THYROID AND PARATHYROIDS 233 



Structure and Development of the Parathyroid 



The four parathyroids, which are either attached to the 

 thyroid or embedded in it, are composed of epitheUal 

 ceils arranged sometimes compactly together, sometimes 

 in lobules which are separated by connective -tissue 



Fig. 32. — Thyroid of another wild rat showing cohininar cells and 

 absence of colloid (Chalmers Watson, from Schafer, The Endocrine 

 Organs). 



liberally endowed with blood-vessels. The cells are 

 mostly small, and may be either clear or granular. A 

 colloid substance is sometimes seen lying between them. 

 This is said to increase in amount after removal of the 

 thyroid. The parathyroids receive the same nerves as 

 the thyroid. 



The upper and lower pairs of parathyroids are developed 



