NATURE OF THE PULSATlON-STlMUtUS. 1 9 



THE NERVOUS OR EPITHELIAL NATURE OF THE STIMULUS WHICH 

 PRODUCES CONTRACTIONS. 



If the sub-umbrella be injured by scraping parts of it away, as in 

 figure 5A, I, or if the margin be cut off as in figure 5A, iii, the removed 

 parts are soon partially regenerated, as shown in the dotted areas, but 

 this newly regenerated tissue is at first epithelial in character, and 

 lacks muscular elements. It therefore can not contract, yet if it be 

 touched with a crystal of KgSO^, or otherwise stimulated, it trans- 

 mits the stimulus across itself to the adjacent muscular tissue of the 

 sub-umbrella, which contracts vigorously, although the newly regener- 

 ated tissue which conducted the impulse does not itself contract. This 



^ Fig. 5A. — Newly regenerated sub-umbrella tissue which lacks muscular elements, 

 and can not itself contract, can still transmit the stimulus to pulsate to normal 

 tissue adjacent to it. In fig. 5A, II, the stimulus crosses areas A and B, which 

 do not contract, while C, D, and S contract In the order named. In fig. 5A, IV, 



the bridge of newly regenerated tissue does not itself contract, but serves nevertheless to transmit the 



stimulus causing contraction in E and F. 



can best be demonstrated by making the newly regenerated tissue 

 serve as a bridge connecting two pieces of uninjured sub-umbrella 

 tissue, as is shown in figure 5A, 11, or 5A, iv. Then, upon touching 

 figure 5A, II, at 5 with a crystal of Z2SO4 or other stimulant, a con- 

 traction wave passes from 5' through B-D-A-C ; but B and A, being 

 newly regenerated tissues without muscular elements, do not contract, 

 although the stimulus which produces contraction passes across them. 

 Similarly in figures 5A, iv, if E, which is normal sub-umbrella tissue, 

 be caused to contract, every contraction is followed by F, although the 

 bridge of newly regenerated tissue which connects them does not con- 

 tract. These experiments tend to show that the stimulus which causes 

 pulsation is transmitted by the epithelial or nervous elements to the 

 muscular elements, and not primarily by the muscular elements them- 

 selves. I have examined many specimens of newly regenerated tissue 



