THE THYROID AND PARATHYROIDS 307 



rabbits. Administration of desiccated thyroid gland substance 

 markedly relieved the symptoms but failed to bring about a 

 complete cure. The cretins proved more susceptible than 

 normal rabbits to the toxic action of thyroid feeding. 



It seems clear from the experiments of Sutherland Simpson 

 and others that in young animals removal of the thyroid (one 

 or more parathyroids being left behind) will bring about a 

 condition resembling cretinism in the human subject. In older 

 animals symptoms may not be observed, but thickening and 

 dryness of the integument with a tendency to loss of hair 

 and wasting followed by adiposity have been described. It is 

 stated that there is loss of muscular tone, and that regeneration 

 of tissues is slower than normal. There may be anaemia. The 

 body temperature is low ; the power of heat regulation is 

 diminished and the animal becomes poikilothermic ; the 

 sexual functions are interfered with. It has been stated that 

 the limit of assimilation of carbohydrates is raised. " The 

 nervous system is markedly affected, dulness and apathy 

 being prominent symptoms. Many nerve-cells, especially those 

 of the cerebral cortex, exhibit a shrunken appearance, and 

 present a strong contrast with those of the normal animal 

 (Schafer). A myxoedematous condition of the skin has been 

 described by some authors, but the present writer has never 

 seen this (vide supra). 



Thyroidectomy in the amphibia interferes with the normal 

 metamorphosis, and retards or completely stops growth and 

 ossification of bone. The operation does not appear to hinder 

 the development of the gonads. 



Several observers have reported a diminution in the alexins 

 and opsonins in the serum after removal of the thyroid. 



L. The Mechanism of the Thyroid Secretion 



It used to be asserted that the secretion of the thyroid gland 

 passes, not directly into the blood-stream, but indirectly by 

 means of the lymphatics. But more recent investigations have 

 rendered this theory very doubtful. 



Asher and Flack utilized the observation of Cyon that the 

 excitability of the depressor nerve is increased by the action of 

 thyroid substance. They believe that the thyroid furnishes 

 an internal secretion which increases the excitability of the 



