THE THYROID APPARATUS 35 



has never been known to be absent. A. Pepere's investigations 

 show that, in all mammals, including man, the development of 

 the thymus is invariably accompanied by a persistent accessory 

 parathyroid system, which is derived from the third branchial 

 cleft. It is seen most clearly in the thymus or in the region 

 of the thymus of live rabbits, where it appears in the form of 

 single groups of epithelial cells, or of relatively large islands 

 composed of parathyroideal tissue. 



As a general rule, all the embryonic beginnings of para- 

 thyroid glands develop into definitive organs. Variations occur in 

 their topographical relationship to the thyroid, and for this reason 

 there is a wide diversity in the anatomical conditions of species 

 and individuals. 



The position of the parathyroids is most constant in cats. 

 The glandula parathyroidea III, representing the external para- 

 thyroid gland, lies upon the dorsal surface of the thyroid gland 

 and generally at its upper pole, though sometimes at the middle. 

 The glandula parathyroidea IV, representing the internal para- 

 thyroid gland, is placed at about the middle of the thyroid, 

 more or less deeply embedded in the tissue. 



A similar arrangement is found in the dog. The external 

 parathyroid body lies in a depression situated at the upper pole 

 of the thyroid, while the internal parathyroid body is found in 

 the upper third or in the middle of the thyroid, and is likewise 

 embedded in the tissue. Supernumerary parathyroid bodies are 

 by no means infrequent in the dog, though in the cat they are 

 scarcely ever met with. 



In ruminants the topographical-anatomical conditions are 

 aberrant. The glandula parathyroidea III of the sheep and the 

 goat is much nearer to the cranium, and is found on the inner 

 surface of the submaxillary gland or at the fork of the carotid. 

 The glandula parathyroidea IV is embedded in the centre of 

 the thyroid. 



In the rabbit, the glandula thyroidea IV also remains in 

 association with the thyroid, and is found in the interior of the 

 gland. The glandula parathyroidea III, representing the ex- 

 ternal parathyroid body, lies from J-iJ cm. below the thyroid 

 gland. Under typical conditions, it lies close to the carotid and 

 depends from a delicate arterial peduncle. It is about 4-6 mm. 

 long, ij mm. broad, and weighs 4-6 mg. Erdheim and Pepere 

 found numerous accessory parathyroids in the thymus of rabbits. 

 Cristiani's investigations show that, in the case of other rodents, 

 one parathyroid body is either embedded in the thyroid tissue or 

 projects from its surface. A glandula parathyroidea III is 

 generally absent, but in the field-mouse there is a free parathyroid 

 gland which lies at either the upper or the lower edge of the 

 thyroid. Erdheim found that, in the rat, in addition to the one 

 parathyroid body embedded in the thyroid tissue, there is fre- 



