THE THYREOID AND PARATHYREOID 409 



or double layer of cuboidal c{)ithclial cells. Two types of cells are 

 recognized. 



One of these is actively secretin.^ colloid and is known as a secreting 

 or colloid cell. The other contains no colloid and is known as a 

 resting or reserve cell. It is probable that their names indicate the 

 relation of these cells to each other and that the reserve cell ultimately 

 becomes a colloid-secreting cell. 



The colloid cell appears in some cases simply to pour out its colloid 

 secretion into the lumen, after which it may assume the character of a 

 resting cell; in other cases the cell appears to be completely trans- 

 formed into colloid, its place being taken by proliferation of the rest- 

 ing cells. In certain alveoli which are much distended with colloid 

 the lining epithelium is flattened. 



That the thyreoid exerts a decided influence upon general body metabolism 

 is shown by the symptoms resulting from congenital absence of the gland (con- 

 genital myxcedema or cretinism) and by the effects of complete removal, the 

 latter giving rise to a train of symptoms known as the "cachexia strumipriva." 



The blood supply of the thyreoid is extremely rich, the vessels 

 branching and anastomosing in the connective tissue and forming 

 dense capillary networks around the alveoli. 



L5miphatics accompany the blood-vessels in the connective tissue. 



Nerves are mainly non-medullated fibres which form plexuses 

 around the blood-vessels and in the connective tissue surrounding 

 the alveoli. Terminals to the secreting cells end in club-like 

 dilatations against the bases of, or between the epithelial cells. A few 

 afferent medullated fibres are found in the plexuses surrounding the 

 blood-vessels. 



Development. — The thyreoid originates as a diverticulum from the ento- 

 derm of the primitive pharynx. It first appears in embryos of 3 to 5 mm. and 

 grows ventrally into the mesoderm of the ventral wall of the neck. Here it 

 forms a mass which lies transversely across the neck. It is composed of solid 

 cords of cells which become hollow to form the alveoli of the gland. At first 

 the gland is connected with the surface by the thyreo-glossal duct. This either 

 disappears entirely or is represented in the adult by such rudimentary structures 

 as the so-called prehyoid, suprahyoid, and accessory thyreoid glands. In- 

 growths of connective tissue divide the solid cords of cells of which the gland 

 at this stage consists into groups or lobules, and at the same time break up 

 the long cords into short segments. Dilatation of the alveoli occurs with the 

 formation of colloid. 



