THE MUSCULAR SYSTEM 65 



former Inhibiliiiii, the latter acceleratiiii^, as nia\' readily be 

 demonstrated experlmentalK'. 



Voluntary Muscle 



The voluntary muscles of the body form the nearest approach 

 to a continuous system of all contractile tissues. They consist for 

 the most part of parallel fibres, the association of which into 

 fasciculi is responsible for the appearance of longitudinal striping 

 when the gross muscle is viewed from the side, and more or less for 

 the grained appearance of the cut surface when the muscle is 

 divided approximately at right angles to the direction of the 

 striping. The control of action is exercised directly from the 

 spinal cord or from the brain. 



A muscle is typically spindle shaped, consisting of a middle 

 fleshy portion, termed the belly of the muscle, and of tapering ends 

 which provide for attachment. The attachment is effected by a 

 strong band of fibrous connective tissue, the muscle tendon (Figs. 

 8, 37). Some muscles, such as those of the abdominal wall, are 

 disposed in the form of flattened sheets, the ends of which are at- 

 tached by tendons in the shape of broad, thin sheets of connective 

 tissue, the aponeuroses. In unipennate muscles the fibres are 

 attached obliquely to the side of the tendon, or in bipennate 

 muscles to both sides, like the vane of a feather. In the so-called 

 biceps, triceps, and quadriceps muscles of the limbs, the origin is 

 divided into two, three, or four portions. 



A typical muscle of the skeleton has the disposition of parts 

 illustrated in Fig. 37 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 attached to the 

 glenoid border of the scapula, the movable tendon, or tendon of 

 insertion, to the lower border 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 in raising the forearm and hand from 

 the ground. The analogous action of the triceps in producing an 

 exactly opposite movement (extension) of the forearm is similarly 

 demonstrated. It will be evident that the immediate result of 

 contraction of the muscles is limited by the form of the joint 

 between the bones to which they are attached. In this case a 



