322 'Journal of Comparative Neurology and Psychology. 



of the spines causes them to lose their tonus ; they become limp. 

 This effect of tension on tonus is common among animals. If then 

 a spine is pressed steadily to one side by the fingers, or by the weight 

 of the animal's body, the muscles on the side pressed lose their 

 tonus. The spine, therefore, becomes loosely movable in certain 

 directions, but not in others. On the other hand, a sudden violent 

 increase of tension, or a mechanical jar, increases the tonus, so that 

 the spines stand out firmly. 



Now, the loss of tonus, caused in the way just described, is con- 

 ducted, doubtless by the nervous network, to the neighboring spines. 

 This conduction occurs in such a way that it is the muscles of corre- 

 sponding sides of the neighl)oring spines that lose their tonus. (This 

 involves complicated conditions in the nervous net ; v. Uexkiill holds 

 that it shows the existence of many independent nets.) Hence when 

 a spine is pressed toward one side, the neighboring spines likewise 

 bend in the same direction. This v. Uexkiill calls the chaining of the 

 reflexes ('^Reflexverkettung"). It shows itself (an important fact) 

 most readily when the spine is bent toward the mouth; the other 

 spines also bend toward the mouth. 



These facts have the following result. When a spot on the body 

 is strongly stimulated, so that the spines bend away from it, the 

 disturbance is not limited to those in the innnediatc neighborhood. 

 The S})incs in bending away press upon the surrounding s])ines, 

 tending to bend them down. They are more easily bent toward' 

 the mouth than elsewhere, so a new set of spines bend over in that 

 direction. They again press on the next spines, bending them in 

 tnrn toward the mouth. Thus a sort of wave })asses toward the 

 mouth from the point stimulated, the spines bending in turn far over 

 toward the mouth, then back again. The entire phenomenon v. 

 Uexkiill calls the wandering of the center of excitation (''Wandcrnng 

 des Erregungsniitt('l])nnkts''' ). 



Another most important fact shows itself. Muscles that are not 

 in tonus are much more easily stimulated to contraction than those 

 which are in tonus. When the muscles lia\'e their usual strong 

 tonus, it re(piii'es a jiowerfnl stimulus to cause tlieni to contract 

 further. But muscles which have lost their tonus as a result of 



