230 SPECIAL ANATOMY. 



465. The Thyroid Gland, Glandula thyreoidea, 



a blood gland on the anterior part of the neck, between the in- 

 ferior half of the thyroid cartilage and the superior rings of the 

 trachea, lying in the shape of a crescent with the horns directed 

 upwards, varying in circumference (in the female larger than in 

 the male), and weight one ounce or more, heavy; of a bluish, in 

 the embryo more of a bright red, colour ; consists of two lobes, 

 lobi, cornua, and a central, inferior, and flat portion, isthmus, 

 from which frequently a third lobe, cornu medium, ascends as 

 far as incisura thyreoidea. But the isthmus is also, frequently, 

 so small, that it appears to be entirely wanting; or it reaches 

 down as far as the sternum. The isthmus is before convex, and 

 separated from the skin by mm. sterno-hyoid. and thyroideus 

 and fascia colli; behind concave, and lying before the rings of 

 the trachea. The lateral lobes are before convex and covered 

 by the m. sterno-thyreoid ; internally concave, and surrounding 

 the inferior part of the thyroid cartilage, the lateral portion of 

 the trachea, the inferior of the pharynx, and the superior of the 

 ossophagus; they are in relation with, behind, the vertebral co- 

 lumn (separated from it by Art. carotis commun., V. jugular 

 int. n. vagus and sympathicus) . The apex of the cornu lies on 

 the outer side of Art. carotis, on the outer and posterior of the 

 thyroid cartilage, and sometimes reaches as far as its superior 

 border. The inferior extremity, thick and round, lies before the 

 fifth and seventh tracheal rings, between the trachea and the 

 carotis commun.; here the Art. thyreoid. infer, enters the in- 

 ferior convex border of the gland; at the superior concave bor- 

 der the art. thyreoid. superior occurs. 



The Parenchyma of the Thyroid gland consists of a spongy uniting tissue, 

 the cells of which are connected by openings with each other, without pos- 

 sessing any excretory duct ; perforated by many vessels it contains a yellowish 

 white serum, the destination of which is not known. Externally the thyroid 

 gland is invested by a dense uniting tissue which is strengthened by fasc. cer- 

 vicalis, M. thyroideus (see before). 



Vessels: Jlrtt. thyreoid. super, (from A. carotis) and inferior (from subdavia), 

 Vv. thyreoid sup. and med. (in V. jugular intern.), inferior (in V. anonyma). 



Nerves: .ZV. laryngeus super, and infer, (from .ZV. Vagus), and branches of 

 gangl. cervic. med. or infimum of N. sympathicus. 



466. 2. The Windpipe, Trachea, arteria aspera, 



is a moveable tube composed of cartilages and membranes, four 

 or five inches long, which, close under the cricoid cartilage, ex- 

 tends from the fifth cervical as low as the third dorsal vertebra, 

 in which region it divides into its two branches, bronchi. In 



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