris, gluteus maximus, and agitator caudce, takes its origin from the 

 whole length of the posterior edge of the ilium (and first two caudal 

 vertebrae) forwards to the commencement of the symphysis iliaca, a, a, a 

 (Fig. 6), and is inserted as follows : — 



(a). The tensor vagina portion is inserted into a broad tendon, pass- 

 ing over the outer side of the knee ; this tendon forming also one of 

 the origins of the gastrocnemius. 



(/3). The gluteal portion of the muscle is inserted, partly like (a), 

 and partly by a muscular slip, into the middle of the vastus externus. 



(7). The caudal portion of this muscle is inserted into the posterior 

 border of the tendon (a), near the knee : the weight of the whole 

 muscle is 46f oz. 



2. If. gluteus medius has the origin marked h, b, in Pig. 6, on the 

 surface of the ilium, and is inserted in the small pit, on the outside 

 of the head of the femur (Fig. 7), and rather behind ; its weight 

 is 13J oz. 



Fig. 6. 



LEFT SIDE OF 



a, a, a, Origin of tensor vagina* femoris, 



glutceus maximus, and 

 agitator caudce, 



b, b, . . glutceus medius. 



c, c, . . ,, glutceus minimus. 



d, . . ,, iliacus. 



e, . . . ,, rectus femoris. 



f, . ,, opponens quadrato fimo- 



ris (or iliocapsularis). 



g, • • 



Origin of sartorius. 



h, h, . 



,, biceps femoris. 



i, i, . 



semimembranosus. 



hh • 



,, semitendinosus. 



k,k, . 



„ adductor magnus. 



1, . . 



„ pectinceus. 



m, m, 



. Obturator foramen. 



n, 



. Ischiadic foramen. 



0, 0, . 



. Origin of gemelli. 



