CHAPTER III. 

 MUSCLE. 



THREE varieties of muscular fibres are found in the body, namely, those 

 forming skeletal muscle, those found in the walls of the blood-vessels, 

 digestive tract, uterus, and other organs, and those in cardiac muscle. 

 The structure of cardiac muscle will be considered later (page 177). 

 The skeletal muscles, which are under the control of the will, are called 

 voluntary muscles, the other kinds of muscle being termed involuntary, 

 since, although they are under the control of the central nervous 

 system, they are independent of the will. 



VOLUNTARY MUSCLE. 



A skeletal muscle consists of fibres bound together by connective 

 tissue. The fibres have an average diameter of 50/x, and vary in length, 

 some being as long as 3 to 4 cm. Each fibre is enclosed in a delicate 

 elastic sheath (sarcolemma), and shows alternating light and dark 

 bands crossing it transversely ; owing to the presence of these stripes, 

 this form of muscle is often called striped or striated muscle. Each 

 fibre contains a number of oval nuclei ; in mammals these lie immedi- 

 ately under the sarcolemma, but in frogs they are scattered throughout 

 the substance of the fibre. In a stretched fibre a narrow clear line, 

 known as Hensen's line, can sometimes be seen running transversely 

 in the middle of each dark band. Frequently, too, a dotted line is 

 visible in the middle of each light band ; it is termed Krause's 

 membrane. 



The fibres consist of fibrils, or sarcostyles^ which run longitudinally 

 and are imbedded in a material known as sarcoplasm. It is probable 

 that each sarcostyle is made up of a number of short segments called 

 sarcomeres, but the exact structure of the sarcostyles and the signifi- 

 cance of Krause's membrane and Hensen's line in different animals is 

 still uncertain. Schafer has shown that the sarcostyles in the rapidly 

 contracting wing muscles of insects are divided by the membranes of 

 Krause into sarcomeres. He considers that the dark central part (or 



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