44 The Endocrine Organs 



this can only be early in life, for under normal conditions of growth and 

 development the thymus is undergoing retrogression whilst the thyroid is 

 becoming more active in its functions. Moreover, the normal variations in 

 both organs are so considerable, and at present, in spite of many observa- 

 tions, so little is accurately known regarding the conditions under which 

 they occur, that it will be wise to reserve judgment as to their mutual 

 relations. 



Influence of the Nervous System on Thyroid Secretion 



It was shown by Cyon that vasodilator nerve-fibres pass from the re- 

 current laryngeal nerves to the thyroid gland. Ossokin showed the gland 

 to be supplied by both superior and inferior laryngeal, and also by pharyn- 

 geal branches of the vagus. Vasoconstrictors pass to it by way of the 

 sympathetic. H. Wiener found extirpation of the inferior cervical ganglion 

 to be followed by atrophy of the gland and diminution of thyro-globulin 

 in the thyroid of the same side. Asher believes that the internal secretion 

 of the thyroid is directly influenced by the nerve-fibres which pass to the 

 gland by the superior and inferior laryngeal nerves. But he used as a 

 test of the outpouring of secretion the increase of excitability of certain 

 nerves (depressor and cardiac vagus), which he states is also produced by 

 injection of thyroid extract into the blood. This, however, I have not 

 found to be the case, and therefore, although it is not improbable that 

 the secretion is under the influence of either the vagus or sympathetic, or 

 both, further evidence is necessary before we can regard this as a proven 

 fact. It has been stated that the iodine content of a thyroid lobe is 

 diminished as the result of excitation of the vagus of the same side, but 

 this also needs confirmation. 



The view which w T as at one time held that the function of the thyroid 

 is merely that of a destroyer of toxic substances circulating in the blood is 

 no longer tenable. The same may be said for an old idea — resuscitated by 

 Cyon — that the organ serves as a kind of shunt to regulate the flow of blood 

 in the cranial cavity. This, indeed, has nothing to support it beyond the 

 extreme vascularity of the gland and its position in close connexion with 

 the carotids. 



