THE STRUCTURE OF THE MOTOR ORGANS 79 



former group, or skeletal muscles, are also from their microscopic 

 characters known as striped muscles, while the latter, or visceral 

 muscles, are called unstriped or smooth 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 involuntary. The heart muscle forms 

 a sort of intermediate link; it is not directly attached to the skele- 

 ton, but forms a hollow bag which drives on the blood contained 

 in it and that quite involuntarily; but in its microscopic struc- 

 ture it resembles somewhat the skeletal voluntary muscles. The 

 muscles of respiration 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 involuntary are 

 not good ones, but so far as we use them they indicate no more 

 than the general fact that the skeletal muscles are as a group re- 

 sponsive to the will while the smooth muscles are not. 



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 white cords which 

 consist almost entirely of white fibrous connective tissue. These 

 terminal cords are called the tendons of the muscle and serve to 

 attach it to parts of the skeleton. In Fig. 38 is shown the biceps 

 muscle of the arm, which lies in front of the humerus. Its fleshy 

 belly is seen to divide above and end there in two tendons, one of 

 which, Bl, is fixed to the scapula, while the other, Bb, joins the 

 tendon of a neighboring muscle (the coraco-brachial, Cb), and is 

 also fixed above to the shoulder-blade. Near the elbow-joint the 

 muscle is continued into a single tendon, B', which is fixed to the 

 radius, but gives an offshoot, B", to the connective-tissue mem- 

 branes lying around the elbow-joint. 



