DIGESTIVE ORGANS AND THEIR ANATOMY. 295 



However, in certain diseases of the thyroid glands, such as 

 goitre, these vesicles may become greatly distended and 

 plainly discernible to the naked eye. 



The wall of these vesicles is made up of a single-layered 

 epithelium, in which the individual cells are somewhat cub- 

 ical or columnar. In the interior there is found a yellowish 

 fluid, which is no doubt the material secreted by these epi- 

 thelial cells. This same fluid also occurs in the areolar tissue 

 between the vesicles. This fluid on account of its appearance 

 is spoken of as a colloid substance. In certain forms of goitre 

 this colloid substance accumulates to such an enormous ex- 

 tent as to increase the size of the gland to many times its orig- 

 inal dimensions. The connective tissue between the vesicles 

 is richly supplied with blood-vessels and lymphatics, which 

 seems to indicate the important part which this gland plays 



Fig. 121. SECTION OF A HUMAN THYROID GLAND. (After Schafer.) 

 Two complete vesicles and portions of three others are shown. The colloid material 

 filling both the vesicles and the spaces between is indicated. In the center of figure is a 

 blood-vessel cut across, next to this a plasma cell. 



in the economy of the body. The composition of this col- 

 loid material has not been satisfactorily determined. By some 

 observers it is stated that it contains proportionately large 

 amounts of phosphorus. More recent experiments have 

 shown that besides containing substances of an albuminous 

 or proteid nature it contains a compound of iodine called 

 thyro-iodine. 



