The Muscular System. 63 



Cardiac muscle, most nearly allied in action to smooth muscle 

 is the mechanical organ of the rhythmical contraction or beat of the 

 heart. The contraction takes place according to the succession 

 of the chambers or the course of the blood, and the rate and strength 

 THE HEART "^ *^^ ^^* '® undsr the control of the vagus and 

 syrnpathetic nerves, the action of which can be 

 experimentally demonstrated. The excised heart in the case of 

 lower vertebrates continues to beat for some time automatically 

 or under stimulation. This behaviour has been interpreted as 

 purely automatic action of the heart muscle, but may depend upon 

 intercardiac nerve connections. 



The voluntary muscles of the body form the nearest approach 

 to a continuous system of all contraccile tissues. They consist 

 VOLUNTARY ^^"^ *^^ most part of parallel fibres, the association 

 MUSCLE. of which into fasciculi is responsible for the appear- 



ance of longitudinal striping when the gross muscle 

 is viewed from the side and more or less for the grained appearance 

 of the cut surface. The attachments, usually of white fibrous 

 connective tissue occurs as concentrated tendons or as flat thin 

 aponeuroses. The control of action is exercised directly from 

 the spinal cord. 



A typical muscle of the skeleton has the disposition of parts 

 illustrated in Fig. 34 by the biceps (a flexor of the forearm) and the 

 long head of the triceps (an extensor of the forearm). The fixed 

 tendon, or tendon of origin, of the biceps is in relation to the 

 „.yp^p,^ glenoid border of the scapula, the movable tendon, 



MUSCLE ^^ tendon of insertion, with the lower border of 



of the ulna. Noting the position of the muscle 

 in front of the elbow joint, it will be seen that its contraction results 

 in flexion, i.e., in bringing the forearm into a position nearer the 

 arm, or of raising the forearm and hand from the ground. The 

 analogous action of the triceps is similarly demonstrated, and 

 it will also be evident that the immediate result of contraction 

 upon the bones is limited by the form of the joint. In this case 

 a hinge-joint confines motion to one plane, while in the case of the 

 shoulder and hip, a joint of the ball-and-socket type allows motion 

 on points at various angles to a plane according to which muscle or 

 group of muscles may be brought into action. 



