EXPERIMENTS ON VERTEBRATES 145 



the performance of one reflex acts at the same time as 

 the stimulus for setting free a second reflex. The tak- 

 ing of food may serve as an illustration of such a chain- 

 reflex. The optic reflex of the moving fly produces 

 the snapping reflex ; the contact of the mouth-epi- 

 thelium with the fly produces the swallowing reflex. 

 Each of these reflexes is purely segmental. By tak- 

 ing into account the act of transmission, complicated 

 acts can thus be resolved into a few segmental 

 reflexes. A second fact must be taken into consider- 

 ation if we wish to trace back the reactions of a frog 

 to segmental reflexes, namely, that the irritability of 

 the organs of its body changes. In the chapter on 

 instincts we shall find how chemical conditions, espe- 

 cially, affect the form of the irritability of the animal, 

 and how all conditions which bring about chemical 

 changes in the body (temperature, food, sexual pro- 

 ducts) also modify its irritability. We shall then un- 

 derstand why the frog burrows at the beginning of the 

 cold weather in autumn and puts in an appearance 

 again with the awakening of spring, or, strictly speak- 

 ing, with the beginning of the warm weather. The 

 segmental reflex in the frog is, however, determined 

 also by the irritability of the peripheral organs and the 

 arrangement of the muscles. The segmental ganglion 

 acts, in the main, simply as the protoplasmic connec- 

 tion between the surface of the body and the muscles. 

 The experiments of Goltz and of Goltz together 

 with Ewald on the spinal cord of dogs prove that this 

 law of segmental reflexes is also correct for dogs. 



