THYROID BODY. 121 



in the man. On cutting into the gland a viscid yellowish fluid escapes. 

 It has not any excretory tube or duct. 



Structure. The thyroid body is not provided with a distinct capsule ; 

 but it is surrounded by areolar and fine elastic tissues, which project into 

 the substance and divide it into masses. 



The substance of the gland consists of spherical or elongated vesicles, 

 which vary in size, some being as large as the head of a small pin, and 

 others only J^th of an inch. These vesicles are simple sacs, distinct from 

 one another, and contain a yellowish fluid with corpuscles. The wall of 

 the vesicles consists of a thin proper membrane with a nucleated epithelial 

 lining. Fine vessels and areolar tissue unite together the vesicles into 

 small irregular masses or lobules of the size of the little finger nail. 



The arteries of the thyroid body are two on each side superior and 

 inferior thyroid. The branches of the external carotids (superior thyroid) 

 ramify chiefly on the anterior aspect ; while those from the subclavians 

 (inferior thyroid) pierce the under surface of the body. A very free com- 

 munication is established between all the vessels ; and in the substance of 

 the thyroid body the arteries form a capillary network around the vesicles. 

 Occasionally there is a third thyroid branch (art. thyroid, ima) which 

 arises from the innominate artery into the thorax, and ascending in front 

 of the trachea assists in supplying the thyroid body. 



The veins are large and numerous ; they are superior, middle, and infe- 

 rior thyroid on each side. The first two enter the internal jugular vein 

 (p. 82). The inferior thyroid vein issues from the lower part of the thy- 

 roid body, and descends on the trachea the two forming a plexuse on 

 that tube beneath the sterno-thyroid muscles ; it enters finally the innomi- 

 nate vein of its own side. 



The TRACHEA, or air tube, is continued from the larynx to the thorax, 

 and ends by dividing into two tubes (bronchi) one for each lung. It occu- 

 pies the middle line of the body, and extends commonly from the fifth 

 cervical to the fourth dorsal vertebra, measuring about four inches and a 

 half in length, and nearly one in breadth. The front of the trachea is 

 rounded in consequence of the existence of firm cartilaginous bands in the 

 anterior wall, but at the posterior aspect the cartilages are absent, and the 

 tube is flat and muscular. 



The cervical part of the trachea is very movable, and has the following 

 relative position to the surrounding parts. Covering it in front are the 

 small muscles reaching from the sternum to the hyoid bone, with the deep 

 cervical fascia : beneath those muscles is the inferior thyroid plexus of 

 veins ; and near the larynx is the isthmus of the thyroid body. Behind 

 the tube is the oesophagus, with the recurrent nerves. On each side are 

 the common carotid artery and the thyroid body. 



The structure of the trachea is described in Section XVII. 

 The CESOPHAGUS, or gullet, reaches from the pharynx to the stomach. 

 It commences, like the trachea, opposite the fifth cervical vertebra, and 

 ends opposite the tenth dorsal vertebra. The tube reaches through part of 

 the neck, and through the whole of the thorax ; and occupies the middle 

 line of the body. In length it measures about nine inches. 



In the neck its position is behind the trachea till near the thorax, where 

 it projects to the left side of the air tube, and touches the thyroid body 

 and the thoracic duct. Behind the oesophagus is the longus colli muscle. 

 On each side is the common carotid artery, the proximity of the left being 

 greatest because of the projection of the oesophagus towards the same side. 



