Jknnings, Uexkiill on Physiology of Behavior. 321 



here two eiitirely (lili'crcnt fuiK'tioiis has, the author believes, led 

 all nerve-muscle physiology into false paths. 



The first reflex shown by the spines is as follows: When a cer- 

 tain spot on the body is moderately stimulated, the surrounding 

 spines bend toward it. The muscles of the side of the spine next 

 the point stimulated contract; that is, their tonus is increased. 

 Thus the points of the spines are directed, for example, toward an 

 approaching enemy. 



But if the stimulus is very intense, the reaction just described is 

 reversed ; the spines bend away from the point stimulated. This 

 result is ])roduced by a decrease in the tcjnus of the muscles on the 

 side of the spine facing the j^oint stimulated. Thus from the same 

 spot on the body opposite effects may be produced, depending on the 

 strength of the stimulus. This phenomenon is called by v. Uexkiill 

 reversal of the reflex ("Eeflexumkehr") ; it is observed in other or- 

 gans of the sea urchin and other animals, nnder various conditions. 

 The author holds it to be due to some sort of ap})aratus in the ganglion 

 cells; an apparatus that he calls the "tonus switch" ("Tonusschal- 

 ter"). This reversal is well seen in the spines when a strong chemical 

 stimulus afl'ects the body. Now, a further consequence of such a 

 powerful stimulus is seen. After such a stimulus, even a weak 

 stimulus, which formerly caused the spines to bend toward the spot 

 stimulated, now causes them to bend away. So the same stimulus 

 on the same spot may cause two diflerent reactions, depending on 

 what stimulus has preceded it. This phenomenon, very common in 

 animals, v. Uexkiill calls the "switching" of the tonus ("Schaltung") ; 

 he holds it due to the same apparatus as the reversal of the reflex. 



The two reactions of the spines serve, under natural conditions, 

 certain functions. The bending toward a stimulated point serves for 

 defense; the bending away under a strong stimulus, particularly a 

 chemical one, preserves the spines from injury, while giving oppor- 

 tunity for the action of certain large ])oisonous pedicellariffi, which 

 tiow bend their envenomed jaws towai'd tlie region attacked and seize 

 whatever is there present. 



Certain other facts in the physiology of the s])ines are of extreme 

 importance. A steady tension, not violent, exercised on the muscles 



