CH. VI.] VOLUNTARY MUSCLE 63 



classification into voluntary and involuntary is shown in the follow- 

 ing table : 



1. Transversely striated muscular fibres : 



a. In skeletal muscle . . . VOLUNTARY. 



b. In cardiac muscle \ 



2. Plain muscular fibres : , 



In blood-vessels, intestine, uterus, | INVOLUNTARY. 

 bladder, etc. . . . J 



All kinds of muscular tissue are therefore composed of fibres, but 

 the fibres are essentially different from those we have hitherto studied 

 in the connective tissues. There the fibres are developed in the 

 intercellular material; here, in muscle, the fibres are developed 

 from the cells; that is, the cells themselves become elongated 

 to form the muscular fibres. 



Voluntary Muscle. 



The voluntary muscles are those which are sometimes called 

 skeletal, constituting the whole of the muscular apparatus attached to 

 the bones.* 



The fibres vary in thickness and length a good deal, but they 

 average -^$ inch in diameter, and about 1 inch in length. Each 



FIQ. 75. A branched muscular fibre from the frog's tongue. (Kiilliker.) 



fibre is cylindrical in shape, with rounded ends ; many become pro- 

 longed into tendon bundles by which the muscle is attached to 

 bone. As a rule they are unbranched, but the muscle fibres of the 

 face and tongue divide into numerous branches before being inserted 

 to the under surface of the skin, or mucous membrane (fig. 75). 

 The fibres in these situations are also finer than in the majority of 

 the voluntary muscles. 



Each fibre consists of a sheath, called the sarcolemma, enclosing 

 a soft material called the contractile substance. The sarcolemma is 



* The muscular fibres of the pharynx, part of the (esophagus, and of the 

 muscles of the external ear, though not under the control of the will, have the 

 same structure as the voluntary muscular fibres. 



