THE THYEOTD 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 accompany the ar- 

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

 The finer fibrils of this plexus end in contact with the epithelium. 

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

 trated between the epithelial cells. 



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

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

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

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

 foramen cecum of the tongue. 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; it varies greatly in length in different indi- 

 viduals; it may even retain a 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 the definitive thyroid arises exclusively from 

 the median anlage. 



Function. Removal of the 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, tliyroidin, 

 is iodin in a special proteid combination. The belief that the thyroid has 

 also an antitoxic role is based chiefly on the observation that thyroidec- 

 tomized animals are extremely liable to certain infections. Gudernatsch's 

 experiments with frog tadpoles show that a thyroid diet accelerates differ- 

 entiation but inhibits growth : the tadpoles metamorphosed prematurely 

 into diminutive frogs a result the opposite of that obtained when thymus 

 is fed. An enlarged thyroid is commonly known as a 'goitre.' Enlargement 

 may be due to increase in the amount of the connective tissue or of the 

 colloid parenchyma ; one results in atrophy of the parenchyma and a condi- 

 tion of hyposecretion, associated with myxedema and in extreme cases with 

 cretinism an apparently hereditary defect; and the other in hypersecre- 

 tion, associated with exophthalmic goitre (Grave's disease). In man and 

 animals the thyroid shows a seasonal enlargement related to the sexual cycle. 



