90 THE ELEMENTARY NERVOUS SYSTEM 



view of the nervous connection of ectoderm with ento- 

 derm is not wholly correct has already been given in the 

 preceding chapter, but much more conclusive physiologi- 

 cal evidence to this effect will be presented here. 



By appropriate lines of incision through the thin walls 

 of sea-anemones it is possible to make preparations by 

 which the courses taken by nervous impulses through the 

 bodies of these animals can be determined with much 

 accuracy (Parker, 1917 a). To test such preparations it 

 is necessary to use a means of stimulation that is both 

 accurately controllable and strictly local. Such a means 

 is found in the mechanical stimulation produced by a 

 delicate blunt glass rod. When the surface of a Metrid- 

 ium is explored by such means the degree of sensitive- 

 ness of the different regions is found to be as follows: 

 Almost insensitive, the surfaces of the pedal disc, the lips, 

 and the oesophagus ; slightly sensitive, the surface of the 

 column between the sphincter and the oral disc, the oral 

 disc between the tentacles and the lips, and the siphono- 

 glyphs; slightly more sensitive, the tentacles and the 

 equatorial portion of the column; fairly sensitive, the 

 surface of the column near the sphincter ; and most sensi- 

 tive, the surface of the column near its pedal margin. 

 Stimulation of any of these regions was followed by a 

 retraction of the oral disc due to a contraction of the 

 longitudinal muscles of the mesenteries chiefly. The re- 

 gions of stimulation, as the description implies, were al- 

 ways in the ectoderm; the response was made by ento- 

 dermic muscles. Hence this particular set of reactions 

 was very appropriate as a means of testing the course of 

 nervous transmission from ectoderm to entoderm. 



If the column wall of a sea-anemone is cut through in 

 a complete ring equatorially, that is, if the column is gir- 



