THE EFFECT OF EXTERNAL INFLUENCES ON CELLS 63 



green coloring matter of plants; for germinating seeds develop in the dark 

 into a white or whitish seedling which becomes green only when it is exposed 

 to the light. Since the chlorophyll bodies are to be regarded as composed 

 of living substance, we have in the latter instance a case where an external 

 stimulus actually effects the formation of living substance. As regards the 

 first example, one might say that the abundant supply of nourishment had 

 given the impetus for a more active formation of living substance; but the 

 matter is not entirely clear, for it might also be that the impulse to mul- 

 tiply in these organisms is just as great with scanty as with abundant 

 nourishment, only with a deficiency of nutrient substances it cannot be 

 manifested. 



From observations on the storage of substance in the bodies of young 

 animals it appears that the inherent growth energy is of much greater im- 

 portance than any form of external stimulation. But in mature animals, 

 if any increase in the living substance takes place under normal circumstances, 

 we think of it at once as being caused by some agency outside the cells them- 

 selves; and hence its consideration properly belongs under the present topic. 

 Now results of metabolism experiments show that ordinarily the cells (of 

 the higher animals at least) destroy practically all of their daily supply of 

 proteid nourishment, but that under certain circumstances (not too great 

 age, and great excess of potential energy in the food, cf. page 120) they store 

 some of the proteid. It appears in fact that in spite of their inner propensity 

 to destroy proteid, an abundant supply of nourishment in some way makes it 

 possible for the cells to change dead proteid into living protoplasm. If this 

 is correct and the question can scarcely be regarded as finally settled this 

 storage would be the consequence of a chemical stimulation brought about 

 by the excess of proteid. 



However this may be, the only really effective way known to us of increas- 

 ing the living substance in the mature higher animals is work; and it is 

 possible to conceive of this also as a special form of chemical stimulation. A 

 grown man may eat ever so much food, his diet may be adapted perfectly to 

 the purpose, but no significant increase of muscle substance will take place if 

 the muscle does not accomplish sufficient work; whereas a working muscle 

 increases both in power and in volume i. e., the work has called forth an 

 increase of living substance. Since now every muscular movement is orig- 

 inated by the motor nerves, and since experience shows that a nonworking 

 muscle always decreases in volume, and a muscle paralyzed by cutting its 

 motor nerves undergoes atrophy and degeneration in a relatively short time, 

 it follows that some kind of a nutrient or trophic influence on the muscle 

 must be exercised by the central nervous system through the motor nerves. 

 What the nature of this influence is, we cannot say definitely. Since, how- 

 ever, the stimuli originating in the body itself are in general of a chemical 

 nature, we may perhaps conclude that the trophic influence mediated by the 

 nerves is a chemical stimulus. Other facts which we shall discuss somewhat 

 in detail in what follows show that an influence of a similar nature is exerted 

 on other organs by the nerves belonging to them. If the cerebral secretory 

 nerve of the submaxillary gland be cut, the gland atrophies. This nerve is 

 therefore of great importance for the maintenance of this part of the body, 



