248 HEAT AND PROTOPLASM [Cn.VIII 



says KUHKE, " was very remarkable, for there was no trace of 

 the protoplasmic network; but the violet cavity of the cell con- 

 tained, in addition to the naked nucleus, a large number of 

 separate round drops and lumps." In this case, the separate 

 pieces eventually became active again, so that the protoplasm, 

 though nearly killed, was not quite so. 



The phenomena seen by KUHKE so closely resemble those 

 produced in the same kind of cells by the galvanic current and 

 other strong irritants as to indicate that cold acts as an intense 

 irritant. We cannot, however, conclude that cold acts in no 

 other way. It is clear that the expansion of forming ice in the 

 vacuoles of the protoplasm must seriously disturb the structure, 

 and, since the whole matter has received little attention, it is 

 possible that a molecular change of some sort takes place when 

 there is much water in the freezing protoplasm. To summarize : 

 Death by freezing results in the higher animals largely from 

 asphyxia, and in the simpler organisms from excessive irritation, 

 mechanical rupture, and, perhaps, other causes. 



I shall now sum up this section on the effect of extremes of 

 heat and cold. As the temperature is elevated above the opti- 

 mum, molecular changes occur in the protoplasm leading to its 

 contraction. The contraction becomes more violent as the tem- 

 perature is still raised, until, finally, a new series of molecular 

 changes occur by which the protoplasm begins to coagulate. 

 At this point the protoplasm begins to lose its irritability. If 

 this process has not proceeded far, the vital activities may, 

 under favorable conditions, return (temporary heat-rigor). 

 Beyond a certain point (death point) recovery is impossible. 

 The death point varies with the species, but lies not far from 

 the maximum natural temperature attained by the medium in 

 which they live. On the other hand, as the temperature is 

 diminished from the optimum, the chemical processes of 

 metabolism decrease in vigor and come to a standstill at about 

 the freezing point of water. Violent contractions accompany 

 the cooling process, concomitantly with which the protoplasm 

 breaks down. From this condition of temporary cold-rigor 

 recovery is still possible ; but a little below, at a point dependent 

 upon the size of the body and the diathermous qualities of its 



