THE THYROID GLAND 



561 



the thyroid in company with the blood-vessels and enter the deep cer- 

 vical lymph nodes. 



Nerves. The nerves of the thyroid are derived from the sympa- 

 thetic system and are mostly non-medullated. They g ^ nr npnny tb Q QT *- 

 teries and form a delicate terminal plexus in the walls of the follicles. 

 The finer fibrils of this plexus end in contact wit.h t.hp ppithplinTfl. 

 Berkeley (1895) describes also occasional fibrils which apparently pene- 

 trated between the epithelial cells. 



Development. The thyrtoid develops from three anlages, a median and 

 a pair of lateral outgrowths from the primitive pharynx. The median 

 anlage sprouts from the flo6r of the pharynx at the level of the first 

 pharyngeal pouches. Its sitte of origin is marked in the adult by the 

 foramen cecum of the tonguel It forms the isthmus and pyramidal lobe 

 (process) of the definitive thyroid. The lateral anlages grow down from 

 the ventral border of the fourth pharyngeal pouches ; they form the lateral 

 lobes of the gland. The pyramidal lobe represents the remnant of the 



embryonic thyroglossal duct; 

 viduals; it may even retain a 



t varies greatly in length in different indi- 

 partial lumen which may be filled with col- 



loid, but it does not open upon the surface. The primary anlages consist 

 of solid cords of cells; the cords subsequently acquire a lumen, and become 

 broken up into lobules and follicles through the invasion of connective 

 tissue. According to Grosser tjhe definitive thyroid arises exclusively from 

 the median anlage. 



Function. Removal of thb thyroid is followed by serious symptoms 

 and frequently fatal results, particularly in the case of the carnivora. Its 

 internal secretion is apparently necessary for normal metabolism and devel- 

 opment. It governs the conditions favoring tissue oxidation (Crile). The 

 essential secretion is probably something apart from the colloid and iodin 

 content of the gland. According to Crile the active constituent, thyroiodin, 



is iodin in a special proteid co 

 also an antitoxic role is based 

 tomized animals are extremely 

 experiments with frog tadpoles 

 entiation but inhibits growth 

 into diminutive frogs a result 

 is fed. An enlarged thyroid is 

 may be due to increase in the 

 colloid parenchyma ; one results 

 tion of hyposecretion, associate 

 cretinism an apparently herec 

 tion, associated with exophtha 

 animals the thyroid shows a seas 



bination. The belief that the thyroid has 

 chiefly on the observation that thyroidec- 

 iable to certain infections. Gudernatsch's 

 show that a thyroid diet accelerates differ- 

 the tadpoles metamorphosed prematurely 

 he opposite of that obtained when thymus 

 mmonly known as a 'goitre/ Enlargement 

 amount of the connective tissue or of the 

 in atrophy of the parenchyma and a condi- 

 with myxedema and in extreme cases with 

 tary defect; and the other in hypersecre- 

 nic goitre (Grave's disease). In man and 

 nal enlargement related to the sexual cycle. 



