THE THYKEOID GLAND. 



1347 



streaks on the ventral surface of the kidney. The greater part of these are made up of columns 

 of cortical cells, but at the borders or ends of the cell columns masses of chromaphil cells occur. 

 This arrangement is transitional between the complete independence of the chromaphil and cortical 

 systems in elasmobranchs and the partial inclusion of chromaphil tissue within cortical char- 

 acteristic of the higher mammals (Swale Vincent). But even in man the union of the systems is 

 far from complete. All the chromaphil tissue except the medulla of the suprarenal gland lies 

 out of touch with cortical substance. It appears therefore (i.) that the paraganglia of the 

 sympathetic trunk are homologous with the segmental chromaphil bodies of elasmobranchs ; (ii.) 

 that the chromaphil bodies of the abdominal plexuses are a new formation confined to the higher 

 vertebrates ; (iii.) that the inclusion of chromaphil tissue within a capsule of cortical tissue, as 

 in the suprarenal glands, is a still later development confined to the highest classes of animals. 



2. THE DUCTLESS GLANDS OF ENTODEEMAL OEIGIN. 



A group of ductless glands, (i.) the thyreoid gland, (ii.) the para thy reoid 

 glands, (iii.) the thymus, and two pairs of inconstant, apparently functionless 

 structures, (iv.) the cervical thymus glands, and (v.) the ultimo-branchial bodies, 

 are developed from the entoderrrial lining of the embryonic pharynx. 



the neck, firmly bound 

 the upper part of the 



Thyreoid cartilage 



Crico-thyreoid 

 ligament 



Superior thyreoid 

 vein 



Cricoid cartilage 

 Internal j ugular vein 



Isthmus of thyreoid 

 gland 



Left lobe of 

 thyreoid gland 



(i.) GLANDULA THYREOIDEA. 



The thyreoid gland (O.T. thyroid body) is placed in 

 by fibrous tissue to the anterior and lateral aspects of 

 trachea and to the sides of 

 the larynx. It is yellowish 

 red, soft, and vascular. It 

 varies in size with age, sex, 

 and general nutrition, being 

 relatively large in youth, in 

 females, and in the well nour- 

 ished. In women it increases 



1 temporarily with menstrua- 



, tion and pregnancy. Its 

 average dimensions are, height 

 5 cm., breadth 6 cm., weight 

 25 gm. ; but these measure- 

 ments are of little value be- 

 caus,e of the range of varia- 

 tion. 



Conventionally, the thyreoid 



gland is said to consist of two 



conical lobes united across 



the middle line by a narrow 



strand of gland tissue, the 

 i isthmus. To many thyreoids 



this description is inapplicable. 

 1 In men and thin elderly spinsters 



the gland is not uncommonly 



horse -shoe 'Shaped; in young Innominate artery 



well-nourished women and in FIG. 1058. DISSECTION OF THE THYREOID GLAND AND OF THE PARTS 

 pregnancy its general contour IN IMMEDIATE RELATION TO IT. 



\ suggests a sphere, deeply notched 



superiorly to accommodate the larynx and deeply grooved posteriorly for the trachea and 

 IJesophagus. Rarely, the gland is in two parts. Not infrequently, it is asymmetrical. 

 Ln about 40 per cent of specimens a process of gland tissue, the pyramidal lobe, extends 

 :rom the upper border of the isthmus, upwards, in front of the cricoid and thyreoid 

 Cartilages, towards the hyoid bone. This process is seldom median, lying more often on 

 Dhe left than on the right. In rare cases, it is double. Less rarely, it is double below 

 md single above. Sometimes it is represented by a strip of fibrous tissue or a narrow 

 nuscle (lig. suspensorium, or m. levator, glandulse thyreoideae). 



Small oval accessory thyreoid glands are common in the region of the hyoid bone, 

 : tnd are occasionally met with in relation to the right and left lobes. 



The relations of the gland are variable, depending upon its size and its relative level. 



Common 

 carotid artery 



Inferior 

 thyreoid vein 



