THE THYROID GLAND. 319 



sent endings on nonstriated muscle tissue. The bronchial arteries 

 have an exceedingly rich nerve supply. 



The visceral and parietal layers of the pleura consist of a layer 

 of fibrous tissue containing numerous elastic fibers. Both layers are 

 covered by a layer of mesothelial cells. The presence of stomata 

 in the pleural mesothelium is denied by Miller. The blood-vessels 

 of the visceral layer of the pleura arise, according to Miller, from 

 the pulmonary artery, these forming a wide-meshed network, which 

 empty into veins which pass into the substance of the lung. Sen- 

 sory nerve-endings, similar to those found in connective tissue, have 

 been observed in the parietal layer of the pleura. 



E. THE THYROID GLAND, 



The thyroid gland is developed from three sources : Its middle 

 portion, the isthmus of the gland, and a portion of the lateral lobes 

 originate as a diverticulum of the pharyngeal epithelium, from what 



S| 



; v 



__- SBB^rf^V&.'St' 

 fc/'^'V $*. ' 



&J:i' f "f^l 



^8E 



h?5' ^ftsfifg 





'- 



Fig. 257. Portion of a cross-section of thyroid gland of a man ; X 3- &dgj Interstitial 

 connective tissue ; bg, blood-vessel ; c, colloid substance ; fs, gland alveoli. 



is later the foramen caecum of the tongue; a part of both lateral 

 portions, the right and left lobes, are formed from a complicated 

 metamorphosis of the epithelium of the fourth visceral pouch. These 

 various parts unite in man into one, so that in the adult the struc- 

 ture of the organ is.continuous. The thyroid gland consists of 

 numerous noncommunicating acini or follicles of various sizes lined 



