920 



THE THYROID BODY. 



ossification ; this commences first in the'thyroid cartilage, then appears in the cricoid, 

 and lastly in the arytenoid cartilages. In the thyroid cartilage the ossification usually 

 begins at the cornua and posterior borders ; it then gradually extends along the whole 

 inferior border, and subsequently spreads upwards through the cartilage. The cricoid 

 cartilage first becomes ossified at its upper border upon each side, near the two pos- 

 terior articular eminences, and the ossification invades the lateral parts of the cartilage 

 before encroaching on it either in front or behind. The arytenoid cartilages become 

 ossified from below upwards. 



DUCTLESS GLANDS ON THE LARYNX AND TRACHEA. 

 1. THE THYROID BODY. 



The thyroid body or gland is a soft reddish and highly vascular organ, 

 situated in the lower part of the neck, embracing the front and sides of the 

 upper part of the trachea, and reaching up to the sides of the larynx. It 

 belongs, like the spleen, to the series of structures known as ductless glands ; 

 and, although its precise function is unascertained, there is reason to believe 

 that it is in some way connected with the elaboration of the blood. 



The thyroid body is of an irregular, semilunar form, consisting of two 

 lateral loles t united together towards their lower ends by a transverse por- 

 tion named the isthmus. Viewed 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-thyroid, 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 



Fig. 644. 



Fig. 614. SKETCH SHOWING THE FORM AND POSITION 

 OF THE THYROID BODY. 4 



The larynx and surrounding parts are viewed from 

 before ; on the right side the muscles covering the 

 thyroid body are retained, on the leftside they are 

 removed ; h, hyoid bone ; th, right thryo-hyoid mus- 

 cle ; oh, omo-hyoid; sh, sterno-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; as, the oeso- 

 phagus appearing behind and slightly to the left of the 

 trachea ; t 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 thyroidese, which is occasionally 

 found in the middle line or to the left side, and which 

 existed in the case from which the figure was taken. 



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. 



Each lateral lobe measures usually two inches 

 or upwards in length, an inch and a quarter 

 in breadth, and three-quarters of an inch in 

 thickness at its largest part, which is below its 

 middle : the right lobe is usually a few lines longer and wider than the left. 

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

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

 terior border of the thyroid cartilage, of which it covers the inferior cornu 

 and adjoining part of the ala. The upper end of the lobe, which is thinner, 



