PHYSIOLOGY: /. F. McCLENDON 
691 
fractory period, K* may inhibit stimulation by giving the cell a single 
stimulus which continues and is manifested by the continuance of the 
refractory period as long as the potassium ion is present. Ca* * on 
the other hand is very toxic and when too liberally applied .causes 
death accompanied by increase in permeability and death rigor, which 
may simulate the stimulated state. 
OH', Na*, and K* lower the threshold for excitation of the jellyfish 
and conch heart, and H', Mg' ' and C ' raise it. When cautiously 
applied, OH', Na', and K* stop the conch heart in systole, and H*, Mg* *, 
and Ca' ' stop it in diastole, but the action of Ca' ' may seem the re- 
verse when not carefully studied. This seemingly uncertain condition 
is probably due to the organism being composed of several kinds of 
irritable elements, each requiring a balance of ions of the two classes. 
A balance for all these elements cannot be maintained with one pair 
of antagonistic ions alone. 
Since the plasma membrane or cell surface is very probably composed 
of emulsoids, or hydrophile colloids,^ the change in permeability must 
be due to a colloid change. Although suspensoids are easily coagulated 
or precipitated, emulsoids have too great an afhnity for water to be 
easily separated from it except in a most gradual manner unless they 
are changed by certain reagents or temperatures so as to approach the 
character of suspensoids. We should therefore not look toward the 
processes of aggregation and dispersion as seen in suspensoids for an 
explanation of change in permeability. The changes in an emulsoid 
membrane are to be regarded as swelling and shrinkage as seen in gela- 
tine or collodion plates. Water soluble substances diffuse easily through 
collodion gels until the shrinkage reaches a certain point, beyond which 
the permeability rapidly decreases with shrinkage and the collodion 
finally becomes impermeable to electrolytes. This is probably due to 
the approximation of the collodion molecules until the pores are too 
small to admit of free diffusion of hydra ted ions through them. It is 
to be distinguished from mere approximation of colloid particles, since 
we may have diffusion through a continuous sheet that has swelled 
in water, and the diffusion probably takes place in the bound or hydrate 
water itself. Bartell and Bigelow have shown that porcelain becomes 
rapidly semipermeable when the pores decrease below O.i micron. 
It was shown by H. N. Morse that KCl and NaCl increase the per- 
meability of colloidal copper ferrocyanide membranes, but it is a gen- 
erally known fact that Ca and Mg salts do not increase the permeability, 
at least not sufficiently for their own diffusion or of that of sugar. Na 
and K salts (especially chlorides) increase the swelling and permeability 
