8 R G. HOSKINS 



tration of th blood is raised. The respiratory center is thereby stimu- 

 lated and the breathing movements become more rapid and deep. Thus 

 more oxygen is supplied and the carbon dioxid more rapidly excreted. 

 Respiration is then carried on at this increased level as long as 

 the increased muscular activity persists and automatically decreases when 

 the activity lessens and thus lowers the rate of carbon dioxid produc- 

 tion. 



The stimulating effect of carbon dioxid is due to the fact that it 

 unites with water to form an acid. There are other acid fatigue products 

 that have a similar influence. Nor is the respiratory center the only 

 structure which reacts to such acid products. The activity of the heart 

 also is increased and the laboring muscles thereby supplied with more 

 blood. A similar adaptive result is attained by local dilatation of the 

 capillaries of the active tissues. These reactions are among the most 

 important adaptations with which biologists are familiar. For non- 

 specific regulatory substances such as carbon dioxid, Gley has proposed 

 the designation " parliormones." 



The internal secretions are entirely different in nature from the 

 ordinary secretory products of gland-cell activity, such as saliva, or the 

 excretions, such as urine. The ordinary secretions are characterized by 

 their ferment or enzyme constituents. Through much study these have 

 become fairly well known. They exist not only in the external output 

 from various cells, but also in the cells themselves and in the circulating 

 fluids. The enzymes are apparently complex in their chemical struc- 

 ture; they are probably proteins. They are sensitive to the presence 

 of hydrogen ions and have an optimum temperature at which they func- 

 tion best. In the presence of water they are readily destroyed by heat 

 and by digestion. They are specifically adapted to the substrates upon 

 which they act. 



The hormones, on the other hand, are, so far as known, much simpler 

 bodies. They are crystallizable and dialyze freely. They withstand boil- 

 ing and, according to Abderhalden, are not destroyed by the action of 

 the digestive juices. Some of them, such as epinephrin, act much more 

 rapidly than do the enzymes, but others, acting as they do upon the 

 developmental processes, show their influence only after a prolonged 

 period. A hormone from the suprarenal glands, epinephrin, has been 

 isolated in a pure form. That the active principles of the thyroid, hy- 

 pophysis, and testes have been similarly isolated has also been claimed. 

 Kpinophriii and thyroxin have been artificially synthesized. Doubtless 

 as their chemical nature becomes better known, other hormones will be 

 similarly produced. None of the internal secretions, so far as we know 

 at present, gives rise to anti-body formation. The individual hormones 

 apparently do not differ among the various species of animals; that is, 

 they show physiologic but not zoologic specificity. Epinephrin, for ex- 



