THE THYROID APPARATUS 37 



serves as the blood reservoir for the brain, and that, when a 

 change takes place in the circulatory conditions, it may either 

 prevent a rush of blood to the arteries of the brain, or it may 

 receive large quantities of blood from the cerebral veins. 



This theory has recently been revived by E. von Cyon. He 

 suggests that iodothyrin, the active substance formed by the 

 thyroid gland, stimulates the nerves of the heart which regulate 

 pressure, and that by this means the blood-pressure is lowered 

 and the circulation retarded. At the same time, the central vagal 

 irritation is conveyed by means of vasodilators to the vessels 

 of the thyroid gland, causing them to dilate and thus still further 

 reduce the flow of blood to the brain. 



When we have discussed the activity of the thyroid extracts, 

 we shall be in a position to judge whether or not there is justifica- 

 tion for this hypothesis. Suffice it for the present that, though 

 experimental proof of changes in the cerebral circulation due to 

 chemical or nervous intervention of the thyroid, if they really 

 exist, ought to be readily obtainable, and would be very con- 

 vincing. Such proof is, up to the present, not forthcoming. 



Another of the older theories has recently been advanced 

 by J. F. Meckel. It postulates an intimate physiological 

 association between the thyroid gland and the female organs of 

 generation. This theory owes its origin to the fact that the 

 thyroid gland of women is larger than that of men, and that it 

 increases in size during menstruation, deflorescence, and 

 pregnancy. 



The swelling of the thyroid is a well-known phenomenon in 

 menstruation and pregnancy. This increase in size may be 

 partly due to general vascular turgescence, but, as we shall see 

 later, there is an actual biological relationship between the thyroid 

 and the sexual glands. 



Tiedemann was the first to suggest that the thyroid is con- 

 cerned in the formation of blood, and that it aids, or may even 

 replace, the function of the spleen. He bases his view upon the 

 histological construction of the thyroid and its structural analogy 

 with the lymphatic glands and the spleen. The latest experiments 

 show that there is some ground for the belief that the thyroid 

 exerts an influence, though indirectly, upon the formation of the 

 blood. 



The functional significance of the thyroid gland and its 

 importance to the life of trie organism, were first demonstrated by 

 the serious nature of the results which follow its extirpation. 



Numerous instances in which goitre in man has been treated 

 by total extirpation of the gland, confirm the results obtained by 

 means of hundreds of experiments w 7 ith animals, and there is no 

 doubt that the removal of the thyroid gland gives rise to severe 

 pathological conditions which often terminate fatally. The 

 results of extirpation may show themselves in two conditions, 



