466 SPLANCHNOLOGY, OR ANATOMY OF THE VISCERA 



become smaller again, and surrounded by areolar tissue and 

 vessels, pass through the nipple to empty on the surface by 

 separate orifices. 



THE DUCTLESS GLANDS 



THYROID GLAND 



The thyroid gland is a very vascular organ, situated at the 

 front of the neck, overhanging the upper rings of the trachea 

 and laterally extending as high as the oblique line on the ala 

 of the thyroid cartilage, and as low as one inch above the upper 

 border of the sternum, when the head is extended. It weighs 

 about one ounce; slightly heavier in the female. It has three 

 lobes two lateral connected by an isthmus; and one third 

 or middle lobe presenting (the former) two surfaces, and an 

 anterior and posterior border. It is firmly attached to the 

 cricoid cartilage and posterior fascia of the trachea by two 

 lateral or suspensory ligaments, and its lobes and isthmus are 

 enclosed within a fibrous capsule derived from the pretracheal 

 portion of the deep cervical fascia. 



The external surface of the lateral lobes is covered by the 

 skin, superficial fascia, a portion of the platysma muscle, 

 deep fascia, sternomastoid, anterior belly of ornohyoid, sterno- 

 hyoid, and sternothyroid muscles. The internal surface is 

 moulded over the thyroid and cricoid cartilages, the trachea, 

 inferior constrictor muscle, and posterior part of cricothyroid 

 muscles, the esophagus (mostly on the left), the superior and 

 inferior thyroid arteries, and recurrent laryngeal nerves. 



The anterior borders are thin, and incline obliquely from 

 above downward &nd inward to the median line. The posterior 

 borders are rounded and rest on the carotid sheath. 



The middle lobe arises from the upper part of the isthmus 

 and left lobe, extending as high as the middle of the hyoid 

 bone in front of the thyroid cartilage. 



The isthmus lies on the second and third ring of the trachea, 

 and measures about J inch in breadth and depth. It has the 

 cricothyroid artery and vein above it, and the lower border 

 is in relation with the inferior thyroid veins. Care should 

 be taken during tracheotomy not to injure these structures. 



