598 VETERINARY PHYSIOLOGY 



moval have developed, a small piece of the tissue of the thyreoid 

 is grafted in a suitable part of the body, blood-vessels may grow 

 in, the tissue may survive and increase, and this may bring about 

 a disappearance of symptoms. On removing the graft the 

 symptoms recur. 



(4) Extracts. — The administration of extracts of the thyreoid 

 subcutaneously, or by the mouth as was shown by Murray, 

 frequently leads to the disappearance of the symptoms of 

 removal or deficiency, and this line of treatment is now uni- 

 versally used in cases of cretinism and myxoedema in man. 

 lodothyreo-globulin or thyreo-oxyindol appears to be the active 

 principle. 



The way in which the thyreoid acts upon the metabolism 

 is demonstrated by the study of the effects of its administra- 

 tion to normal animals. When given in large doses over 

 long periods, all the symptoms of hyperthyreoidism may be 

 produced. 



Tadpoles fed on thyreoid tissue undergo a very rapid develop- 

 ment, and Hoskins and Herring have shown that in young white 

 rats the continued administration of thyreoid causes an extra- 

 ordinary increase in the suprarenals, heart, kidneys, and pancreas 

 with a decrease in the size of the pituitary in the female. The 

 main effect is to increase metabolism. 



A further effect is to activate the terminations of the 

 true sympathetics and of the para-sympathetics of all the 

 visceral nerve fibres. Hence both the augmentor and the 

 inhibitory terminations in the heart are activated, and the 

 heart responds more readily to stimulation of the vagus or of 

 the augmentor. The same seems to be the case with the nerve 

 terminations in the blood-vessels, e.g. the reflex response to the 

 depressor nerve (p. 418). Since these thyreoid preparations 

 act upon the true sympathetic termination, they facilitate the 

 action of adrenalin. Further, the suprarenals are supplied by the 

 splanchnic nerves, and apparently the active principle of the 

 thyreoid facilitates the action of these, and so increases the 

 output of adrenalin. 



(5) Hyperthyreoidism. — {Increased Functional Activity). — 

 This condition in man is known as Graves Disease or exoph- 

 thalmic goitre. It is characterised by a condition of hyper- 

 excitability and sleeplessness, rapid action of the heart, a 



