IIO INTERNAL SECRETION 



are brought about by a single homogeneous hormone capable of 

 producing two opposite sets of effects, according to the apparatus 

 influenced. The nervous system supplies the only field for such 

 activity, and so the further question suggests itself : Is the 

 hormonic activity of the thyroid gland direct, or is it effected 

 only by the agency of the nervous system ? That it is possible 

 for an organ to send out chemical stimuli which, in different parts 

 of the body, produce entirely different results in one part pro- 

 moting assimilation and in another inhibiting it cannot be 

 doubted. It must further be taken into account that the end- 

 results will also be different in cases where the influence upon 

 the end-organ is not direct, but is effected by the agency of 

 another internal secretory organ. Thus, for instance, the changes 

 which take place in the sexual organs after suppression of the 

 thyroid might easily result, not directly from the absence of an 

 assimilatory thyroid hormone, but indirectly and as secondary 

 results, due to a change in the function of the hypophysis. The 

 solution of these problems demands a far more complete know- 

 ledge of the interactivities of the internal secretory organs than 

 we at present possess. 



The second theory, which explains the antagonistic activities 

 of the thyroid gland by a single thyroid hormone, acting primarily 

 upon the nervous system, demands serious consideration. Certain 

 phenomena seem to point to the conclusion, that the changes in 

 organic function brought about by the agency of the thyroid 

 secretion, are, in reality, only the physiological expression of a 

 changed condition of irritability of the vegetative nervous system. 

 Although the thyroid secretion principally stimulates the sym- 

 pathetic system, it does not affect it alone, for it also stimulates 

 certain portions at least of the autonomous system ; moreover, the 

 sympathetic and autonomous innervation is, certainly in many 

 parts, antagonistic. This antagonism is sufficiently explained by 

 the assumption of a single homogeneous thyroid hormone, which 

 exercises a disassimilatory effect upon the vegetative nervous 

 system in other words, increases its functional activity. But it 

 is very important to remember that our knowledge of the manner in 

 which the nervous system acts upon the processes of metabolism, 

 and especially upon the new formation of tissue and the processes 

 of growth, is extremely meagre. Investigation of recent date, 

 especially that undertaken by the Vienna school of scientists, has 

 been concerned solely with the chemistry of metabolism, while the 

 morphological aspects of these, processes have up to now received 

 but little attention. 



THE THYMUS GLAND. 



THE thymus (German : Briesel) develops in man as a paired 

 sac-like diverticulum of the ventral portions of the third and 

 fourth, and perhaps also partly from the second, pharyngeal 



