352 GENERAL PHYSIOLOGY 



With all those organisms that do not exist in a constantly 

 uniform nutrient medium, that rather must seek their food, food 

 is available only at irregular intervals. Periods of lack and periods 

 of superfluity alternate with one another. If such an organism 

 has had no food for some time, if, e.g., an Amceba, which nourishes 

 itself upon Algce, has been deprived of food for some time and by 

 chance comes to a place where Algae exist, these food-organisms 

 operate as a stimulus upon it and cause it to creep toward and 

 ingest them. Here food acts as a stimulus, although it is a 

 necessary vital condition. Analogous cases exist in the cell- 

 community. The simplest example is afforded by the green 

 plants. Light forms one of their most important vital conditions. 

 Without light no cleavage of carbonic acid, no formation of starch, 

 no assimilation, takes place in the green parts of the plant ; the 

 plant dies. Yet this condition undergoes the widest variations in 

 intensity, for light continually alternates with darkness and, 

 therefore, acts as a stimulus. Not only can the process of 

 assimilation be regarded as a phenomenon of stimulation, but the 

 light-stimulus produces, in addition, a series of other, very evident 

 reactions which express themselves in motion. In the animal 

 cell-community, also, cases in which stimuli are a vital condition 

 are known in great number. The stimulating impulses that 

 are produced in the central nervous system become transmitted 

 to the tissue-cells through the nerve-fibres. A muscle, e.g., moves 

 only when a stimulus is conducted to it from the brain or the 

 spinal cord through its nerve. If the nerve be cut or in any other 

 way be made incapable of transmitting the impulse from the 

 central nervous system, the muscle can no longer move, and after 

 a time atrophies. In less degree a muscle becomes feeble and 

 decreases in mass when it is used little, i.e., when few impulses 

 are sent to it from the central nervous system. This condition is 

 termed atrophy from disuse. This is true not only of muscle- 

 cells, but of all tissues to which, through their nerves, stimulating 

 impulses are no longer conducted. In cases where, by disease, 

 a portion of a nerve has become temporarily impassable to stimuli, 

 medical treatment endeavours, often with success, to hinder the 

 atrophy of the tissue supplied by the nerve by stimulating it arti- 

 ficially by electrical currents, and in this action of the galvanic 

 current lies the sole therapeutic importance of electricity. The 

 strengthening of an organ by use belongs also in this category. By 

 continued use, as every gymnast, fencer, oarsman, and mountain- 

 climber knows, a muscle of moderate strength can be transformed 

 in a short time into one of marked strength and endurance, the mass 

 increasing very considerably. The effect of all exercise depends 

 upon the fact that stimulating impulses are sent continually into 

 the organ in question, putting it into activity. 



From these examples it is evident that certain stimuli can be 





