256 THE ENDOCRINE GLANDS 



ratiis. In higher ^'ertebrates there is a tendency for a combination 

 of the two portions into one gland. In mammals the thyroid is 

 located below the larynx on the ventral surface of the trachea 

 adjacent to the upper tracheal cartilages. The form varies in 

 different mannnals. In man there is an oval-shaped mass on 

 each side of the anterior face of the trachea connected by a median 

 "isthmus." 



The thyroid begins its development as an evagination of cells 

 from the raid-ventral wall of the pharynx in the region of the first 

 pharyngeal pouches or somewhat posteriorly. This mass of early 

 cells, the thyroid "anlage," grows into a long stalk connected with 

 the mouth cavity at the base of the tongue, behind the papillated 

 region, and forms a distal larger mass which grows into the isthmus 

 of the mature gland. Later the stalk atrophies, thus shutting off 

 the "isthmus" from the digestive system. However, the site of 

 origin remains in the adult as the foramen cecum, a small depression 

 at the base of the tongue on its upper surface, just back of the most 

 posterior vallate papillae. In addition to the above, lateral anlages 

 grow down from the lower surface of each foiu'th pharyngeal ])ouch 

 and establish a connection on either side with the isthmus to form 

 the lateral lobes 



Connective tissue grows around the outside, forming an outer 

 loose sheath and an inner firm capsule of fibroelastic connective tissue 

 from which extensions are carried in between groups of gland cells. 



In young embryos the gland cells arrange themselves in the form 

 of more or less spherical follicles resembling the terminal end-pieces 

 of an alveolar gland. 



The mature thyroid consists of follicles which are structurally 

 independent of one another. (Fig. 158.) As they develo]), a clear 

 yellow viscid colloidal fluid with an affinity for acid dyes collects 

 inside each follicle. The follicular wall is composed of cuboidal 

 epithelial cells, although the form varies somewhat with age, the 

 breeding season, and the type of food. Both mitochondria and 

 Golgi apparatus have been demonstrated in the follicle cells. The 

 inner border of these cells is cuticular, and there is no basement 

 membrane supporting them. The nuclei are large, round, and 

 regularly placed, and the side boundaries of the cells are indefinitely 

 indicated. Between adjacent follicles is loose fibroelastic connec- 

 tive tissue and a reticular network. The connective tissue sup])()rts 

 a very rich capillary network, so that the gland receives a relati\ely 

 large quantity of blood. 



