VARIETIES OF MUSCLE. 11? 



-consciousness; some moving without our having any direct 

 knowledge of the fact. This is especially the case with cer- 

 tain muscles which are not fixed to the skeleton but sur- 

 round cavities or tubes in the Body, as the blood-vessels and 

 the alimentary canal, and by their movements control the 

 passage of substances through them. The former group, or 

 skeletal muscles, are also from their microscopic characters 

 known as striped muscles, while the latter, or visceral mus- 

 cles, are called unstriped or plain muscles. The skeletal 

 muscles being generally more or less subject to the control 

 of the will (as for example those moving the limbs) are 

 frequently spoken of as voluntary, and the visceral muscles, 

 which change their form independently of the will, as invol- 

 untary. The heart-muscle forms a sort of intermediate 

 link ; it is not directly attached to the skeleton, but forms 

 a hollow bag which drives on the blood contained in it and 

 that quite involuntarily; but in its microscopic structure it 

 resembles the skeletal voluntary muscles. The muscles of 

 respiration might perhaps be cited as another intermediate 

 group. They are striped skeletal muscles and, as we all 

 know, are to a certain extent subject to the will; any one 

 can draw a deep breath when he chooses. But in ordinary 

 quiet breathing we are quite unconscious of their working, 

 and even when attention is turned to them the power of 

 control is limited; no one can voluntarily hold his breath 

 long enough to suffocate himself. As we shall see hereafter, 

 moreover, any one or all of the striped muscles of the Body 

 may be thrown into activity independently of or even 

 against the will, as, to cite no other instances, is seen in the 

 "fidgets" of nervousness and the irrepressible trembling of 

 extreme terror; so that the names voluntary and involun- 

 tary are not good ones. The functional differences be- 

 tween the two groups depend really more on the nervous 

 connections of each, than upon any essential difference in 

 the properties of the so-called voluntary or involuntary 

 muscular tissues themselves. 



The Skeletal Muscles. In its simplest form a skeletal, 

 muscle consists of a red soft central part, the belly, which 

 tapers at each end and there passes into one or more dense 



