522 
in the rate of production of its secretion. 
As is well known, this stimulatory increase in 
secretory products is always accompanied by 
large increases in the amount of oxygen con- 
sumed, CO, produced and heat evolved. In 
the case of muscle, the amount of lactic acid 
produced furnishes a certain criterion of the 
metabolic rate of the muscle. In the resting 
state, the lactic acid content is small; after 
the stimulation of injury and after contrac- 
tion, it is greatly increased. And although 
molecular oxygen does not appear to be con- 
sumed in this process, the production of lactic 
acid from carbohydrates is nevertheless chem- 
ically an oxidation. In nerve, where it was 
long believed that no increase in metabolism 
occurred during the passage of the impulse, it 
is now known, thanks to the researches of 
Tashiro,” that a relatively great increase in 
carbon dioxide output is associated with the 
process. He also presents some evidence that 
the oxygen consumption is likewise accel- 
erated, during the conduction. In this con- 
nection it may be mentioned that Alexander 
and Revyesz!® found that the oxygen consump- 
tion and the carbon dioxide elimination of the 
brain are increased when the retina is stimu- 
lated by light. In nervous, as in other tissues, 
the difference between the resting and the 
stimulated state is then largely quantitative. 
Further significant facts are that the rate of 
the passage of the current of action along the 
nerve bears a direct relation to the known 
irritability of the nerve and the rate at which 
it conducts the impulse; and as Tashiro’s 
work shows that these factors are also directly 
related to rate of carbon dioxide production 
(more irritable nerves in the resting state 
giving off more of the gas than the less ir- 
ritable ones), 1t becomes obvious that metab- 
olic condition is the primary factor in the 
causation of the current of action. 
The evidence then clearly indicates that 
each cell is always carrying on a specific kind 
of metabolism and that stimulation consists 
essentially in the temporary acceleration of 
17 Loe. cit., Chaps. II. and III., also p. 53. 
18 Biochem. Zeitsch., XLIV., p. 95. 
SCIENCE 
[N. S. Von, XLVIII. No. 1247 
the rate of metabolism. From this point of 
view regarding the nature of stimulation, the 
eurrent of action is readily explicable. Any 
stimulated region is a region whose metabolic 
rate has been temporarily increased by the 
stimulation, and as in the preceding eases, it 
must necessarily form with non-stimulated 
regions of lower rate a concentration cell with 
respect to rate of metabolism. It must there- 
fore be electronegative to such non-stimulated 
regions. The various kinds of bioelectrice cur- 
rents are thus conceived as referable to the 
same cause, differences in metabolic rate; and 
they are, apparently, merely the consequence 
of such differences, and of no significance in 
themselves. 
This explanation of the current of action 
has been long held by Waller (loc. cit.), who 
on scanty and indirect evidence and in the 
face of skepticism from other physiologists 
maintained that the metabolism of nerves is 
the same as that of other protoplasm, that the 
current of action is due to sudden increase in 
their rate of metabolism, and that this current 
is merely a sign of that metabolic change. 
Waller’s idea has received strong confirmation 
through Tashiro’s work. 
Certain interesting corollaries follow if this 
conception of the nature of stimulation is 
true. Thus if an increase in metabolic rate 
is the essential feature of stimulation, it fol- 
lows that any organ or cell whose rate of 
metabolism is already sufficiently high will 
function without stimulation—in other words, 
it will be automatic. Some physiologists deny 
that there are any truly automatic organs, but 
surely the facts that are known about the 
heart, the medullary centers, and the digestive 
tract sufficiently prove that automaticity is a 
real phenomenon. Consider, for instance, the 
embryonic heart, which in all known cases, has 
a beat of myogenic origin. The metabolic 
rate of this young muscle tissue is in all prob- 
ability so high that it contracts in the absence 
of stimulation from without. Later, as the 
rate falls with age, the aid of the nervous sys- 
tem must be evoked to keep the apparatus go- 
ing. The nervous system is, indeed, the auto- 
matic structure par excellence of the body and, 
