116 The Vertebrate Organ Systems 



Smooth muscle cells are grouped in several ways, depending upon 

 their location and function. Those located in the walls of hollow 

 organs are arranged in layers or bundles ; those found in the skin are 

 arranged singly or in small groups ; and those at the bases of the in- 

 dividual hairs are grouped as small bundles. (Fig. 21.) 



Smooth muscles are capable of prolonged, rhythmical contractions. 

 The force exerted by them is often very great ; for example, during 

 parturition, the smooth muscles of the uterus can exert a force of from 

 50 to 55 pounds pressure. These fibers have greater powers of ex- 

 tensibility than do those of other muscle tissues. The expansion of 

 the urinary bladder is made possible because of this highly developed 

 property. Smooth muscle is not under voluntary control. 



Skeletal Muscle (Fig. 34, C and D). — The individual muscle 

 fibers or cells of skeletal muscle are considerably larger than those of 

 smooth. They vary in length from about 1 mm. to several centimeters. 

 In some cases, the individual fiber is as long as the entire muscle. 

 In general, their shape is cylindrical with the ends being tapered or 

 notched. Scattered throughout its length, each cell contains numerous 

 nuclei which in mammals lie near the surface. The surface of each 

 fiber is covered by a thin sheath, the sarcolemma. Contractile threads, 

 the myofibrils, like those found in the smooth muscle, run the length 

 of the individual fiber. Each myofibril consists of alternating light 

 and dark portions. All the fibrils within a single cell have these alter- 

 nating portions in the same position, giving the fiber a striated appear- 

 ance. For this reason, skeletal muscle is also called striated muscle. 



The striated muscles form the "flesh" of the body. They are at- 

 tached to the bones, and by their contractions, bring about movements 

 of the skeletal system. These muscles also compose the diaphragm, 

 portions of the tongue, and the muscles of the eyeball and pharynx. 



Each muscle fiber is surrounded by the sarcolemma. Many muscle 

 fibers are arranged parallel to one another and are grouped together 

 into small bundles known as fascicles. These fascicles are held together 

 by sheaths of connective tissue fibers. Numerous fascicles are further 

 grouped together to form the large muscles, which likewise are covered 

 and held together by an external sheath of connective tissue, the fascia. 



The movement of skeletal muscle, in contrast to those of smooth 

 muscle, are rapid. Also the individual fibers are not nearly so extensi- 

 ble as are those of smooth muscles and they fatigue more easily. 

 Finally these muscles are under voluntary control. 



