a THE TERY ROLED BODY. 1155 
in front of the second, third, and fourth rings of the trachea, and unites the bases 
or lower ends of the two lateral lobes. 
A third lobe is frequently found in connexion with the thyroid body. This is 
the pyramidal or middle lobe (Fig. 790). When present it assumes the form of 
an elongated slender process 
which springs from the upper 
border of the isthmus on one 
or other side of the mesial 
plane Qmore usually on the 
left side) and extends up- 
wards for a variable distance 
towards the hyoid bone wpon 
the cricoid and thyroid car- 
tilages. A strand of fibrous | 
tissue, or perhaps a narrow 
shp composed of muscular 
fibres levator glandule 
thyreoideve), connects it to 
the body of the hyoid bone. 
The thyroid body is 
firmly attached to the parts 
on which it les, and there- 
fore follows the larynx in 
all its movements. 
_ Thyroid cartilage 
Crico-thyroid. 
_7 Tuembrane 
vein 
Cricoid cartilage 
—q Internal jugular vein 
Isthmus of thyroid 
be dy 
Lateral lobe of 
thyroid body 
_ Common 
Sa carotid artery 
‘4 — 
3g _\nterior 
thyroid vein 
Variations.—Small detached 
portions of the thyroid tissue 
placed in the neighbourhood of 
the lateral lobes or in the vicinity 
of the hyoid bone are not un- 
Innominate artery 
common. Such glandular masses 
are termed accessory thyroid Fic. 789.—Drissecrion or THE THyroIp Bopy AND OF THE PARTS 
bodies. The isthmus is the part IN IMMEDIATE RELATION TO IT. 
of the organ which is most sub- 
ject to variation. Its size differs greatly in different individuals, and it not infrequently happens 
that it is absent. 
Blood-vessels.—Four large arteries, and occasionally a fifth smaller vessel, convey blood to the 
thyroid body. ‘Two superior thyroid branches spring from the external carotid arteries. Each 
of these divides at the apex of the lateral lobe into three branches for its supply. Two 
inferior thyroid branches from the thyroid axis of the subclavian artery distribute their terminal 
branches to the basal portions and deep surfaces of the lateral lobes. The occasional artery is the 
thyroidea ima, a branch of the innominate, which ascends upon the trachea to reach the isthmus 
of the thyroid body. The thyroid arteries anastomose freely with each other. 
The veins which drain the blood from the thyroid body are still more numerous. They are 
three in number on each side—viz. the superior and middle thyroid veins, which join the internal 
jugular ; and the inferior thyroid, which descends in front of the trachea and joins its fellow of 
the opposite side to form a large common stem which opens into the left immominate vein. 
Numerous large veins ramify on the surface of the organ and lie in grooves in its substance. 
It is from this plexus that the inferior thyroid veins take origin. 
The nerves which go to the thyroid body accompany the vessels. They are derived from 
| the middle and inferior cervical ganglia of the sympathetic. 
| Structure of the Thyroid Body.—The thyroid body is enveloped by a closely 
applied thin capsule of connective tissue. From the deep surface of this numerous pro- 
cesses penetrate into the substance of the organ, and divide it into lobes and lobules. 
From the septa which separate the lobules fine lamellze proceed which form the boundaries 
of vast numbers of closed vesicles or alveoli of different sizes and shapes. Some of the 
vesicles are spherical or polyhedral, whilst others are oval or flattened and branching. All| 
are lined by a layer of cubical or columnar epithelial cells, and most of them contain a 
viscid semi-fluid colloid material. 
The blood-vessels traverse the organ in the fibrous-tissue septa, whilst the capillary 
network is disposed on the outer surface of the various vesicles. Numerous lymphatic 
vessels arise external to the alveoli, and Baber has shown that they frequently contain 
colloid material similar to that in the interior of the vesicles. 
Development of the Thyroid Body.—The thyroid body is formed from three 
