THE THYROID BODY. 



295 



extends along the whole inferior border, and subsequently spreads upwards 

 through the cartilage. The cricoid cai-tilage first becomes ossified at its upper 

 border upon each side, near the two j^osterior articular eminences, and the ossifica- 

 tion invades the lateral parts of the cartilage before encroaching either in front 

 or behind. The arytenoid cartilages become ossified from below upwards. 



DUCTLESS GLANDS ON THE LARYNX AND TRACHEA, 



THE THYROID BODY. 



The thyroid body or gland (fig. 198) is a soft reddish and highly 

 yascular organ, consisting of two lateral lobes, united together towards 

 their lower ends by a transverse portion named the islhmus. Viewed 



Fig. 198. 



Fig. 198. — Sketch showing the Form and Position 

 OF THE Thvroid Body (Alien Thomson). One-Half 

 THE Natural Size. 



The larynx and surrounding parts are viewed from 

 before ; on the right side the muscles covering the 

 thyroid body are retained, on the left side they are 

 removed ; /(, hyoid bone ; th, right thyro-hyoid muscle ; 

 oh, omo- hyoid ; ish, stemo-hyoid ; st, sterno-thyroid ; 

 c, on the crico-thyroid membrane above the cricoid 

 cartilage, points by a dotted line to the right crico- 

 thyroid muscle ; tr, the trachea ; a, the oesophagus 

 appearing behind and slightly to the left of the trachea ; 

 t, the right lobe of the thyroid body partially seen 

 between the muscles ; t', the left lobe entirely exposed ; 

 i, the isthmus ; It, the fibrous or muscular band termed 

 levator thyroidea;, which is more rarely found in the 

 middle line or to the right side, and which existed 

 in the case from which the figure was taken. 



as a whole, it is convex on the sides and in 

 front, forming a rounded projection upon 

 the trachea and larynx. It is covered by 

 the sterno-hyoid, sterno-thvroid, and omo- 

 hyoid muscles, and behind them it comes 

 into contact with the sheath of the great 



vessels of the neck. Its deep surface is concave where it rests against 

 the trachea and larynx. It usually extends so far back as to touch the 

 lower portion of the pharynx, and on the left side the oesophagus also. 



The general direction of each lobe is, from below, upwards and 

 backwards, reaching from the fifth or sixth ring of the trachea to 

 the posterior border of the thyroid cartilage, of which it covers the 

 inferior cornu and adjoining part of the ala. The upper thin end of 

 the lobe, which is sometimes called the cornn, is usually connected to 

 the side of the thyroid and cricoid cartilages by areolar tissue. 



The transverse part, or isthmus (i), which connects the two lateral 

 lobes a little above their lower ends, commonly lies across the third and 

 fourth rings of the trachea, but is very inconstant in size, shape, and 

 position, and the part of the trachea covered by it differs accordingly. 

 From the upper part of the isthmus, or from the adjacent portion 

 of either lobe, a slender conical process, named, from its shape and 

 position, the jjyramid, or middle lobe, often proceeds upwards to the 

 hyoid bone, to which its apex is attached by loose fibrous tissue. 



