THYROID GLAND 579 



the pressure of the secretion causes a separation or rupture of 

 the cell-wall, and the colloid is discharged into the surrounding 

 lymphatics ; these vessels, by means of the thoracic duct, convey 

 their contents into the blood. According to some observers the 

 colloid substance is absorbed also by the capillary blood-vessels. 



The nervous supply of the gland is derived from the laryngeal 

 nerves ; no structural changes, however, have been observed in the 

 cells as the result of stimulation or section of these nerves or of 

 the vago-sympathetic nerve. Vaso-constrictor nerves to the 

 thyroid are found in the cervical sympathetic nerve of the 

 monkey and in the anterior roots of the third thoracic nerve of 

 the rabbit, cat, and dog. According to His, the blind tube which 

 passes from the region of the foramen ca3cum of the tongue 

 towards the hyoid bone is to be regarded as a rudiment of the 

 duct of the thyroid gland the ductus thyreoglossus. The thyroid 

 is developed as a median evagination of the floor of the pharynx 

 between the first and second branchial arches. The parathyroids 

 arise as thickenings of the epithelium in the dorsal aspect of the 

 third and fourth visceral clefts. The structure of the thyroid 

 gland is essentially the same in all classes of the vertebrates, 

 although its size and position may show great variations : the 

 invariable presence of the colloid substance in closed acini sur- 

 rounded by simple glandular epithelium is an interesting fact. 



The natural stimulus to the gland is unknown, but appears- 

 to be the absence or presence of some substance in the blood. 

 The removal of five-sixths of the gland leads to structural changes 

 in the remaining portion of the gland which indicate increased 

 activity. 



Chemical examination of the gland shows that it contains two 

 proteids, a nucleo-albumin, and the colloid substance, which may 

 be regar&ed as a compound proteid possessing iodine in consider- 

 able amount ; the percentage of iodine varies, but on an average 

 is 0-3 per cent, of the dried substance. The colloid substance is 

 not a nucleo-proteid, for it yields no nuclein on gastric digestion. 

 When the colloid matter is subjected to digestion, only those 

 products which contain iodine possess active properties. The 

 active substance is named iodothyrin and its effects upon the 

 metabolism of the body appear to be similar to those exerted by 

 the gland substance itself. Iodothyrin is a brown amorphous 

 substance, almost insoluble in water, but readily soluble in weak 



