THE FROG — A REPRESENTATIVE VERTEBRATE 401 



girdle to the trunk, for there is no direct connection between the girdle 

 and the vertebral column. 



The sternum is divided into four midventral pieces, two of which 

 extend anteriorly from the clavicles and two posteriorly from the cora- 

 coids. The terminal piece at each end is unossified. 



The forelimb is composed of a humerus extending from the shoulder 

 to the elbow joint; a radio-ulna (fusion of a radius and ulna) continuing 

 to the wrist joint; a series of small wrist bones, the carpals, lying in the 

 proximal part of the hand; four long metacarpals in the region of 

 the palm; and a series of small segments known as phalanges in each 

 of the four digits. Although the first finger is not apparent in an entire 

 frog, its vestigial metacarpal can often be seen in the skeleton. 



The pelvic girdle is attached to the sacral vertebra and provides a 

 solid support for the pelvic appendages (Fig. 21.5). Each side of the 

 girdle consists of a long ilium extending posteriorly from the sacrum to 

 the ischium and pubis. The ventral pubis is unossified. A concavity, the 

 acetabulum, is situated where the three join, and serves for the articula- 

 tion of the hind limb. 



The femur extends from the acetabulum to the knee, and a fused 

 tibio-fibula from the knee to the ankle joint. Ankle bones, the tarsals, 

 form the proximal part of the foot. These are followed by five meta- 

 tarsals in the region of the sole, and a series of phalanges in each digit. 

 The frog foot is unusual in that the two proximal tarsals are elongated 

 and form, in effect, an extra segment to the limb. These elongated 

 tarsals are followed distally by two small and inconspicuous ones. A 

 bone called the calcar supports the prehallux. The fusion of the radius 

 and ulna and of the tibia and fibula, and the extra leverage provided 

 by the elongation of the tarsals, are adaptions for jumping. 



183. Muscular System 



Smooth muscles are found in the walls of many visceral organs, 

 cardiac muscles in the wall of the heart and striated muscles attach to 

 the skeleton. The striated muscles, which are generally under voluntary 

 control, form the bulk of the muscular system. Most of these are attached 

 to bones by tendons. The origin of the muscle is its fixed end; the 

 insertion is the end attached to the structure that moves when the muscle 

 contracts. The origin is generally the end nearer the longitudinal axis of 

 the body, or, in the case of longitudinal muscles, the more anterior 

 end; the insertion is the peripheral or posterior end (Fig. 21.6). 



Muscles can induce movement only by contracting or shortening, 

 hence the muscles of the body are grouped into antagonistic sets. One 

 set of muscles is responsible for moving a part in one direction, whereas 

 movement in the opposite direction entails the relaxation of the first 

 set of muscles and the contraction of an antagonistic set on the opposite 

 side of the part. Various terms are used to describe movement in dif- 

 ferent directions. For example, flexion is the bending of a joint with a 

 consequent diminishing of the angle between the bones, as occurs at the 

 knee or elbow; extension is the opposite movement, i.e., a straightening. 



