THE DUCTLESS GLANDS. 193 



The bloodvessels of the thyroid are abundant, and form a rich 

 plexus in the areolar tissue between the alveoli. The lymphatics 

 are also abundant and large, forming a network of rather large ves- 

 sels in the same situation. The nerves accompany the vessels, are 

 destitute of ganglia, and have been traced to the bases of the epi- 

 thelial cells, whence they may occasionally send minute terminal 

 twigs with enlarged ends between the epithelial cells. 



2. The Parathyroids (Figs. 167, 168, 169).— These are two bodies 



Fig. 1GS. 





m 



- 



■■■:'■■ . 



J P ■ i 



■■■ .• 4 ■■ 

 : 1 • "■■€ » V& ] ^ ft- 



m 



Section of a portion of the external parathyroid of a kitten two months old. (Kohn.) Show- 

 ing the columns of epithelial cells separated by a delicate, vascular areolar tissue. The 

 nuclei between the columns of epithelium belong chiefly to capillary bloodvessels, m, 

 in, nuclei exhibiting karyokinetic figures. 



of identical structure, which are developed in conjunction with 

 the thyroid gland ; but, while the latter progresses in its devel- 

 opment until it attains the structure already described, the para- 

 thyroids retain a structure similar to that of the embryonic thyroid. 

 They are composed of solid columns of epithelial cells, which anas- 

 tomose with each other, but are elsewhere separated by a small 

 amount of vascular areolar tissue. They are enclosed in a very 

 thin capsule of areolar tissue, but are in very close relation to the 

 neighboring tissues of the thyroid gland (Figs. 167 and 169), and 

 frequently also with isolated follicles of thymus tissue. 



Different observers vary in their opinions respecting the para- 

 thyroids. Some regard them as reserve thyroid tissue, remaining 

 dormant while the thyroid is functionally competent, but developing 



13 



