THE STRUCTURE OF THE MOTOR ORGANS 77 



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. 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. 42 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 membranes lying around the elbow-joint. 



The belly of every muscle possesses the power of shortening 

 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 at- 

 tached, 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 arc 

 merely passive cords and are sometimes very long, as for instance 

 in the case of the muscles 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 same 

 bone the muscle would clearly be able to produce no movement, 

 unless by bending or breaking the bone ; the probable result in such 

 a case would be the tearing of the muscle by its own efforts. In 

 the Body, however, the two ends of a muscle are always attached 

 to different parts, usually two bones, between which more or less 



