ANATOMY AND EMBRYOLOGY 



175 



low as against the upper pole of the thymus. Statements even exist that 

 accessory parathyroids are pushed down to the pericardial fatty tissue 

 (Vassale and Piano). The bodies, especially the upper, are bound to the 

 thyroid gland by connective tissue, and the upper may also in part be inter- 

 polated in the thyroid tissue. We find in the different species of animals 

 great variations in the number and position of the parathyroids. In the 

 cat, the upper parathyroid is always imbedded in the thyroid tissue; in the 

 dog it is sometimes fully imbedded there (inner parathyroid, Kohn), so that 

 extirpation of the parathyroids alone may be attended with great difficulty. 



60S. C. 



, *'"' " " '''.''.' ":*:-. ^ - 



C. C. 

 FIG. 24. Parathyroid gland. F. C. = fat cells; eos. C. = eosinophilic cells; C. C. = chief cells. 



It is especially important, moreover, that in herbivorous animals the para- 

 thyroids are entirely apart, spatially from the thyroid gland. 



The parathyroids are very abundantly supplied with blood. Between 

 the individual cell groups are found numerous strikingly wide capillaries. 

 The glands are supplied by the inferior thyroid artery. 



Histologically the parathyroids are characterized by great richness in 

 cells. According to Welsh, large polygonal chief cells and smaller so-called 

 oxyphilic cells are to be distinguished from each other. 



Embryology. The parathyroids are entodermal glands (Verdun). Fig. 3 

 shows how the parathyroids and the thymus gland originate from the 

 branchial arches. 



The epithelial bodies (e 3 and 4 ) come from the dorsal part of the third 



