242 HEAT AND PROTOPLASM [Ch. VIII 



epithelium of the frog ceased its movements at 0°. Muscles 

 of the frog were found by Kuhnb ('64, p. 3) to become at 

 — 3° to — 7° a solid lump which did not, however, wholly lack 

 irritability. The evidence of all these cases shows that activity 

 nearly ceases in protoplasm at or near 0° C. 



Another effect produced on protoplasm by cold — an effect 

 which often immediately precedes quiescence — is violent con- 

 traction. This has been repeatedly observed. The protoplasm 

 of Tradescantia hairs, which has been in cold-rigor, was found 

 by KiJHNB ('64, p. 101) to lie in separated rounded drops and 

 lumps, — an appearance like that resulting from excessive stimu- 

 lation. The rapid freezing of muscle gives rise, according to 

 Hermann ('71, p. 189), to violent contractions. The sciatic 

 nerve of the frog's leg when cooled to — 4° to — 8° causes clonic 

 contractions of the muscle, lasting two minutes. (Apfana- 

 siBFF, '65, p. 678, and others.) It is clear, then, that cold acts 

 as a violent stimulus to protoplasm. 



The final result of temporary rigor is thus clearly brought 

 about by the cooperation of two causes : (1) the diminution in 

 the chemical processes upon which metabolism and movement 

 depend, and (2) the directly stimulating effect of the cold, 

 which acts like contact or excessive heat. Both causes work 

 to produce a quiescence which may be replaced by activity 

 when the causes are withdrawn. 



The fact that cold-rigor usually occurs close to the zero- 

 point indicates that the activities of protoplasm are closely 

 determined by the fluid state of water. This fact is not to be 

 explained on the ground that freezing prohibits all chemical 

 change — many chemical changes take place below the freez- 

 ing point of water, but, apparently, few of those which are 

 involved in metabolism. Nor is the rigor due to the change 

 which the freezing of the protoplasmic fluids brings, because 

 as the temperature approaches the zero-point, but while the" 

 water is still perfectly fluid, metabolism diminishes ; and it 

 diminishes at such a rate as to cease just where water begins to 

 freeze. The critical point for vital activity has been adjusted 

 to this critical point of water. 



So, too, the composition of protoplasm is such that at a tem- 

 perature, lying below the normal and above the freezing point 



