. The endocrine organs; an introduction to the study of internal secretion . th the corre-sponding lobe of the thyroid near its upperand dorsal aspect, whilst the other, thelower (parathyroid III), lies either in con-tact with the corresponding lobe of thethyroid near its lower and mesial aspector is removed a greater or less distancefrom it, sometimes as low down as thethymus. Both upper and lower para-thyroids are usually supplied with bloodby branches of the inferior thyroid artery,with which they are anatomically closelyconnected. But there is a good deal ofvariation in this, as well as in
. The endocrine organs; an introduction to the study of internal secretion . th the corre-sponding lobe of the thyroid near its upperand dorsal aspect, whilst the other, thelower (parathyroid III), lies either in con-tact with the corresponding lobe of thethyroid near its lower and mesial aspector is removed a greater or less distancefrom it, sometimes as low down as thethymus. Both upper and lower para-thyroids are usually supplied with bloodby branches of the inferior thyroid artery,with which they are anatomically closelyconnected. But there is a good deal ofvariation in this, as well as in their posi-tion relatively to the thyroid and to oneanother (Halsted and Evans, Geis). Inmany animals (dog, cat, rabbit, monkey)the upper parathyroid is deeply embeddedin the substance of the thyroid (Kohn), andit has on that account then been termedthe internal parathyroid, the lower beingdesignated external. The expressionsparathyroid III and parathyroid IV, whichindicate the source of each gland, are onthe whole the least ambiguous and oughtif possible to be FIG. 1.—Sketch of a preparation ofhuman larynx and oesophagus fromthe dorsal aspect, showing the thy-roid and parathyroids. The thyroid may also contain a portion of tissue resembling thymusgland, and, like that, containing lymphocytes and corpuscles of portion of thymus tissue is developed from the same branchial pouch(IV) as the upper parathyroid, whereas the main thymus is developed 12 The Endocrine Organs from pouch III, which gives origin to the lower parathyroid (fig. 2). Accessory thyroids, usually quite small, occasionally occur in the tissues of, the neck and in the anterior mediastinum. Accessoryparathyroids also occur;they are common in therabbit, and are in this animaloften found to a consider-able number embedded inthe thymus. This is alsothe case with the rat. In the rat, mouse, andguinea-pig only the parathy-roid III is ordinarily present,and this varies greatly in posi-tion rela
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