50 THE CELL 



tion of starch are bound up with electromotive phenomena. If a shaded and 

 an exposed part of a green leaf (Fig. 17) be connected with a galvanometer, 

 when the light falls on the exposed part, an electric current is observed which 

 runs in the leaf itself from the insolated to the shaded part. The deflec- 

 tion begins after some three to ten seconds and lasts as long as the illumina- 

 tion continues, if that time does not amount to more than about five min- 

 utes. The effect is least in diffuse daylight, greatest in direct sunlight and 

 is abolished by boiling the leaves. All such effects are absent in flower parts 

 devoid of chlorophyll. 



Whether these electrical variations have any general significance for the 

 cells exhibiting them, or for the individuals containing the cells we cannot 

 say definitely. But the matter is clearer in the case of the well-known elec- 

 tric fishes, of which there are many different species. In most of these 

 fishes the electrical organs are metamorphosed muscles, but in Malapterurus 

 it represents a transformation of the glands of the skin. In rest the organ 

 shows no current; but under the influence of the specific nerves, as well as 

 by direct stimulation it develops very strong electric currents, the force of 

 which in the cramp fish (Fig. 30) amounts to thirty-one volts. That such 

 currents must be of great service for these animals in their struggle for exist- 

 ence is at once perfectly obvious. 



3. THE EFFECT OF EXTERNAL INFLUENCES ON CELLS 

 A. ON STIMULI IN GENERAL 



The fundamental property to which we trace the total activity of the liv- 

 ing substance is its irritability i. e., its ability under the influence of all 

 kinds of agents to change its metabolism, and therefore to change its trans- 

 formation of energy, in one direction or another. All those agents which 

 have the power to evoke a change of this kind are called stimuli. Among 

 them are to be included different chemical reagents, mechanical agents of all 

 kinds, heat, light and electricity. 



The change in metabolism produced by a stimulus is either dissimila- 

 tive or assimilative (cf. supra page 22). In the former there is a produc- 

 tion of kinetic energy and the process taking place in the cell is described as 

 an excitation. In the latter we have a storing of potential energy, and the 

 change is often described as a trophic effect. The stimulus may however 

 check metabolism for a longer or shorter time, may bring it to a standstill 

 momentarily, or stop it altogether. We speak then of a paralysis of the cell. 

 Paralysis represents a negative condition with respect either to the assimilative 

 or the dissimilative responses. 



By stimulation of certain nerves the dissimilative action of the innervated 

 organ may be abolished. Of such phenomena, described as inhibition, the influ- 

 ence of the vagus on the heart has been most closely studied. When the vagus 

 is stimulated artificially the heart beats more slowly, and with a stimulus strong 

 enough it stops in diastole (the brothers Weber, 1846). This inhibition is not 

 a kind of paralysis, for phenomena to be described later (Chapter VI) show that 

 while the heart in vagus standstill is inexcitable, it is not paralyzed. 



