METABOLISM, NUTRITION AND DIETETICS 563 



plasia (goitre), and also the compensatory hyperplasia which 

 follows partial removal of the thyroid. It exercises a curative 

 effect on active hyperplasias. The physiological and thera- 

 peutical activity of thyroid substance varies directly with the 

 amount of iodine in it in organic combination (thyroiodin) . 



As in the case of other glands forming an internal secretion, 

 it has been debated whether the function of the thyroid is to 

 destroy toxic bodies or to form substances indispensable or 

 advantageous to the organism. While the precise role played 

 by the organ in the economy remains obscure, it is evident that 

 in most animals and in man its secretion is of great importance, 

 whether it be solely the quasi-external secretion of ' colloid/ 

 containing the thyroiodin, that collects in its alveoli and slowly 

 passes out of them by the lymphatics, or perhaps, in addition, 

 some other substance, which, like the glycogen of the liver, 

 never finds its way into the lumen of the gland-tubes at all. 

 It may also be admitted that, by aiding in the maintenance of 

 the normal level of general nutrition, particularly that of the 

 central nervous system, the ability of the organism to cope with 

 toxic substances introduced from the outside or manufactured 

 in the body is favoured. There is, however, no evidence that 

 an actual destruction or neutralization of toxic substances 

 occurs in the gland itself. 



Although no clear proof has yet been given that the secretion 

 of the thyroid is influenced by nerves, it is probable that this is 

 the case. Section of the superior and inferior thyroid nerves 

 going to the gland is followed by degenerative changes in it. 



Suprarenal Capsules. It had been observed by Addison that 

 the malady which now bears his name, and in which certain 

 vascular changes, with muscular weakness, anaemia, and pig- 

 mentation or ' bronzing ' of the skin, are prominent symptoms, 

 was associated with disease, usually tuberculous, of the supra- 

 renal or adrenal bodies. This clinical result was soon supple- 

 mented by the discovery that extirpation of the adrenals in 

 animals is incompatible with life (Brown-Sequard). Our know- 

 ledge of the functions of these hitherto enigmatic organs was 

 greatly extended by the experiments of Oliver and Schafer, who 

 investigated the action of extracts of the suprarenals (of calf, 

 sheep, dog, guinea-pig, and man), when injected into the veins 

 of animals. The arteries are greatly contracted, and this mainly 

 through direct action on the vaso-motor nerve-endings or some 

 structure intermediate between them and the smooth muscle of 

 the vessels, but partly through the vaso-motor centre. The 

 blood-pressure rises rapidly, although the heart may be inhibited 

 through the vagus centre. The heart is at the same time directly 

 stimulated, so that, although it beats slowly, the beats are 



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