98 



SCIENCE 



[N. S. Vol. XXXI. No. 786 



definition. The secretin formed in the in- 

 testinal epithelial cell is conveyed to the 

 pancreas in the blood and brings about a 

 correlation between the activity of this 

 gland and that of the duodenum, but on 

 the other hand some substance contained 

 in the pancreatic juice and conveyed to the 

 duodenum in the stream of secretion ex- 

 cites the formation of the enterokinase, 

 and thus correlates the activity of the duo- 

 denum with that of the pancreas. The 

 two actions seem to be so similar, except 

 for the means of transport, that one would 

 naturally put them in the same class. By 

 the same reasoning we might be justified 

 in designating the hydrochloric acid of the 

 'gastric juice as a hormone in reference to 

 its action in causing a formation of secre- 

 tin in the epithelial cells of the duodenum. 

 One can imagine that a similar transporta- 

 tion may occur in the secretions of the 

 reproductive or respiratory passages, in 

 the cerebro-spinal fluid, as seems to be the 

 case for a time at least with the secretion 

 of the pars intermedia of the pituitary 

 gland, or even along the axial stream of a 

 nerve fiber. If, as seems to me, the idea of 

 correlation or coordination is the essential 

 point rather than the assumption that the 

 product must constitute an internal secre- 

 tion, we might modify the definition so far 

 as to designate as hormones those sub- 

 stances in solution which, conveyed from 

 one organ to another through any of the 

 liquid media of the body, effect a corre- 

 lation between the activities of the organ 

 of origin and the organ on which they 

 exert their specific effect. As regards the 

 nature of the action of the hormones on the 

 organ affected we know too little to make 

 any safe generalization. In the case of the 

 secretin it seems most probable that the 

 hormone arouses the pancreatic cells to an 

 act of secretion and therefore it has in 

 this instance the value of a chemical stim- 



ulus. But in other cases the effect of the 

 hormone may be rather of the nature of an 

 activation. This at least would seem to 

 be true for the hormone, of unknown na- 

 ture, given off by the pancreas and con- 

 cerned in the glycolysis of sugar in the 

 organism. The effect of the hormone adre- 

 nalin upon the musculature innervated by 

 the sympathetic system may also be of the 

 nature of an activation rather than of a 

 chemical stimulation. 



The substances of known composition 

 which may be regarded as playing the role 

 of hormones are few in number, three or 

 four at most as follows: First, the carbon 

 dioxide formed in the tissues, particularly 

 in muscle during contraction. It seems 

 agreed now that the carbon dioxide acts as 

 the normal stimulus to the respiratory 

 center. "When produced in the working 

 muscles in such quantities as to raise per- 

 ceptibly the carbon dioxide tension in the 

 alveoli of the lungs and the blood of the 

 pulmonary veins, the respiratory center is 

 excited to greater activity and the excess 

 above the normal contents is thereby re- 

 moved; second, the adrenalin of the 

 adrenal glands which in some way, directly 

 or indirectly, makes possible the full func- 

 tional activity of the involuntary muscu- 

 lature of the body; third, the hydro- 

 chloric acid produced in the stomach which 

 stimulates the foi-mation of secretin in the 

 duodenal epithelium; and fourth, possibly 

 the iodothyrin of the thyroid gland with 

 its dynamogenic effect upon the neuro- 

 muscular apparatus of the body. In addi- 

 tion there are a number of hormones of un- 

 known composition which have been either 

 proved or assumed to exist, and which are 

 held responsible for certain well known 

 correlations of function. The pancreatic 

 secretin formed in the epithelium of the 

 duodenum or jejunum which stimulates 

 the flow of pancreatic secretion ; the gastric 



