THE DUCTLESS GLANDS 1189 



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 hypertrophy 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 functions of almost all 

 the tissues of the body. If this secretion is lacking we obtain the condition 

 known as myxoedema in adults, as cretinism in young children. If it be 

 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 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. 548). 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 

 herbivora respectively. Forsyth has shown that, in man, the situation of 

 the parathyroids corresponds almost exactly with the places in which are 

 found occasionally accessory thyroids ; and, according to Edmunds, after 

 excision of the thyroid, the parathyroids undergo histological alteration and 

 are converted into thyroid tissue, the cells taking on an alveolar arrangement 

 and producing colloid material. According to this view the parathyroids 

 would represent simply immature thyroid tissue. On the other hand, it has 

 been suggested (Biedl) that the parathyroids have a function entirely dis- 

 tinct from that of the thyroid gland, removal of the thyroids producing 

 simply a condition of cachexia and the changes associated with my x oedema, 



