88 ESSENTIALS OF ZOOLOGY 



rudimentary fifth near the thumb, the prepollex, consisting of only 

 a single bone. Each of the four digits, or fingers, extends from a 

 metacarpal bone. This is followed in digits II and III by two 

 phalanges and in digits IV and V by three phalanges. 



The hindlimhs have essentiallj^ the same structure as the fore- 

 limbs. The large bone which joins the girdle at the socketlike 

 acetabulum is known as the femur. This bone articulates with the 

 tihio-fibula, which, like the bone of the forearm, is a fusion of two 

 bones. The tarsus or ankle differs from the wrist, being composed 

 of two long bones, the tihiale (astragulus) and fihulare (calcaneum), 

 and two small tarsals. There are also two extremely small bones form- 

 ing the prehallux, or rudimentary sixth toe. Distal to the tarsals are 

 five long metatarsals. Each foot contains five complete digits, each 

 following a metatarsal bone. In digits I and II are two phalanges, 

 in digits III and V three phalanges, and in digit IV four phalanges. 



Muscular System 



Muscular tissue controls the movements and positions of various 

 parts of the body of the bullfrog. This it does by contracting, that 

 is, by shortening and thickening its elements. 



Movements may be under voluntary control, as the skeletal muscles, 

 involved in moving the limbs, in which case the muscle fibers are 

 striated and are known as voluntary muscle. Other movements, such 

 as the heartbeat and the peristaltic movements in the intestines, are 

 not under control of the will. Muscles concerned in these actions 

 are kno\vn as involuntary and are usually made up of smooth muscle 

 fibers except in the heart, which contains striated cardiac muscle. 



Most voluntary muscles are attached to bones at one end or at both 

 by specialized connective tissue bands known as tendons. The end of 

 the muscle which is attached to a relatively fixed and immovable part 

 is called the origin; the end which is attached to the part which 

 moves when the muscle contracts is known as the insertion. A typical 

 voluntary muscle is made up of three parts : the tendons attached at 

 its ends; the membrane surrounding the muscle, known as the fascia; 

 and the belly, or fleshy part, of the muscle. 



The different actions performed by the various skeletal muscles 

 give rise to descriptive names applied to them. Some of these are as 

 follows : 



Extensor — one that straightens a part, such as extending the foot. 

 Flexor — one that bends a part, such as a joint. 



