544 TEXT-BOOK OF PHYSIOLOGY 



brought the hand toward the chest as far as the pit of the stomach. Neither 

 the thumb, which was partially bent toward the palm of the hand, nor the 

 fingers, which were half bent over the thumb, presented any movements. 

 The arm being replaced in its former position, we saw it again execute a 

 similar movement on scratching the skin, in the same manner as before, a 

 little below the clavicle. This experiment succeeded four times, but each 

 time the movement was less extensive; and at last scratching the skin over 

 the chest produced only contractions in the great pectoral muscle which 

 hardly stirred the limb" (Dalton). 



Reciprocal Action of Antagonistic Muscles. It has been stated in a 

 foregoing paragraph that the reflex movements taking place through 

 the spinal cord are not only complex but coordinated, purposive and 

 protective in character. An analysis of the characteristic movements 

 of a limb in any one of its parts, or in its entirety, has shown that they are 

 the resultant of the reciprocal activity of muscles acting in opposite 

 directions on a common lever for a mechanical advantage. Such an 

 anatomical condition is seen in the action of the flexors that play over a 

 hinge joint, such as the elbow and knee joints, the lever acted on in the 

 two cases being the bones of the forearm and leg respectively. 



It has been shown experimentally that when a flexor muscle of the 

 arm or leg is excited to activity or has its tonus increased or augmented the 

 opposing extensor muscle is simultaneously relaxed or has its tonus dimin- 

 ished or inhibited; and that when the extensor muscle is excited to activity 

 the flexor muscle is inhibited. 



Moreover if the arm or leg assumes any marked attitude of flexion 

 it has been found that all the flexors are in a state of contraction and all 

 the extensors in a state of more or less inhibition; and if the limb assumes 

 any marked attitude of extension, all the extensors are in a state of con- 

 traction and the flexors in a state of inhibition. The extent of the reaction 

 in the two groups of muscles will be proportional within limits to the 

 strength of the stimulus. Similar conditions are observed when the two 

 limbs are in opposite states of flexion and extension, as in the act of walking, 

 but with this difference, that when the flexors of the progressing limb are 

 in condition of contraction the symmetrical flexors of the supporting limb 

 are in a condition of inhibition; and when the extensors of the progres- 

 sing limb are in a condition of inhibition, the symmetrical extensors of the 

 supporting limb are in a condition of contraction. Similar conditions 

 are observed in the muscles of the eyeballs when they are turned to the 

 right or left. 



Reciprocal Innervation of Skeletal Muscles. The reciprocal action of 

 antagonistic muscles is accomplished by the coordinate activity of the 

 spinal motor centers brought about by the arrival of nerve impulses 

 developed by peripheral stimulation constituting what has been termed 

 reciprocal innervation, central in character. Reciprocal innervation may 

 therefore be denned as the innervation of two oppositely acting mechanisms 

 in virtue of which an increase or augmentation in the activity of the one 

 is accompanied by a decrease or inhibition in the activity of the other. 



Inasmuch as inhibitor nerves for skeletal have never been demonstrated 

 it is believed that the reciprocal activity of the mechanism is central; 

 that the motor center in the spinal cord for each oppositely acting muscle 



