524 TEXTBOOK OF ZOOLOGY 



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 known 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 insertio7i. 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. 



Adductor — one that draws the limb toward the median ventral line. 



Abductor — one that draws the limb away from the median ven- 

 tral line. 



Levator — one that raises a part, such as the lower jaw. 



Depressor — one that lowers a part, such as the lower jaw. 



Rotator — one that rotates one part on another. 



The 'pectoral muscles cover the chest and ventral portion of the 

 upper body region ; the rectus al)dominis extends along the median 

 ventral region ; the paired ohliquus externus and internus cover most 

 of the sides of the trunk. The muscles of the limbs are numerous. 

 There are some eighteen separate muscles which control various move- 

 ments of the legs. A detailed description of these and other muscles 

 of the frog would be confusing to the elementary student and there- 

 fore is not included. The major muscles of the hind leg are illus- 

 trated in Fig. 284 and can be clearly understood after a careful dis- 

 section in the laboratory. 



