5 o8 TEXT-BOOK OF PHYSIOLOGY. 



upper part of the thigh, I scratched with the point of a scalpel the skin of 

 the chest at the areola of the nipple, for a space of loor n centimeters in 

 extent, without making any pressure on the subjacent muscles. We im- 

 mediately saw a rapid and successive contraction of the great pectoral 

 muscle, the biceps, probably the brachialis anticus, and lastly the muscles 

 covering the internal condyle. The result was a movement by which the 

 whole arm was made to approach the trunk; with rotation inward and half- 

 flexion of the forearm upon the arm; a true defensive movement, which 

 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). 



Laws of Reflex Action (Pfliiger). 



1. Law oj Unilaterality. If a feeble irritation be applied to one or more 



sensory nerves, movement takes place usually on one side only, and 

 that the same side as the irritation. 



2. Law oj Symmetry. If the irritation becomes sufficiently intense, motor 



reaction is manifested, in addition, in corresponding muscles of the 

 opposite side of the body. 



3. Law oj Intensity. Reflex movements are usually more intense on the side 



of irritation; at times the movements of the opposite side equal them in 

 intensity; but they are usually less pronounced. 



4. Law oj Radiation. If the excitation still continues to increase, it is pro- 



pagated upward, and motor reaction takes place through centrifugal 

 nerves coming from segments of the cord higher up. 



5. Law oj Generalization. When the irritation becomes very intense, it is 



propagated to the medulla oblongata; motor reaction then becomes 

 general, and it is propagated up and down the cord, so that all the mus- 

 cles of the body are thrown into action, the medulla oblongata acting 

 as a focus whence radiate all reflex impulses. 



Special Reflex Movements. Among the reflexes connected with the 

 more superficial portions of the body there are some which are so frequently 

 either increased or diminished in pathologic conditions of the spinal cord 

 that their study affords valuable indications as to the seat and character of 

 the lesions. They may be divided into: 



1. The skin or superficial reflexes. 



2. The tendon or deep reflexes. 



3. The organ reflexes. 



The skin reflexes, characterized by contraction of underlying muscles, 

 are induced by stimulation of the skin e.g., pricking, pinching, scratching, 

 etc. The following are the principal skin reflexes: 

 i. Plantar reflex consisting of contraction of the muscles of the foot, induced 



