i INTEENAL PEOTECTIVE SECEETIONS 23 



that dogs often bear up well against the effects of bilateral thyroid- 

 ectomy, provided there are no lesions of the sensory nerves of the 

 region, and that the total occlusion of the thyroid vessels was 

 equally without effect. 



The direct confutation of Munk's opinion is given more 

 particularly by the experiments on dogs of Fuhr (1886), Fano 

 (1893), and Vassale (1893), which prove that lesions of the nerves 

 and vessels of the neck, translocation of the thyroids and the 

 grafting of them subcutaneously, have no sequelae, and that 

 suppression or complete ablation of the gland are alone capable of 

 producing the symptoms described by Schiff. Vassale proposed 

 an experimentum crucis : if the thyroid lobe be excised from a dog 

 on one side, and the sympathetic nerves divided on the other, this 

 double operation is not followed by phenomena of cachexia and 

 tetania thyreopriva. 



At a later time E. Cyon (1897-98) formulated a new hypo- 

 thesis to explain the effects of thyroidectomy. His theory at first 

 seemed highly suggestive, but it proved fallacious in face of 

 many facts that have received experimental confirmation. Cyon 

 attempted to fuse the old theory of Schreger-Liebermeister with 

 that which attributes a secretory antitoxic function to the thyroid, 

 directed, i.e., to the removal of some toxic matter from the body 

 as a whole, and more particularly from the nervous system. 



His hypothesis may be summed up as follows : 



(a) The function of the thyroid gland is to form and pour 

 into the blood a special substance designed to stimulate or keep 

 up the functional tonus of the nerve centres which regulate the 

 beats of the heart. This substance is thyro-iodine (discovered, as 

 we shall see, by Baurnann among the active substances of the 

 thyroid, infra, p. 30). 



(5) In proportion as thyro-iodine is formed by the activity of 

 the glandular epithelia, the iodine salts circulating in the blood 

 which have a paralysing action upon the regulatory apparatus of 

 the beats of the heart (Barbera and Cyon), are withdrawn from the 

 circulation and remain innocuous, forming organic combinations. 



(c) By means of the depressor nerves and cardiac branches of 

 the recurrent nerve, the heart exerts a direct control over the 

 thyroid function, determining the formation of the amount of 

 thyro-iodine that is necessary for its normal activity. 



(d) The thyroid, which lies at the entrance of the carotids into 

 the cranium, is a protection against an excessive flow of blood to 

 the brain, since it can carry off a great quantity of blood through 

 its vessels in a very short time. It therefore acts as a secondary 

 circulation of low resistance. 



(e) This regulatory function of the cerebral circulation attri- 

 buted to the thyroid is also controlled by the heart, since the 

 depressors are able to determine the active dilatation of the 



