REFLEX ACTIONS. 



141 



from the skin are carried to the cord by two different varieties of 

 fibers. It may well be that one variety of sensory neuron, the 

 so-called pain fibers, effects both results, because of the opportuni- 

 ties in the cord for connections with different groups of nerve cells. 

 The Reflex Frog. — The motor reflexes from the spinal cord 

 can be studied most successfully upon a frog in which the brain 

 has been destroyed or whose head has been cut off. After such 

 an operation the animal may for a time suffer from shock, but 

 a vigorous animal will usually recover and after some hours will 



Fig. 66. — Kolliker's schema to show 

 the direct reflex arc. It shows the pos- 

 terior root fiber (black) entering the 

 cord, dividing in Y. and connecting with 

 motor cells (red) by means of collater- 

 als. 



Fia;. 67. — Kolliker's schema to 

 show the reflex arc with intercal- 

 ated tract cells. Posterior root fiber, 

 black; intercalated tract cell, blue; 

 motor cells, red. 



exhibit reflex movements that are most interesting. The funda- 

 mental characteristics of reflex movements in their relations to the 

 place, intensity, and quality of the stimulus can be studied with 

 more ease upon an animal whose cord is thus severed from the 

 brain than upon a normal animal. In the latter case the connec- 

 tions in the nervous system are more complex and the reactions 

 are therefore less simple and less easily kept constant. 



Spinal Reflex Movements. — The reflex movements obtained 

 from the spinal cord or from other parts of the central nervous system 

 may be divided into three groups by characteristics that are physio- 

 logically significant. These classes are: (1) Simple . reflexes, or 

 those in which a single muscle is affected. The best example of 



