1318 THE SKIN, MAMMARY GLANDS AND DUCTLESS GLANDS 



pre-larjiigeal glands and cephalo-laterally along with the superior thyreoid artery to the deep 

 cervical glands. From the caudal part of the lateral lobes and isthmus efferent vessels pass 

 caudally to some small pre-tracheal glands and caudolaterally to the deep cervical glands. 



The nerves of the thyreoid gland are probably all derived from the sympathetic and arise 

 from the middle and inferior cervical ganglia and accompany the arteries to the gland. 



Development, — The thyreoid gland is first seen in very young embryos as a prominence on 

 the ventral wall of the pharj'nx. This becomes a stalked vesicle and divides into lateral lobes. 

 The stalk elongates forming the thyreoglossal duct of His. Later the lumen is obliterated and 

 the duct is then represented by an epithelial cord which soon loses its connection with the 

 pharynx. It opens at first cephalic to the regular second branchial arch on the summit of the 

 tuberculum impar but later shifts to its caudal boundary (Grosser). It is represented in the 

 adult onljr bj' a short blind pouch, the foramen caecum but very rarely a considerable duct may 

 be present. The bilobed mass appears to shift caudally, increasing in size and spreading 

 laterally and dorsally. The median cord of cells formed from the stalk becomes the isthmus 

 and the pjTamidal lobe, when this is present, the lateral portions form the lateral lobes. The 

 gland is now composed of irregular, in general transversely disposed cords of cells. More rapid 

 growth later occurs in the centres of the lateral lobes and the cell cords become closely packed 

 with little connective tissue between. Lumina appear in different places in the cell cords and 

 the cell cords are broken up into groups of cells; in these the lumina continue to appear even 

 upjnto early childhood. On each side, diverticula from the more caudal pharyngeal pouches, 

 the ultimobranchial bodies, come into contact with the dorsal and lateral parts of the anlage 

 of the thyreoid gland and become partly enclosed in the neighbourhood of the transversely 

 running cell cords. This core of cells becomes either a compact body or an irregular group of 

 cells and is probably not transformed into thyreoid tissue. 



THE PARATHYREOID GLANDS 



The parathyreoid glands are small masses of epithelial cells found in the neigh- 

 bourhood of the dorsal surface of the thyreoid gland but quite distinct from it and 



Fig. 1073. — Parathyreoid Glands, Viewed From Behind (Natural Size). 



1 —Pharynx 



I — Common carotid ort. 



; ^ Branch of sup. 

 ■"" thyreoid art. 



— Internal jugular V. 



Superior parathyreoid 

 !— Vagus nerve 



I — Lateral lobe of thyreoid 



1 



— Inferior parathyreoid 



— Inferior thjrreoid art. 



Recurrent (inferior 

 laryngeal) nerve 



Trachea 

 CEsophagus 



of different structure. They arc ductless glands and although very small they are 

 essential to life. 



The usual number is four, two on each side, in relation with the lateral lobes 

 of the thyreoid gland (fig. 1073). In colour they are yellowish with more or less of 

 a reddish or brownish tint but lighter than the thyreoid gland. Their consistency 

 varies somewhat but usually it is softer than that of the thyreoid gland. The 

 shape of the majority of the glands is a flattened ovoid, sometimes tai)ering at one 

 or both ends, rarely a flattened circular disc. At some place on the surface there 

 is usually a depressed hilum where the artery enters and the vein leaves. The 

 average size of the glands is 6 to 7 mm. in length; 3 to 4 mm. in width and 1 to 2 

 mm. in thickness. Occasionally they may be found 15 mm. in length. They 



