THE DUCTLESS GLANDS 1329 



large doses either to man or animals, it quickens the pulse, even 

 causing violent palpitation, and increases the metabolic activities of 

 the body, so that the appetite is increased, the nitrogenous output 

 rises above the intake, and the subcutaneous fat is diminished 

 or disappears. It is possible that a moderate degree of thyroid 

 inadequacy is not infrequent and that the beneficial effects on general 

 health, in removing excessive corpulence and in promoting the growth 

 of hair, which are observed on administering the drug to people of 

 middle life, may be due to the actual replacement of a function which 

 is being insufficiently discharged. The symptoms caused by excessive 

 doses of thyroid gland are strikingly similar to those occurring in the 

 disease known as exophthalmic goitre, where there is a true hyper- 

 trophy of the gland associated with cardiac palpitation, proptosis 

 (bulging of the eyes), wasting, and muscular weakness. 



All these facts warrant us in including the thyroid body among 

 the glands with an internal secretion, the presence of which in the 

 blood-stream is a necessary condition for the normal growth and func- 

 tions of almost all the tissues of the body. If this secretion is lacking 

 we obtain the condition known as myxcedema in adults, as cretinism 

 in young children. If it be present in excess the symptoms of exoph- 

 thalmic goitre are produced. The exact character of the internal 

 secretion cannot be regarded yet as de finitely 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 Reid Hunt has 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. 546). Considerable diver- 

 gence 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 



84 



