342 DUCTLESS GLANDS. 



It has been demonstrated, as already stated, that grafting a 

 thyroid or part of it into an animal before the removal of its 

 thyroid will prevent the myxedematous symptoms. It is interest- 

 ing to know that if, later, this graft is removed, the symptoms will 

 then supervene. Injections of an extract of the thyroid gland 

 and also feeding the gland itself are followed by the same results 

 as the grafting process. 



There are two theories which have been advanced to explain 

 the results of removal of the thyroid : (1) autotoxication and (2) 

 internal secretion. 



Autotoxication. This theory supposes that there are toxic sub- 

 stances normally in the blood which, being removed or rendered 

 harmless by the thyroid, accumulate when that organ is removed 

 and produce the effects that follow thyroidectomy. In support 

 of this, it is stated that the urine of animals operated on is more 

 toxic than that of unoperated animals, and that their blood is also 

 toxic for others. 



Internal Secretion. We have already seen that the pan- 

 creas, in addition to the pancreatic juice, which is its external 

 secretion, also produces an internal secretion. This is also be- 

 lieved to be true of the thyroid, and it is this secretion which is 

 taken up by the blood or the lymph in its passage through the 

 gland and carried to the tissues, where it is, in some way not under- 

 stood, connected with their metabolic processes. From the dis- 

 turbances in the nervous system and connective tissues which occur 

 on ablation of the gland, it is probably especially related to their 

 metabolism. When, therefore, after extirpation of the thyroid or 

 in cases of myxedema where the gland is diseased, injections of the 

 extract or injections of the gland itself are followed by beneficial 

 results, it is, doubtless, due to the introduction into the blood of this 

 internal secretion. It is also possible that some of the toxic products 

 of metabolism may be destroyed by the gland or its secretion. 



Several observers have maintained that the thyroid was inti- 

 mately connected with the regulation of the supply of blood to 

 the head, and have reasoned thus from its great vascularity and 

 direct connection with the blood-vessels of the head ; and Cyon has 

 demonstrated that the nerves supplying the thyroid when stimu- 

 lated lower the blood-pressure in the carotid, by virtue of vaso- 

 dilators, which are contained in the trunks of the nerves. These 

 nerves are called into action when the cut ends of the vagi, of the 

 depressors, or of the cardiac branches of the recurrent laryngeal 

 nerves are stimulated. 



To which of the constituents of the thyroid extract its effects 

 are due is still a mooted question. Indeed, the most recent 

 researches seem to indicate that the gland forms more than one 

 substance, each one having its own action ; thus there seems to be 

 no doubt that both iodothyrin and thyreo-antitoxin are produced, 



