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. Accessory 

 parathyroids also occur ; 

 they are common in the 

 rabbit, and are in this animal 

 often found to a consider- 

 able number embedded in 

 the thymus. This is also 

 the case with the rat. 



Tons// 



Parathyn/// 



T/?y/n. /// 



V 



PosC.ohnch. 

 Tfyym./V b oc fy 



S 



cib 



In the rat, mouse, and 

 guinea-pig only the parathy- 

 roid III is ordinarily present, 

 and this varies greatly in posi- 

 tion relatively to the thyroid. 



The thyroid is repre- 

 sented in all Vertebrates. 

 In Amphioxus and Am- 

 mocoetes it retains an open 

 connexion with the phar- 

 ynx. Parathyroids do not 

 occur below Amphibia. 1 



Structure and Develop- 

 ment of Thyroid 

 Structure. — The thyroid 

 proper is an organ consist- 

 ing of small closed vesicles 

 (fig. 3) of varying size and 

 shape, but for the most 

 part irregularly spheroidal. 

 Each vesicle is lined by epi- 

 thelium, the cells of which 

 are columnar, cubical, or 

 flattened in accordance 

 with the state of distension of the vesicles. There is no definite base- 

 ment membrane separating the epithelium from the connective tissue 

 and blood-vessels. The vesicles are generally filled by the so-called 

 "colloid," a viscid fluid in the fresh organ, which is coagulated into a 



1 For the structure and morphology of the thyroid and parathyroids, the paper by the 

 late Mrs F. D. Thompson in the Phil. Trans, for 1910 may be consulted. The following 

 works deal with the whole subject : — Leopold -Levi, Physio-pathologie du corps thyroide, 

 1908; L. Lannoy, Thyroides, Parathyroides, Thymus, 1914. For the parathyroids, see 

 D. A. Welsh, Journ. Anat., 1898 ; D. Forsyth, Trans. Path. Soc, 1907 ; and Guleke, Chirurgie 

 der Nebenschilddriisen, 1913. 



Tfiyro/d 



Fig. 2.— Diagram to illustrate the origin of the cleft organs 

 (thyroid, parathyroids, thymus, etc. ) in the mammalian 

 embryo. 



1, II, III, IV, branchial pouches; Parathyr.iii, origin of lower 

 parathyroid from cephalic aspect of third pouch ; Parathyr.iv, 

 origin of upper parathyroid from cephalic aspect of fourth 

 pouch; Thym.iii,iv, origin of thymus from caudal aspect of 

 third and fourth poucheg (Thym.iv usually forms the part 

 associated with the thyroid) ; Post.branch.body, post-branchial 

 body, which forms a separate organ in lower Vertebrata, 

 but in mammals either disappears or is incorporated with 

 the thyroid. 



