xvin PHYSIOLOGY OF NERVOUS SYSTEM 519 



which run to the ectoderm and enter into connexion with the 

 plexus there. Eomanes 1 scraped away the radial cords and found 

 that the spines still converged when a point on the ectoderm 

 was stimulated, but that, on the other hand, if definite locomotor 

 movements were to be carried out, the presence of these 

 cords was a necessity ; hence he concluded that the superficial 

 plexus sufficed for ordinary reflexes, but that for purposeful move- 

 ments the central nervous system was necessary. 



Von Uexkull 2 has made an exhaustive study of the physio- 

 logy of the nervous system in the Echinoidea. He points 

 out that all the organs controlled by the nervous system, 

 spines, pedicellariae, tube-feet, and (see below) Aristotle's lantern, 

 give two opposite reactions in response to the same stimulus 

 according as it is strong or weak, bending away from the point 

 of stimulation when it is strong and towards it when it is weak. 

 This reversal of reaction can only be due to the action of the 

 neuron in altering the effect of the stimulus on the muscles, and 

 this Uexkull regards as its fundamental property. Thus in 

 Preyer's 3 experiments with Starfish the strong form of stimulation 

 is obtained by directly applying the stimulus to the radial cord or 

 to the tube -feet, the weak form by stimulating the back, when 

 of course the stimulus has to traverse a longer path before 

 affecting the tube -feet, and is consequently weakened. Yon 

 Uexkiill also introduces the conception of " tone " with regard 

 to the nervous system. This term has been used to denote the 

 amount of chronic contraction in a muscle, and it is to be dis- 

 tinguished from the fleeting contractions which cause movement. 

 The more tone there is in a muscle the less responsive it is to 

 stimuli tending to bring about movement. As applied to the 

 nervous system " tone " denotes a condition when it is not 

 receptive to small stimuli, but when it is maintaining a con- 

 dition of tone in a muscle by which of course its own tone is 

 measured. Tone in a neuron can therefore be measured by the 

 produced tone in the muscle, and the one is to be discriminated 

 from the other only by using stimulants, such as caffeine, which 

 have no direct action on muscle. Tone can also be measured by 

 the amount of stimulus necessary to irritate the neuron. When 



1 "Jellyfish, Starfish, and Sea-urchins," Intern. Sci. Series, 1885, p. 302 et seq. 



" "Die Physiologie des Seeigelstachels," Zeitschr. fur Biol, xxxix. p. 73. 

 3 " Bewegungen von Stelleriden," Mittk. Zool. Stat. Neapel, vii. 1886-7, p. 22. 



