THE SPINAL CORD 539 



able, it is highly probable that the human spinal cord would execute similar 

 movements. Thus it was observed by Robin in a man who had been decapi- 

 tated that reflex muscle contractions could be elicited by stimulating the skin 

 after the lapse of an hour after execution. " While the right arm was lying 

 extended by the side, with the hand about 25 centimeters distant from the 

 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 10 or 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 of 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 of 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 of 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 of 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 of 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. 



Reflex Irritability. The general irritability or quickness of response of 

 the mechanism involved in reflex action can be approximately determined 

 by observation of the length of time that elapses between the application of 

 a minimal stimulus and the appearance of the muscle response. The method 

 of Turck is sufficiently accurate for general purposes. This consists in 

 suspending a frog, after removal of the brain, and immersing the foot in 

 a 0.2 per cent, solution of sulphuric acid. The time is determined by means 

 of a metronome beating one hundred times a minute. Stimulation of the 

 skin can also be effected by the induced electric current, as suggested 

 Gaskell A single shock is, however, ineffective. The currents must 



