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the activity of the responding organ ; rather will its ultimate effects be, 

 by building up the responding organ, to increase its activity. 



The most familiar example of a chemical correlation evoking the 

 building up of tissues is that presented by the thyroid gland, though the 

 effects of the chemical substance formed by the thyroid are so widespread, 

 and differ to such an extent according to the age of the animal employed, 

 that a physiological analysis of its results is still difficult to give. In the 

 growing animal the chemical substance secreted by the thyroid evidently 

 influences the growth of tissues, among others of the bones, and it is a 

 familiar fact that injection or administration of thyroid to cretins will 

 result in a restoration of the child towards normal, in increased growth of 

 bones, and in development of various functions, including those of the 

 brain and central nervous system. On the other hand, in adults, the 

 pronounced effect of injection of thyroid is increased activity of the 

 chemical changes of the body, as instanced by the increased nitrogenous 

 metabolism, and disappearance of all over-growth in the subcutaneous 

 connective tissue such as is present in myxcedema. Although, therefore, 

 the main result of thyroid treatment is to restore normal growth where 

 such has been previously wanting, it is difficult to say whether its primary 

 effect should be regarded as dissimilative or assimilative. The fact that 

 the thyroid gland can be administered by the mouth shows that the 

 active principle is not destroyed by the gastric juice, and would, there- 

 fore, remove this from the proteid class of bodies, and would diminish 

 very largely any probability of the hormone furnished by this gland being 

 of the nature of a toxin. Whether it is represented by the thyroiodin, 

 the organic iodine compound extracted from the gland by Baumann, is 

 still doubtful, and we can only conjecture that in all probability, when 

 isolated, it will be found to belong to the drug class rather than to the 

 toxin class. We are still quite without knowledge as to the conditions 

 which determine the amount of active substance produced in the thyroid 

 gland. All we know is that the activity of the thyroid, like that of the 

 suprarenal gland, is essential to the normal development of the functions 

 of the body. Whether we are dealing here with a constant process, or 

 with a chemical reflex similar to those we have studied in the alimentary 

 canal and evoked by some event affecting directly the thyroid gland, we 

 cannot say. 



The largest group of correlations between the activity of one organ 

 and the growth of others is formed by those widespread influences exer- 

 cised by the generative organs on the body as a whole and on parts of the 

 body. The effects of removal of the testes in the male animal on the 

 growth and disposition of the individual have been known for centuries, 

 and marked additions to our knowledge of this subject have been made 

 recently by Mr. S. G. Shattock. According to Ancel and Bouin, the inter- 

 stitial cells of the testis provide an internal secretion, in the absence of 

 which the sexual characters of the male are not developed. The experi- 

 ments of Shattock and Seligmann show that the formation of the so-called 

 secondary sexual characters must be due to chemical influences from the 



