THE ENDOCRIXE SYSTEM 



133 



from the lining of the body cavity. The homologous tissue forms 

 a mass (interrenal gland) associated with the mesonephros in 

 lower vertebrates. Its secretion is necessary for life. It appears to 

 be concerned in maintenance of normal functioning of the kidneys, 

 in the regulation of inorganic substances in the body, especially 

 sodium and potassium, and in the conservation of muscular strength 

 and ability to resist fatigue. 



The Thyreoid Gland 



Like the pancreas, the thyreoid gland develops as an outgrowth 

 of the lining of the digestive tube, in this case from that of the 

 pharynx, but it becomes completely detached therefrom. It gives 

 directly into the blood a secretion (thyroxin) which takes part in 

 the regulation of growth and stimulates metabolism in the body 

 generally. This substance has a high content of iodine in combi- 

 nation with colloid material. Thyreoid deficiency in man is as- 

 sociated with the conditions known as cretinism and myxoedema, 

 and hyperactivity with exophthalmic goitre. 



The Parathyreoid Gland 



The parathyreoid bodies are minute cell-masses lying in, or 

 immediately outside of, the thyreoid. They are produced embry- 



onically from the dorsal part of 

 the epithelial lining of certain gill 

 pouches. They have been shown 

 to have essential functions in con- 

 nection w^ith calcium-phosphorus 

 metabolism. 



The Thymus Gland 

 The thymus is developed in 

 mammals from the ventral part of 

 the epithelial lining of the third 

 pair of embryonic pharyngeal gill 

 pouches. It is thus paired in ori- 

 gin, but in mammals the masses 

 of the two sides become associated ventrally and migrate backwards 

 to a position near the heart. The functions of the thvmus are 



Fig. 80. The parts_ of the hypophysis 

 of the rabbit and adjacent structures as 

 seen in sagittal section: d, pars distalis 

 (anterior lobe) ; e, median eminence of 

 tuber cinereum; i, pars intermedia; 1, 

 residual lumen; m, mamillary body; n, 

 infundibular process (neural lobe) ; o, 

 optic chiasma; r, infundibular recess; s, 

 infundibular stalk; t, pars tuberalis. 



