II22 A MANUAL OF ANATOMY 



Thyroid Body. — The thyroid body is situated on either side of 

 the upper part of the trachea and the larynx, and a small portion of 

 it lies in front of the upper part of the trachea. Its size is subject 

 to much variation ; its weight is rather more than i ounce ; and it 

 is larger in the female than in the male. It consists of two lateral 

 lobes and an isthmus. 



Each lateral lobe is conical, the rounded base being directed 

 downwards, and it measures about 2 inches in length. It extends 

 from the inferior cornu and adjacent portion of the ala of the 

 thyroid cartilage to about the level of the fifth ring of the trachea. 

 Its superficial surface, which looks forwards and outwards, is some- 

 what convex, and is covered by the stemo-thyroid, sterno-hyoid, 

 and anterior belly of the omo-hyoid muscles. It is also overlapped 

 by the anterior border of the sterno-cleido-mastoid. Its deep 

 surface is concave in adaptation to the trachea and lar5nix. The 

 anterior border is thin, and towards its lower part is connected with 

 that of the opposite lateral lobe by means of the isthmus. The 

 posterior border is thick, and is in contact with the pharynx and 

 oesophagus. The lateral lobe slightly overlaps the common carotid 

 artery, enclosed in the carotid sheath, and is frequently grooved 

 by that vessel. Interiorly it overlaps the recurrent laryngeal nerve 

 and inferior thyroid artery. The apex of the lateral lobe rests upon 

 the inferior constrictor muscle of the pharynx, and the superior 

 thyroid artery enters it superficially. 



The isthmus is inconstant as regards size and position. Its 

 depth ranges from J to i inch, and its breadth is about | inch. It 

 connects the lower parts of the anterior borders of the lateral lobes, 

 but does not reach quite so low as their bases. It lies in front of 

 the trachea, usually upon the second and third rings, sometimes 

 upon the upper three rings, or it may be the second, third, and 

 fourth rings. It fits closely to the rings upon which it rests. 

 Along its upper border there is a branch of the superior thyroid 

 artery, known as the artery of the isthmus, which anastomoses with 

 its fellow of the opposite side. From its lower border several veins 

 issue, which take part in the inferior thyroid plexus of veins in front 

 of the trachea. 



In some cases an additional lobe is present, called the pyramidal, 

 or middle, lobe. It takes the form of a long pyramid which is 

 attached by its base to the upper border of the isthmus, usually 

 at its junction with the left lateral lobe. Its apex is attached to 

 the body of the hyoid bone by a fibrous band, which sometimes 

 contains muscular fibres, known as the levator glandulce thyroidece 

 muscle. 



The thyroid body is invested by a fibrous sheath which is derived 

 from the pretracheal lamina of the deep cervical fascia. 



When a portion or portions of the pyramidal lobe, or of a lateral 

 lobe, become detached, the isolated masses are known as accessory 

 thyroids. 



Blood-supply — Arteries. — ^The thyroid body is characterized by 

 great vascularity. The arteries on either side are (i) the superior 



