THE FROG 21 



numerous fibres which either run parallel with one another or 

 diverge outwards in a fan-shaped manner from the point of 

 attachment to the line of insertion (e.g. the muscles of the 

 breast). In the limbs the muscles are more rounded or 

 spindle-shaped, their component fibres being gathered into 

 bundles which terminate at each end in a tendon. The 

 general character of a muscle may most conveniently be 

 studied in the calf-muscle or gastrocnemius of the frog. This 

 muscle lies on the inner surface (that surface which in man 

 would be hindermost) of the leg. It is stout and spindle- 

 shaped, tapering towards either extremity, but most markedly 

 towards the lower end, where it passes into a stout glistening 

 white tendon, the tendo Achillis. It is attached at the upper 

 end by a broad tendon to the bone of the thigh, and to the 

 upper end of the bone of the leg ; the tendon at the lower end 

 passes round the place where the heel would be, if such a 

 structure were present in the frog, and is continuous with a 

 broad sheet of connective tissue spread over the sole of the 

 foot. Such a sheet of connective tissue serving for the attach- 

 ment of muscle or tendon is called an aponeurosis, and this 

 particular one is known, as the aponeurosis plantaris. The 

 gastrocnemius muscle by its contraction pulls upon the back 

 of the ankle and the sole of the foot, and tends to bring the 

 foot and ankle in a line with the leg. The bones to which the 

 upper end of the muscle is attached are not moved by its con-^ 

 traction, and this fixed end of the muscle is called its origin. 

 The opposite end, which is moved when it contracts, is called 

 its insertion. Most of the voluntary muscles are attached, at 

 one of their ends at least, to bones. The bones of an animal 

 constitute its skeleton, and afford a system of levers actuated 

 by the muscles, and they also form a solid framework which 

 supports and protects some of the organs. 



We have already been able to recognise the fact that the 

 skeleton of the frog is internal, in this respect resembling our 

 own, and differing from the skeleton of such animals as lobsters 

 and insects, which have their skeleton on the outside. 



In the frog's skeleton we may recognise two main parts, an 

 axial and an appendicular skeleton. There is also a branchial 

 skeleton, well developed in the tadpole, but reduced and of 

 minor importance in the adult frog. The axial skeleton, so 

 named because it lies along the main longitudinal axis of the. 



