THE STRUCTURE OF THE MOTOR ORGANS. 113 



subject to the will; any one can draw a deep breath when he 

 chooses. But in ordinary quiet breathing we are quite un- 

 conscious of their working, and even when attention is turned 

 to them the power of control is limited; no one can voluntar- 

 ily 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 independ- 

 ently 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. The functional differences 

 between 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 

 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 bony or 

 cartilaginous skeleton. In Fig. 51 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, Bl), 

 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 membranes lying around the elbow- 

 joint. 



The belly of every muscle possesses the power of shorten- 

 ing forcibly under certain conditions. In so doing it pulls 

 upon the tendons, which being composed of inextensible 

 white fibrous tissue transmit the movement to the hard parts 

 to which they are attached, just as a pull at one end of a rope 

 may be made to act upon distant objects to which the other 

 end is tied. The tendons are merely passive cords and are 

 sometimes very long, as for instance in the case of the mus- 

 cles of the fingers, the bellies of many of which lie away in 

 the forearm. 



If the tendons at each end of a muscle were fixed to the 



