114 A MANUAL OF ANATOMY. 



between the lower trunks of the brachial plexus; to reach 

 a position under the levator anguli scapulae muscle ; from 

 here on its course is as described on page 370. 



The Thyroid Gland. Fig. 1 8. 



The thyroid gland is a ductless gland, composed of two 

 symmetrical halves, which are joined by a narrow isthmus. 



The halves, or lobes, of the gland rest upon the antero- 

 external surface of the trachea, and the isthmus crosses its 

 front. 



Each lobe is about two inches long, and one and one- 

 half inches broad, and three-fourths of an inch thick. The 

 lower end of the lobe is broader than its upper end. Its 

 antero-external surface is convex, and its inner surface 

 adapted to the trachea, against which it rests. 



It reaches from the fifth or sixth tracheal ring upward to 

 cover the lower fourth of the thyroid cartilage. 



The isthmus is about one-half an inch wide and crosses 

 the second and third tracheal rings. 



The entire gland weighs from one to two ounces, and is 

 usually heavier in females than males. 



Relations. In front : The integument, platysma, small 

 part of the sternomastoid, deep fascia (two layers), omo- 

 hyoid, sternohyoid, and sternothryoid. 



Laterally : The sheath of the common carotid, internal 

 jugular, and pneumogastric. The superior and inferior 

 thyroid arteries. 



Internally : The larynx and trachea, recurrent laryngeal 

 nerves. 



Posteriorly : Pharynx, and on the left side the oeso- 

 phagus. 



The arteries of the gland : The superior thyroid from the 

 external carotid, the inferior thyroid from the thyroid axis, 



