320 'Journal of Comparative Neurology and Psychology. 



the aninial as a whole the parts do influence one another. The 

 author therefore took in hand a thorough study of the changes in 

 the reflexes and the way they influence each other, undertaking to 

 formulate and define precisely the underlying phenomena. It is 

 here that we find the origin of the most important concepts in von 

 ITexklill's highly original formulation of behavior. It will be worth 

 while therefore to examine carefully a sample of the author's method 

 of analysis; for this purpose we select certain features in the 

 physiology of the spines. 



Ty1'1CAT> EXA.MI'LK OF MeTUOD OF ANALYSIS. 



It will be recalled that the rounded shell of the sea urchin is 

 covered with long spines. Each spine is a tapering calcareous rod, 

 with a concavity at its base, by which it articulates with a hemi- 

 si)herical elevation of the shell. The spine is held in position by 

 two circles of muscles radiating from the circumference of its base 

 to the shell. These muscles, and particularly the inner circle, are 

 steadily pulling upon the spine, thus holding it stiffly in position. 

 This pulling takes place without external stimulus ; it is due to a 

 certain amount of tension which forms the normal condition of the 

 muscles and continues without any such repeated contractions or trem- 

 ors as are called tetanus. That is, the muscles have a certain normal 

 tunus. This tonus becomes the central concept in v. Uexklill's 

 formulation of the physiology of movement. The iinier layer of 

 muscles is devoted chiefly to maintaining by its constant tension a 

 certain position of the spine ; it is an example of one of the two great 

 tyjies of muscular action, — the ''Sperrung" or tension, as dis- 

 tinguished from actual contraction, involving shortening. The outer 

 circle of muscles is more active in its changes; they shorten quickly 

 and readily, thus moving the spine in various ways. They exemplify 

 tlie other great type of imiscidar iiction, — ^'Vcrl-iirziinrj," — shorten- 

 ing or contraction. Tension and contraction v. T^exkiill shows occur 

 quite independently of each other, ar.d this independence, with all its 

 theoretical and practical consequences, conu^s to play a very great 

 part in the later development of v. ITexkiill's views; li(^ finds it 

 throughout animals (see 33). The neglect of the fact that we have 



