700 THE MUSCULAR SYSTEM 



A. Introduction 



1. Definition 



The muscular system produces mobility of the various body parts. As such, 

 it is composed of cells specialized in the execution of that property of living 

 matter which is known as contractility. Since contractility is a generalized 

 property of living matter, it may occur without the actual differentiation of 

 muscular tissue. In the developing heart of the chick, for example, contractures 

 begin to occur as early as 33 to 38 hours of incubation before muscle cells, 

 as such, have differentiated (Patten and Kramer, '33). 



2. General Structure of Muscle Tissue 



Muscle cells are elongated, fibrillated structures, known as muscle fibers. 

 They contain many elongated fibrils, called myofibrils, extending longitudi- 

 nally along the muscle fiber. The myofibrils may possess a series of cross 

 striations in the form of light and dark transverse bands as in skeletal or 

 striated muscle and cardiac muscle, or the transverse bands may be absent 

 as in smooth muscle (fig. 325A-C). In smooth muscle, the myofibrils are 

 extremely fine, whereas in striated muscle they are seen readily under the 

 microscope. 



a. Skeletal Muscle 



In skeletal muscle, the muscle fibers are elongated, cylinder-shaped struc- 

 tures; the ends are rounded; and a row of nuclei extend along the periphery 

 of the muscle fiber or cell, and are more numerous at the ends of the cell 

 than in the central portion. The cell, as a whole, is filled with myofibrils, 

 embedded in a matrix of sarcoplasm. The latter contains fat droplets, gly- 

 cogen, interstitial granules, amino acids, mitochondria, and Golgi substances. 

 The surrounding cell membrane is a delicate structure and is known as the 

 sarcolemma. 



The myofibrils are composed of dark and light transverse bands, a dark 

 band alternating with a light band. The bands are arranged along the myofibrils 

 in such a manner that the dark band of one fibril is at the same level as 

 the dark bands of other fibrils. The light bands are arranged similarly. This 

 arrangement presents the effect shown in figure 325A. 



Two types of muscle fibers are found in skeletal muscle. In one type, the 

 red or dark fiber, there is an abundance of sarcoplasm with fewer myofibrils. 

 The myofibrils possess weaker transverse markings or striations. In the second 

 type, the pale or white fiber, there is less of the sarcoplasm present with a 

 larger number of highly difi'erentiated myofibrils, having well-defined trans- 

 verse striations. This muscle fiber is larger in transverse diameter than the 

 red type. In many animals, such as man, these two sets of fibers are inter- 

 mingled in the various skeletal muscles, but in some, such as the breast 



