750 THE ENDOCRINE ORGANS, OR DUCTLESS GLANDS 



tions, and the original solid column becomes hollowed out. The two 

 lateral branches of the original column divide again and again so as to 

 form a system of hollow tubes lined with epithelium. These afterward 

 become cut up so as to form the closed vesicles characteristic of the 

 gland. Each vesicle is more or less spheroidal in shape, and has no 

 basement membrane, but its walls are formed by a layer of epithelial 

 cells, which may be columnar, cubical, or flattened in shape. Each vesicle 

 is filled with the so-called colloid material, which is peculiar in con- 

 taining iodine, and between the vesicles is a layer of connective tissue 

 often containing small cells, some of which are not unlike those of the 

 parathyroid. The connective tissue also contains the blood vessels, 

 which are very numerous indeed, the thyroid, in proportion to its size, 

 receives more than five times as much blood as the kidneys, the only 

 tissue that surpasses it in this regard being the medulla of the adrenal 

 gland (see page 211). The nerves arise from both the vagus and the 

 sympathetic systems and have been traced to the secreting epithelial 

 cells. The above description applies to a strictly normal gland. 



THE THYROID GLAND 



Condition of the Gland 



In the crowded communities of the Great Lakes Basin of this conti- 

 nent, it has been found that in most animals the thyroid gland is more or 

 less abnormal. In Cleveland, for example, Marine has found this to be 

 the case in well over 90 per cent of the dogs brought to the laboratory. 77 

 The condition usually goes under the name of simple goiter, which in- 

 cludes all thyroid enlargements except those of exophthalmic goiter. 

 In man the goiter originates usually about the age of adolescence and 

 more frequently in girls than in boys. It may sometimes pass over into 

 the exophthalmic type. The exact pathological changes in the goitrous 

 gland vary with the species of animal and with the duration of the dis- 

 ease. In man, besides the cystic or colloid goiter an adenomatous type 

 is very common although rare in other animals. 



From the numerous observations that have been made on the glands of 

 domestic animals, it has been clearly established that the very earliest 

 sign of goiter is a diminution in the iodine content of the gland; fol- 

 lowed by an increase in the epithelial cells and in the blood supply and a 

 decrease in the colloid. Such hyperplasia may be induced in what re- 

 mains after removal of a large part of a normal gland (compensatory 

 hyperplasia), or if a similar operation be performed early in pregnancy, 

 the young when born will be found to have hyperplastic thyroids. A 

 certain degree of hyperplasia exists as an accompaniment of pregnancy, 



