MtJSCULAR SYSTEM. 169 



the trunk being supported by these members alone in walk- 

 ing. From the extent, and the fixed condition of the dorsal 

 part of the pelvis, the extensors of the femur have a high 

 origin and a lengthened form, and the glutens maximus, me- 

 dius, and minimus are developed in this region, as in qua- 

 drupeds. The glutens majcimus here reaches along the femur, 

 as low as the fibula, passing below and behind the knee-joint. 

 The only distinct abductor of the femur is the gemellus supe- 

 rior^ which rotates the thigh outwards, and this muscle is 

 deficient in many aquatic birds. The pyriformis, quadratus 

 femoris, and obturator externus extend the femur, and are 

 aided by a muscle analogous to the pectineus. The peroncus 

 longus and the tibialis posticus, arising from the back part of 

 pelvis, are connected below with the extensor tendon of the 

 heel. The rectus femoris arising from the upper part of the 

 pubis, continues its long tendon over the extensor tendon of 

 the knee, and unites below with the common perforated flexor 

 of the toes. The gracilis arises very low from the femur, and 

 acts as an extensor of the knee, and the popliteus, directed 

 transversely, is situated high on the tibia. The foot is raised 

 by the tibialis anticus, which passes from the tibia to the 

 metatarsal bone, and rotates the foot inwards in the climb- 

 ing movements of the zygodactylous birds ; these birds have 

 likewise the peroneus brevis of considerable size, which arises 

 from the exterior of the tibia and fibula, and rotates the foot 

 outwards. The gastrocnemius, (Fig. 76. u,) arises from the 

 condyles of the femur, and passes down thick and fleshy to 

 the broad aponeurotic tendon of the heel, which forms a 

 groove for the passage of the flexor tendon of the toes, and 

 is inserted below into the long metatarsal bone. Behind the 

 tibialis anticus (Fig. 76. r,) arises the fleshy head of the ex- 

 tensor longus digit orum, (Fig. 76- ,) the tendon of which, 

 bound down by strong ligaments in its course, divides at the 

 lower end of the metatarsal bone, into three separate ten- 

 dons, which proceed along the upper part of the toes to be 

 inserted into the base of each terminal phalanx. The pos- 

 terior toe has a distinct extensor muscle which descends from 

 the upper part of the metatarsal bone, and is inserted into 

 both phalanges. The long flexor of the toes descending from 

 the upper part of tibia exterior to the gastrocnemius, is con- 

 nected with two cartilages at the heel-joint, which represent 



