THE SPINAL CORD 547 



The organ reflexes, e.g., the activities of the genito-urinary organs, the 

 stomach, intestines, gall-bladder, etc., which are induced by peripheral 

 stimulation have been considered in connection with the physiologic action 

 of these organs. The genito-urinary center is located in the lumbar region 

 of the spinal cord. In diseased conditions of this region the genito-urinary 

 reflexes are sometimes increased, at other times decreased or even abolished. 



Direct or Cerebral Excitation. The activity of the emissive cells of 

 the spinal cord segments, due to the arrival of nerve impulses descending 

 the spinal cord from the cerebrum, in consequence of psychic states of a 

 volitional or of an affective or emotional character, will be considered, in a 

 subsequent paragraph entitled " encephalo-spinal conduction." 



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 on 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 Tlirck 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 by 

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

 follow each other with a rapidity sufficient to give rise to a summation of 

 effects in the nerve-centers which will then be followed by a muscle response. 

 It is highly probably that the chemic stimulation gives rise to a similar sum- 

 mation of effects. 



The total reflex time, i.e., the time elapsing between the application 

 of a stimulus and the response of the muscle is a variable factor in different 

 individuals and depends largely on the degree of irritability of the intra- 

 spinal mechanisms. If this is increased above an average normal the 

 total reflex time will be shortened; if it is decreased below an average 

 normal, it will be lengthened. The true reflex time, however i.e., the 

 time occupied in the passage of the nerve impulses through the spinal 



