16 



The 



Endocrine Organs 



again and again to form a system of hollow epithelium-lined tubes, and 

 later these become cut up into the closed vesicles which are characteristic 

 of the gland. The thyroid bud is joined by outgrowths from the ventral 



Fig. 7. — Section of parathyroid (cat). Magnified 400 diameters, 

 illustration of the compact type of gland. 



This is a good 



wall of the pharynx on each side behind the fourth branchial pouch (post- 

 branchial bodies) ; these may be involved in the thyroid, but it is doubtful 

 if they contribute to the formation of its vesicles. 



Structure and Development of Parathyroids 



Structure. — The parathyroids (fig. 1) are four minute organs averaging 

 6 to 7 mm. in length, 3 to 4 mm. broad, and 1*5 to 2 mm. thick (Welsh). 

 Each weighs about 003 gramme (half a grain). Rarely there are more 

 than four, sometimes fewer. As already stated, one (the lower) may 

 occur altogether detached from the thyroid, 1 but usually in man both are 

 closely attached to it. The upper of the parathyroids may be completely 

 embedded in the substance of the thyroid (fig. 13). 



The parathyroids were described by Sandstrom in 1880. Their physio- 

 logical independence and distinction from the thyroid proper was first re- 

 cognised by Gley in 1891, and confirmed by Vassale and Generali in 1896, 

 but is not fully accepted by Forsyth, Vincent, and some other authorities. 



1 This is constantly the case, as Gley pointed out, in the rabbit, and partly accounts 

 for the fact that when thyroidectomy is performed in that animal, two parathyroids being 

 left behind, tetany does not occur until these are removed. Even then tetany sometimes 

 fails to show itself ; this may perhaps be accounted for by the presence of accessory para- 

 thyroids in the thymus. 



