The Muscular System 119 



Muscles which bring about these types of movements derive their 

 names from the movements. Thus a muscle which produces flexion 

 is known as a flexor; one which produces extension, is an extensor; 

 etc. One exception to this general practice is the frequent use of the 

 term sphincter for a constrictor. 



Fig. 35. — Diagrams of parts of muscles and opposing actions. A, Extension 

 of the forearm; B, flexion of tlie forearm. 



Muscle Arrangement. — The evolutionary origins of the various 

 muscles of the mammalian body are very difficult to trace. This is 

 in part due to the apparent ease with which muscles can alter their 

 functions and general appearance. Homologies such as were seen 

 clearly in the skeleton are thus difficult to find among the muscles. In 

 general, however, the muscles of the vertebrates are fundamentally 

 segmental in their arrangement. 



This segmental arrangement is best seen in fish where the trunk 

 muscles are in groups known as myotomes (Fig. 196). These myotomes 

 are clearly visible in the embryos of mammals, but in the adult, most of 

 them are highly modified. Only among certain back and rib muscles 

 is this original segmentation still visible in the adult. The most highly 

 modified muscles of these original myotomes are those of the appendages. 



The muscles of the body never cross either the ventral or dorsal 

 midline. Ventrally the muscles meet at a line of fascia known as the 

 linea alba. 



The Naming of Muscles. — There are no general rules for naming 

 individual muscles. The names used are derived from numerous sources. 

 Sorhe muscles, for example, are named according to their origins and 

 insertions : the sternomastoid and the sternohyoid. At times the name 

 is derived from the shape : the deltoid of the shoulder region. Other 

 times, the direction of the fibers may be the source of the name : the 

 external and internal obliques of the abdominal wall. The action per- 

 formed may be the source of the name: the levator labii raises the 



