Groupings of the Chief Endocrine Glands 7 



of the blood-vessels and inhibition of that of the intestines. But in both cases 

 the action may be regarded as that of a hormone or exciting agent, for both 

 effects are produced by stimulation of the end substance of the sympathetic nerves. 

 In extreme dilution (1 to several millions) in mammals, and in a less dilute form 

 in birds, adrenalin is said to cause inhibition instead of contraction of the muscle 

 of the blood-vessels. But this again may be produced by excitation of inhibitory 

 nerve fibres by the more dilute solution, and if so, the action would still be 

 hormonic. Nevertheless, the possibility of the same autacoid substance acting 

 under some circumstances as a hormone or excitant, and under other circumstances 

 as a chalone or depressant, must be borne in mind. This, indeed, serves to illustrate 

 the drug-like nature of these principles, for such inversion of action under difference 

 of circumstance is known to occur with some drugs. 



Noel Paton (Regulators of -Metabolism, 1913) takes a view of the action of the 

 principles of the internal secretions different from that usually adopted. He regards 

 them as playing the same sort of part in the metabolism of the various organs 

 they influence as Sydney Ringer showed to be the case with the salts of blood in 

 maintaining the activity of the heart. " A certain minimum amount of each seems 

 to be essential, and some proportion between the amounts of each must be main- 

 tained if the metabolism is to continue in its normal course. ... It seems to me that 

 such a conception is more in accordance with the facts which we possess than that 

 of a series of hormones or excitors directly calling forth the activity of the various 

 tissues." 



GROUPINGS OF THE CHIEF ENDOCRINE GLANDS 



Most of the purely endocrine glands can be grouped under the three 

 main heads of thyro-parathyroid apparatus, suprarenals, and pituitary, 

 It is noteworthy with regard to these groups that each of the organs 

 forming the group is compounded of two distinct but usually closely inter- 

 grown parts. Thus the thyro-parathyroid group is composed of thyroid 

 proper and parathyroids ; the suprarenal of cortical and medullary portions ; 

 the pituitary of epithelial and epithelio-neural parts. And in each case 

 the functions of the two portions, so far as is known, appear to be in no 

 way similar, difficult as it is to believe that tissues so closely connected 

 anatomically should have no sort of functional connexion. The close 

 anatomical connexion is, however, sometimes absent, as with those para- 

 thyroids which are altogether detached from the thyroid, and as with the 

 paired bodies and the interrenal body of Elasmobranch fishes. A close 

 anatomical relationship without any apparent functional bearing is, how- 

 ever, not without examples in other organs, e.g. in the frog there exists 

 a very intimate anatomical connexion of the suprarenal with the kidney, 

 although there is no particular reason to believe that any special functional 

 relationship obtains between the two. 



It is further noticeable that in each of the above groups one of the two 

 parts has a more evident, and in a sense a more active, function than the 

 other. Thus the removal of the parathyroids occasions symptoms which 

 are far more acute than those produced by the corresponding operation on 



