314 HANDBOOK OF PHYSIOLOGY. 



the thyroid when that body is removed. They frequently exist in 

 pairs, hut there may be more than two, lying along the carotid in the 

 region of the thyroid. The parathyroids are thought to be immature 

 thyroids. 



Functions of the Ttiyroid. The colloid material which is formed 

 within the thyroid vesicles, and is believed to be their secretion, finally 

 ruptures through their walls into the lymph channels and thus gains en- 

 trance to the circulation. The secretion of the thyroid falls into the 

 class known as internal secretions, and exerts a profound influence upon 

 the metabolic processes of the body, probably through the agency of the 

 central nervous system. Complete extirpation of the thyroid, at least in 

 some animals, produces death, preceded by a group of characteristic 

 symptoms. In man and the monkey, the symptoms after removal come 

 on slowly and resemble the disease known in man as myxoedema. 



This disease is known definitely to be due to disease of the thyroid, 

 whereby its function is interfered with. Moreover, if a piece of thyroid 

 of sufficient size be grafted into an animal from which the glands have 

 been removed, and the graft takes, the symptoms of thyroid removal are 

 lessened in intensity or disappear altogether. And, likewise, thyroid 

 feeding or the administration of thyroid extracts relieves the symptoms 

 of the disease myxoaderna. 



The above facts show that the thyroid gland must perform some im- 

 portant function in the animal economy, and it is believed that this is 

 accomplished by virtue of its internal secretion. The colloid material 

 of the gland has been submitted to much chemical study, and a substance 

 called iodothyrin has been isolated as its active principle. Baumann and 

 Koos state that iodothyrin exists in the gland in combination with pro- 

 teid bodies. Iodothyrin relieves the symptoms of thyroid removal much 

 to the same extent as thyroid feeding. It is a very resistant substance, 

 and is not injured by the action of the gastric juice or by boiling with 

 10 per cent sulphuric acid for a long time. 



The Supra-renal Capsules or Adrenals. These are two flat- 

 tened, more or less triangular or cocked-hat shaped bodies, resting by 

 their lower border upon the upper border of the kidneys. 



Structure. The gland is surrounded by an outer sheath of connective 

 tissue, which sometimes consists of two layers, sending in exceedingly 

 fine prolongations forming the framework of the gland. The gland 

 tissue proper consists of an outside firmer cortical portion, and an inside 

 soft dark medullary portion. 



The finer structure of the supra-renal capsules is incompletely known. 



(1.) The cortical portion is divided into (fig. 225) an external narrow 

 layer of small rounded or oval spaces, the zona glomerulosa, made by the 

 fibrous trabecula?, containing polyhedral cells (&). The second layer of 

 cells is arranged in columns radiating from the- medulla, the zona fascic- 



