THE THYROID GLAND. 125 



to call forth or regulate the activities of tissues which are not 

 immediately concerned in general metabolism but rather with 

 special processes, such as the excitability of the nervous system 

 (e. g., adrenalin), the behavior of the reproductive glands (e. g., 

 in the secretion of milk), the growth of certain tissues (e. g., of 

 subcutaneous tissues, of hairs) or the atrophy of others, (e. g., 

 of the uterus after pregnancy is terminated). For such hor- 

 mones, special manufacturing centres are provided in the duct- 

 less glands. The thyroid and thymus glands in the neck, the 

 pituitary in the brain, the spleen and adrenal glands in the ab- 

 domen are good examples. None of these has any duct, but they 

 discharge the products of their activity internal secretion 

 into the blood stream, by which it is carried to the tissue or organ 

 on which it acts. Internal secretions may also be produced by 

 certain cells of the digestive glands, as, for example, the so-called 

 Isles of Langerhans of the pancreas (see p. 72), and likewise 

 there are certain organs whose main functions are of quite a 

 special nature, such as the ovaries and testes, that can produce 

 very powerful internal secretions. 



We shall confine our attentions for the present, however, to 

 the strictly ductless glands. Their function is ascertained ex- 

 perimentally either by removing the gland by operation or by in- 

 jecting an extract of it and then observing the behavior of the 

 animal. Much can also be learned by observing patients in which 

 the gland is diseased. 



The Thyroid and Parathyroid Glands. The thyroid gland 

 consists of two oval lobes situated one on either side of the 

 trachea just below the larynx or voice box, -and connected to- 

 gether over the trachea by an isthmus of thyroid tissue. Em- 

 bedded in the substance of each lobe of the gland on the poste- 

 rior surface are the two very small parathyroid glands. Minute 

 examination shows the thyroid glands to be composed of vesicles 

 lined by low columnar epithelium and filled with a clear glossy 

 substance called colloid. The parathyroids have an entirely dif- 

 ferent structure, being composed of elongated groups of poly- 

 hedral cells with no colloid material. 



The functions of the two glands are probably essentially dif- 



