428 PROCESSES INFERRED FROM INDIRECT OBSERVATTON 



is used up in consequence of their excitation. On the other hand, 

 Carlson's experiments with the heart ganglion of Limulus and the 

 above-cited experiments on the effects of heating the respiratory center 

 show that chemical changes play a predominant role in the activities 

 of Nerve Cells, and, as a matter of fact, the consumption of oxygen 

 by the brain is very large, and it is the first tissue to suffer from lack of 

 oxygen, indicating a very high level of metabolic activity in the cellular 

 elements of the nervous system. The conducting fibers and the nerve- 

 cells from which they issue stand therefore in sharp contrast to one 

 another in respect to the metabolic foundations of their functional 

 activity, and we are thus led to recall the fundamental difference 

 between their susceptibilities to the various classes of chemical stimu- 

 lants to which reference has been made in a previous chapter. Nerve- 

 fibers are powerfully stimulated by salts which precipitate calcium, 

 nerve-cells are insensitive to these reagents. Nerve-cells are stimu- 

 lated by a variety of specific substances, by polyphenols and by 

 Creatine, for example, to which nerve-fibers are indifferent. 



The phenomena of Muscular Contraction and the change which trans- 

 forms the nervous impulse into a muscular stimulus at the myoneural 

 junctions are, it would appear, conditioned in their speed by underlying 

 chemical reactions. Thus Burnett has determined the influence of 

 temperature upon the Latent Period of indirect muscular stimulation 

 (i. e., through the intermediation of a motor-nerve) and finds that the 

 period consumed in the transformation of the nervous into the muscular 

 stimulus is halved or even more reduced by a rise of ten degrees in the 

 temperature. Similarly the changes involved in the stimulation of 

 Sensory Nerve Endings are determined by chemical factors, since T. E. 

 Moore has shown that the temperature-coefficient of the reaction to 

 cutaneous stimulation by heat is of the chemical magnitude. Corre- 

 sponding with these facts we find that nerve-endings readily undergo 

 fatigue. We have seen that the rate of the heart-beat is doubled or 

 more than doubled by a rise of 10 and the same thing has been found 

 to be true for other rhythmic muscular contractions. The rate of 

 conduction of the Action-current in Muscles, however, appears, from 

 the investigations of Lucas, to be a process analogous to the conduc- 

 tion of a nervous impulse, comparatively little affected by temperature 

 (coefficient from 1.45 to 1.65). 



It is a general characteristic of Photochemical Reactions, and a pecu- 

 liarity which distinguishes them from all other types of chemical 

 transformation, that they are practically unaffected by temperature, 

 the temperature-coefficients being usually unity, and at any rate not 

 in excess of the magnitudes commonly obtained in purely physical 

 phenomena. This being the case it is a very significant fact that the 

 temperature-coefficients of the phenomena induced by light in living 

 organisms are usually high and distinctly of the order indicating the 

 involvement of chemical reactions of the ordinary type. Thus the 

 phototropic bending induced by light in Avena sativa has been shown 



