THE CAUSE OF PULSATION 



483 



' « D 



^ D 



Fig. 2. Showing how a Pulsation-wave may be 



Circuit of Tissue. 



transmits them is almost certain to be more complete on one side than 

 on the other of any stimulated point. Of course the waves meet as in 

 Fig. 2, B, and then the strong wave destroys the weak one and con- 

 tinues around the ring. There is then only one wave left in the cir- 

 cuit and this travels constantly around (Fig. 2, C) for hours or days 

 until something stops it. such as the cutting of the circuit or a fresh 

 stimulus which produces a wave that meets and destroys it. 



The weak wave was destroyed by the strong one in the above experi- 

 ment because a weak stimulus can not set tissue into pulsation, which 

 has been caused to pulsate through a strong stimulus, until after an 

 appreciable interval of rest. Thus a weak stimulus following im- 

 mediately after a strong one will produce no contraction, whereas a 

 strong stimulus may cause tissue to pulsate even immediately after it 

 has responded to a weak one. 



It is now evident that the disk without its sense-organs can pulsate 

 in sea-water if only a wave be once started in it, but that under normal 

 conditions there is nothing to start a wave, and thus the disk remains 

 quiescent. In other words, the sea-water is indifferent, and neither 

 stimulates nor inhibits pulsation. 



It is now time for us to determine why it is that the sea-water does 

 not stimulate the disk when its sense-organs are removed. In the first 

 place we must know the composition of sea-water, and chemical analysis 

 shows that it consists of a mixture of sodium chloride (common salt), 

 magnesium chloride and sulphate, potassium chloride, and calcium 

 chloride and sulphate. 



Numerous experiments show us that the common salt is a strong 

 stimulant to both nerves and muscles. On the other hand, magnesium, 

 calcium and potassium, all inhibit and do not stimulate the disk. 

 Indeed, the stimulating effect of the common salt in the sea-water is 

 exactly offset by the subduing tendency of the magnesium, calcium and 

 potassium; and thus it is that the sea-water as a whole neither stimu- 

 lates nor inhibits the pulsation of the jellyfish. The sea-water main- 



