THE MUSCULAR SYSTEM 



47 



As an illustration of this, lay the left forearm on a table before you, and 

 grasp the mass of flesh which forms the front of the upper arm with the right 

 hand. Now gradually raise the left forearm, still keeping the elbow on the 

 table, and notice how the muscle thickens as the hand rises. The chief 

 muscle engaged in this motion is one called the biceps muscle (Lat. hi, two ; 

 and caput, a head), because at its upper extremity it has two heads with 

 separate tendons. These tendons are united to the scapula at the shoulder- 

 joint, while the lower tendon is fastened to the radius, very near the elbow- 

 joint. The points of union with the fixed bone (the scapula) are called the 

 points of origin of the mus- 

 cle ; the point at which the 

 muscle is attached to the 

 bone to be moved (the 

 radius) is called its point 

 of insertion ; and the thick, 

 fleshy, middle part of the 

 muscle is known as its 

 body. Some few of the 

 muscles have no tendons, 

 the muscular fibres being 

 connected directly with the 

 bone ; while others, instead 

 of being of the form just 

 described, are broad and 

 flat muscular sheets. Some 

 muscles are not connected 

 in any way with bones. 

 These often enclose cavi- 

 ties, in which case they 

 are called hollow muscles, 

 and are generally of the 

 involuntary kind. Exam- 

 ples are to be seen in the 

 heart and stomach. 



After a bone has 

 been moved by a mus- 

 cle, it is brought back 

 to its former position 

 by the contraction of a 

 second muscle on its 

 opposite side. Hence 

 we find that muscles are generally arranged in pairs, each muscle 

 of a pair being antagonistic in its action to the other. 



Those muscles which are used to bend the limbs are called 

 flexors; while those which straighten the limbs are called ex- 

 tensors. 



Fig. 50. Side View of the Muscles of the 

 Face and Neck. 



Muscles may be made to contract by the application of some kind of 

 stimulus or irritant. The stimulus is generally conveyed to the muscular 

 fibres by means of the nerve which distributes its branches among them. But 

 muscular contraction may also be produced by mechanical irritation, such as 



