124 HUMAN PHYSIOLOGY. 



of the pupils from intestinal irritation (worms), and many pathological 

 phenomena. 



Laws of Reflex Action. (Pfluger.) 



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 upon 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 the 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 propagated 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 gen- 

 eral, and it is propagated up and down the cord, so that all the muscles of 

 the body are thrown into action, the medulla oblongata acting as a focus 

 whence radiate all reflex movements. 



Special Reflex Movements. 



There are a number of reflex movements taking place through the spinal 

 cord, a study of which enables the physician to determine the condition of 

 its different segments. They may be divided into, I. Skin or superficial, 

 and 2. Tendon or deep reflexes. The skin reflexes are induced by irritation 

 of the skin and mucous membranes, e.g., pricking, pinching, scratching, etc. 

 The following are the principal skin reflexes : 



1. Plantar reflex, consisting of contraction of the muscles of the foot, 

 induced by stimulation of the sole of the foot ; it involves the integrity of 

 the reflex arc through the lower end of the cord. 



2. Ghiteal reflex, consisting of contraction of the glutei muscles when 

 the skin over the buttock is stimulated ; it takes place through the segments 

 giving origin to the fourth and fifth lumbar nerves. 



3. Cremasteric reflex, consisting of a contraction of the cremaster muscle, 

 and a retraction of the testicle toward the abdominal ring, when the skin on 

 the .inner side of the thigh is stimulated; it depends upon the integrity of 

 the segments giving origin to the first and second lumbar nerves. 



4. Abdominal reflex, consisting of a contraction of the abdominal mus- 

 cles when the skin upon the side of the abdomen is gently scratched ; its 



