THE DUCTLESS GLANDS 1241 



present in excess the symptoms of exophthalmic goitre are produced. 

 The .exact character of the internal secretion cannot be regarded yet as 

 definitely established. It seems possible that it is identical with a substance 

 containing iodine in organic combination, which was isolated by Baumann 

 from the thyroid gland and is known as iodothyrin. In certain experiments 

 the results of administration of iodothyrin were found to be identical with 

 those obtained by giving the whole gland. Doubt has been thrown on the 

 specific nature of this body on account of the fact that iodine may be 

 wanting in the thyroid gland in certain animals, though Keid Hunt has 



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FIG. 561. Section of parathyroid. (KoHN.j 



e/>, secreting epithelium ; p/gr, cells containing pigment ; cap, si 

 capillaries ; end, endothelial cells. 



shown that the physiological effects of thyroid extract are proportional to 

 the amount of iodine contained therein. 



SIGNIFICANCE OF THE PARATHYROIDS. The parathyroids are small 

 bodies, varying in number, situated on the border of the thyroid gland 

 or actually embedded in its substance. In histological appearance they 

 differ widely from the thyroid, and consist of solid masses or columns of 

 epithelial cells surrounded with connective tissue and richly supplied with 

 blood vessels (Fig. 561). Considerable, divergence of opinion still exists as to 

 the significance of these bodies. In some animals e.g. in the dog, where 

 they are embedded in the gland, they will be necessarily removed in any 

 operation for the extirpation of the thyroid. In others such as the rabbit, 

 where they lie outside the gland, it is easy to avoid them in the excision of 

 the thyroid. To this varying distribution of the parathyroids have been 

 ascribed the different results of extirpation of the thyroid in carnivora and 



