40 MANUAL OF ELEMENTARY ZOOLOGY 



The bones of the leg correspond closely to those of the 

 arm. The thigh-bone or femur has a long, slender, slightly 

 curved shaft with a rounded head to fit into the acetabulum 

 and a wide condyle for articulation with the shank-bone, 

 os cruris, or tibio-fibula. The latter, like the radio-ulna, 

 corresponds to two bones in man and many other animals, 

 showing traces of being formed by the fusion of an inner 

 or anterior shin-bone or tibia and an outer or posterior 

 fibula. The ankle, like the wrist, consists of two rows of 

 bones, which are here called tarsals. The first row contains 

 two bones, the tibiale, astragalus, or talus and the heel-bone, 

 fibulare, or calcaneus. These bones are joined at each end 

 by a piece of cartilage. The second row consists of two 

 small distal tarsals. The metatarsus contains six meta- 

 tarsals, one minute and corresponding to a small extra toe, 

 the prehallux or calcar, which lies inside the first toe or 

 hallux, but does not project from the foot. The calcar has 

 one phalanx, the first two toes have each two, the third and 

 fifth toes three, and the fourth toe, which is the longest, has 

 four. 



It will be seen that the fore- and hind-limbs and girdles 

 are built upon a common plan. The skeleton of this is 

 shown in Fig. 19. It may be traced in all animals which 

 are pentadactyle — that is, have fingers and toes. Neither of 

 the limbs of the frog conforms to it exactly. 



The movements of the body and of its organs are brought 

 about by means of a tissue known as muscle. 

 This tissue is classed according to its function 

 as voluntary when it is under the direct control of the 

 will and involuntary when it is not under such control. 

 Involuntary muscle generally forms part of the wall of 

 some internal organ such as the stomach, bowel, bladder, 

 or heart, and by its contraction brings about changes in 

 the width of this organ and thus movement of the fluid 

 it contains. Voluntary muscle is usually found in the 

 form of distinct organs or muscles, which are attached at 

 their ends to two parts of the skeleton and by their 

 contraction change the relative position of these parts 

 and thus of the regions of the body which they support. 

 Sometimes the end of a muscle may be attached by a stout 

 band or aponeurosis of connective tissue to another muscle. 



