PROTOPLASM AND PHYSICAL AGENTS. 39 



deatli ensues at about 40 C., and the body becomes stiff and 

 rigid (/'Igor caloric) from the coagulation of the muscle-sub- 

 stance. The lower forms of animal life agree well "with plants 

 in their " fatal temperatures,' which in many cases lie between 

 40 and 50 G. 



Lastly, it appears to be true that there is a certain most 

 favorable or optimum temperature for the protoplasm of each 

 species of plant and animal, this optimum differing considerably 

 in different species. Probably the highest limit occurs among 

 the birds, where the uniform temperature of the body may be 

 as high as 40 C. The lowest occurs among the marine plants 

 and animals of the Arctic seas, or of great depths, where the 

 temperature seldom rises more than a degree or two above the 

 freezing-point. Between these limits there appears to be great 

 variation, but 35 C. may perhaps be taken as the average op- 

 timum. 



Moisture. Protoplasm always contains a large amount of water, of 

 which indeed the lifeless portion of living things chiefly consists. (Se 

 table on p. 34.) All plants and animals are believed to be killed by com- 

 plete drying, though some of the simpler forms resist partial drying for a 

 long time, becoming quiescent and reviving again when moistened, some- 

 times even after the lapse of years. Hence water appears to be an essen- 

 tial constituent of protoplasm, although, as in the case of mineral matters, 

 we do not know the nature of its connection with the other elements or 

 compounds present. 



Electricity. It has been shown that many forms of vital action are ac- 

 companied by electrical disturbances in the protoplasm. It is therefore 

 not surprising that the application of electricity to living protoplasm should 

 have a marked effect on its actions. If the stimulus be very slight, proto- 

 plasmic movements are favored. Colorless blood-corpuscles creep more 

 actively, and ciliary action increases in vigor. Stronger shocks cause a 

 spasmodic contraction of the protoplasm (tetanus), from which it may or 

 may not be able to recover, according to the strength of the shock. 



Poisons. Towards certain agents protoplasm is indifferent or seemingly 

 so, but towards others it behaves in a very remarkable manner. The mat- 

 ters known as poisons modify or destroy its activity, as is well known from 

 the familiar effects of arsenic, opium, etc. Disease may also interfere with 

 its normal activity ; but the consideration of these phases of the subject 

 belongs to the more exclusively medical sciences, such as toxicology and 

 pathology. 



Other Physical Agents. The more highly specialized forms of proto- 

 plasm are affected by a great variety of physical agents, such as light, 



