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 
of the beat is under the control of the vagus and 
sympathetic 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 for the 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 
A TYPICAL glenoid border of the scapula, the movable tendon, 
MUSCLE. or 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. 
THE HEART. 
