220 ANATOMY OF VERTEBRATES. 



outer side, 34, to the triceps extensor cruris, and more especially 

 to the rectus femoris, as having its chief origin from the ilium ; 

 whilst its tendon expands over the fore part of the knee joint, as 

 that of 23 passes over the fore part of the elbow joint ; and both 

 without having any sesamoid lever developed therein. 



The retractors or extensors of the thigh and leg, ib. 33, answer- 

 ing to the retractors of the arm and fore-arm, 24, arise from the 

 hinder and outer part of the ilium, and are inserted partly into the 

 femur, partly into the outer part of the head of the tibia. A 

 muscle, fig. 141, 38 (sacro-plantaris}, forming part of this system, 

 has a special extent and disposition, favouring the effective back- 

 ward stroke of the foot in swimming : it arises from the sacral rib 

 and is inserted into the plantar fascia. It is a ' flexor ' of the leg, 

 like the ' biceps flexor cruris :' it is an ( extensor ' of the foot, like 

 the ' plantaris.' And here a few remarks may be offered on the 

 terms ( flexion ' and ' extension,' as applied to the ( fore-arm ' and 

 6 leg; ' in higher air-breathing Vertebrates and in Man. 



C? ~ ~ 



The fore and hind limbs of the Salamander are figured extended 

 in corresponding positions, in fig. 140, as those of the Plesiosaurus 

 are represented in fig. 45. The ulna is external or posterior in the 

 arm, the fibula in the leg. If, in the dead newt, the fore-arm be 

 moved upon the arm to and fro, in the direction of the trunk's axis, 

 it can be bent at an angle with the arm either way ; and the like 

 would most probably be the case in the Plesiosaur : there is 

 no bony configuration of the elbow-joint to prevent this in either 

 reptile ; only the ligaments favour the forward bend more 



than the hinder one, in the batrachian. In the hind limb the les; 



o 



can be bent at an angle with the thigh, both forward and back- 

 ward; but the ligaments of the joint offer more resistance to the 

 forward than to the backward bend. As we ascend the verte- 

 brate scale in the comparison of limbs, a bone of the fore-arm 

 sends a process across the back part of the elbow-joint which 

 fits into a cavity in the bone above the joint when the two are 



brought into the same line ; and the fore-arm cannot be bent 



~ * 



back at an angle with the arm without fracture of the inter- 

 locking bar or ' olecranon.' In the leg the contrary bend, at an 

 angle forward upon the thigh, is prevented by configuration of 

 the knee-joint, with interarticular cartilages and ligaments. 



Thus the forward bend is favoured in the fore-limb ; the back- 

 ward bend in the hind limb. 



In quadrupeds the limbs are habitually retained with the first 

 and second segments more or less bent in the directions favoured 

 by the configuration of the elbow-joint and knee-joint respec- 



