654 INTERNAL SECRETION ENDOCRINE GLANDS 



It has long been known that vaso-motor fibres for the dog's thyroid run 

 up in the cervical sympathetic to the superior cervical ganglion, and 



Sample. 



Control. 



Thyroid. 



Fig. 207. Effect of Desiccated Human Thyroid on Tadpoles of Different Ages 

 The upper row shows the older, the lower row the younger tadpoles. The older 

 ones had posterior leg buds slightly over 2 centimetres long, but no forelegs. The 

 younger tadpoles had just visible posterior leg buds and no foreleg buds. The indi- 

 viduals of each age were alike at the beginning of the experiment as to size and 

 condition of development. One of each age was killed in formalin at the beginning 

 of the experiment (designated as ' Sample ' in the figure). Another of each age 

 was used for control, and fed fresh liver every second day. A third specimen of 

 each age (under ' Thyroid ' in the figure) was fed 50 milligrammes of desiccated 

 human thyroid every second day, alternating with fresh liver. The thyroid 

 showed the condition of multiple adenoma and contained 4-3 milligrammes of 

 iodine per gramme of the dry weight. The difference at the end of the experi- 

 ment between the thyroid-fed and control tadpoles of the younger set is especially 

 seen in the atrophy of the tail and the appearance of the hind-legs in the former. 

 In the older set, at each stage in the experiment differentiation was more ad- 

 vanced in the thyroid-fed specimen. This is still the case at the end, especially 

 in the much greater atrophy of the tail. The difference is less than in the younger 

 set, simply because even the control is approaching the end of the metamor- 

 phosis (Graham). 



thence to the lobe of the same side. These were first discovered by 

 the effect produced by their stimulation on the thyroid circulation 

 time (p. 135). Some evidence of the existence of secretory fibres has 



