34 INTERNAL SECRETION 



held that, not the thyroid, but the thymus, is derived from ihe 

 lateral branchial clefts and that, as a matter of fact, the later.-il 

 lobes of the thyroid gland have not a separate area of origin. 

 This view is confirmed by discoveries made by Maresch and 

 Peucker. They each describe an instance in man in which the 

 thyroid was absent, while the parathyroids and the remains of the 

 thymus were present in their full number. It is highly probable, 

 therefore, that the lateral portions of the thyroid gland are derived 

 from a diverticulum of the primitive median thyroid process. 



The parathyroid glands are developed from the dorso-cranial 

 thickenings of the branchial clefts, the parathyroid III from the 

 third branchial cleft, and the parathyroid IV from the fourth 

 branchial cleft. In the neighbourhood of the latter is a cyst-like 

 hollow lined with pavement epithelium, which is called the post- 

 branchial body. This is derived from a lateral portion of the 

 fourth branchial cleft. 



In a further stage of development, the entire complex evolved 

 from the fourth branchial cleft becomes very intimately associated 

 with the thyroid and is, eventually, entirely covered by it. In 

 many species of animals, the parathyroid IV, together with the 

 thymus IV and, in some cases, the post-branchial body, are 

 found within the thyroid gland and, on account of their topo- 

 graphical position, are described as " internal." 



It would be more accurate if we were to follow the example 

 of French authors and call these branchial organs by a name 

 expressing their origin, namely, glandular branchiales parathy- 

 mica3 and parathyroideas. And it would serve still further to 

 elucidate the matter if we were to add the numbers III and IV, in 

 accordance with the branchial process from which the organ 

 takes its origin. 



The descriptions " internal " and " external " cannot be 

 applied to the parathyroid glands of man, because these are all 

 placed externally to the thyroid. The glandula branchialis 

 parathyroidea III is situated at the aboral pole of the thyroid, and 

 represents the inferior parathyroid body. The glandula 

 parathyroidea IV, which represents the internal parathyroid 

 gland of many animal species, lies, in man, on the dorsal surface 

 and near the upper edge of the thyroid and is called the superior 

 parathyroid gland. 



COMPARATIVE ANATOMY OF THE PARATHYROID 



GLANDS. 



The organs of the neck, even in the embryo, do not always 

 occur in the classic number. The most constant is the thymus 

 III, and the origins of the parathyroids are also of fairly regular 

 occurrence. The primordial beginning of the parathyroid III 



