8 PHYSIOLOGY CHAP, 



with water, and coagulates after death, when it resembles the 

 fixed content of the vesicles. 



Lewandowsky (1902) came to the same conclusion from histo- 

 logical work on the thyroid of dog, cat, rabbit, ape, lamb, and 

 hedgehog. He found that the secretion from the vesicular 

 epithelium was quite fluid, and indistinguishable under the 

 microscope from other protein solutions. According to this, there 

 are no colloid cells which secrete preformed colloidal substances. 

 This secretion first assumes the properties of a colloid in the 

 vesicular lumen. 



The blood-vessels, which are numerous and large in proportion 

 to the size of the organ, penetrate the cavities of the interstitial 

 connective tissue, where they ramify rapidly, and come into 

 intimate relations with the walls of the alveoli, round which they 

 form a capillary network that is in perfect contact with the 

 epithelium. The lymphatics arise from the spaces of the inter- 

 lobular and interalveolar connective tissue, forming a number of 

 large trunks that anastomose into plexuses at the surface of the 

 organ. 



The nerves that supply the thyroid come from the two 

 laryngeals, superior and inferior, from the vagus, and from the 

 superior cervical ganglion of the sympathetic. Their mode of 

 termination in the muscle cells of the vessels and in the epithelial 

 cells is unknown. 



Embryology shows that the thyroid originates from three 

 epithelial diverticuli of the primitive intestine, two of which form 

 the lateral lobes, and the third the isthmus and pyramid of 

 Morgagni. The epithelial cells of the three diverticuli are grouped 

 into small masses which are then transformed into vesicles or 

 alveoli. The peripheral cells of each mass constitute the epithelium 

 of the alveolus ; the central cells become granular, and on breaking 

 up form the colloidal content of the primitive alveolus. 



The primitive epithelial masses are mainly grouped together 

 to form the principal thyroid gland ; but there are almost invari- 

 ably certain nodules which do not fuse with it, and which give 

 rise to small accessory thyroids, these in the successive phases 

 of embryonic development may wander to a considerable distance 

 from their origin and enter into relation with various organs 

 derived from the cephalic end of the foetus. On a careful compu- 

 tation of the accessory thyroids found in various places, they 

 exist in the tongue and in the sub-maxillary, retro-pharyngeal, 

 retro-oesophageal, laryngo-tracheal, hyoid, crico-thyroid, bronchial, 

 aortic, and inediastinic regions (D'Aiutolo, 1890). The accessory 

 thyroids are perfectly similar in structure to the principal thyroid 

 body, and they also exhibit alveoli filled with colloidal substance, 

 blood-vessels, lymph spaces and vessels, and nerve filaments. 



The Parathyroids (glandulae parathyreoideae] differ completely 



