THE THYREOID GLAND 1315 



The pyramidal lobe, though usually single, may be double or bifid at its caudal end, one 

 process joining each lateral lobe. It may be attached in the angle between the isthmus and 

 one of the lateral lobes, or to the lateral lobe itself. It may be cylindrical, band-like, or swollen 

 at its centre or cephalic end and is occasionally entirely separate from the rest of the thyreoid 

 or divided into separate detached parts, thus forming accessory thyreoids. The apex in some 

 cases extends to the middle of the thyreohyoid membrane or rarely to or beyond the hyoid 

 bone or the process may be quite short. In the thyreoid ligament, attached to the apex, 

 muscle fibres are sometimes found, aberrant parts of the infrahj^oid muscles, the levator of the 

 thyreoid gland. 



The thyreoid lobes, right and left, are placed on each side of the trachea and 

 larynx (figs. 1068, 1069, 1070). Each lobe is somewhat pyramidal in shape and 

 presents for examination a base, an apex, a medial, a ventro-lateral, and a dorsal 

 surface. 



The base is roughly convex or pointed, rarely flattened, usually at the level of 

 the fifth or sixth ring of the trachea (figs. 1068, 1069). 



It is separated from the jugular notch of the sternum by a distance of 1.5 to 2 cm. but when 

 the head is extended the distance is greatly increased. It is in relation with the inferior thy- 

 reoid artery and numerous veins, mostly tributaries of the inferior thyreoid vein. 



The apex is pointed or rounded (figs. 1068, 1069). It is directed cephalo- 

 dorsally and is situated at the dorsal border of the lateral lamina of the thyreoid 

 cartilage at the level of its caudal, or rarely its middle, third. 



It is covered by the sterno-thyreoid muscle beneath which the superior thyreoid artery 

 accompanied by the corresponding vein crosses the apex to reach the gland. It is also crossed 

 in this situation by the external ramus of the superior larjoigeal nerve as it passes to the crico- 

 thyreoid muscle. 



The medial surface of the lateral lobe is concave and intimately bound to the 

 trachea and cricoid cartilage (fig. 1070). Toward the apex it becomes more 

 flattened where it comes into contact with the lateral lamina of the thj'reoid 

 cartilage. 



At the border where this surface joins with the dorsal surface it is in relation with the 

 oesophagus and pharjmx, and in the angle between these structures and the trachea and larynx 

 it is close to the recurrent larjTigeal nerve. 



The dorsal surface (fig. 1070) is broad and rounded caudally, but toward the 

 apex is reduced to a mere border. It lies upon the fascial sheath containing the 

 common carotid artery, the jugular vein, and vagus nerve, most intimately 

 related to the common carotid artery which usually produces a groove in it. 



The inferior thjTeoid artery sends large branches over this surface. The inferior thyreoid 

 veins also have large branches here. Imbedded in the connective tissue in relation with this 

 surface the parathyreoid bodies are found, and in some cases the recurrent nerves are placed so 

 far laterally that they also touch this surface. In many cases the sj^mpathetic trunk and the 

 middle cervical ganglia of the sympathetic with the cardiac branches are closely related to the 

 dorsal surface of the gland. 



The ventro-lateral surface is convex and is separated by loose connective tis- 

 sue from the overlapping sterno-thyreoid, sterno-hyoid, and omo-hyoid muscles. 



More superficial on its lateral aspect is the sterno-cleido-mastoid muscle. The above 

 muscles are enclosed by the superficial and middle sheets of the cervical fascia. In the subcu- 

 taneous tela the platysma muscle spreads OA^er the gland. This surface of the gland is 

 covered by a plexus of veins and by branches of the superior thjTeoid arter}'. 



Accessory thyreoid glands are small masses of glandular tissue one or more of which may 

 be found situated in the median line or at one side of it anywhere between the isthmus and the 

 root of the tongue. They vary considerably in size and represent parts of the pyramidal lobe 

 or isthmus which have become completely separated from the rest of the gland. In structure 

 they are composed of the same tissue as the rest of the gland. 



Fixation. — In addition to the connective tissue which binds the thyreoid gland 

 to the trachea, it is attached by the connection of its capsule with the cervical 

 fascia and by the fibrous prolongations from the capsule. 



These prolongations are found medially attaching the isthmus and adjoining portions of 

 the lateral lobes to the ventral surface of the cricoid cartilage, the caudal border of the thyreoid 

 cartilage, and the sheath of the crico-thyreoid muscles, and laterally attaching the lateral lobes 

 to the trachea and lateral surface of the cricoid cartilage. In addition to these the connection 

 of the vessels and nerves to the gland helps to fix it in position. 



Structure. — The thyreoid gland like other glands is composed of a connective- 

 tissue stroma supporting an epithelial secreting parenchyma. 



