1388 SURFACE AND SURGICAL ANATOMY. 



median plane for fear of wounding the superior laryngeal vessels and nerve which 

 pierce the thin lateral portions of the thyreo-hyoid membrane. 



The wound in suicidal cut-throat is generally at this level. The more important structures 

 which are usually divided are : more or less of the left sterno-mastoid muscle, the superior 

 thyreoid vessels, the thyreo-hyoid membrane, the base of the epiglottis, and, less frequently, the 

 carotid vessels, the internal jugular vein, and the superior laryngeal nerve. When the wound 

 is above the hyoid bone, the lingual and external maxillary vessels and the muscles of the tongue 

 are the more important structures injured. 



At the level of the middle of the anterior border of the thyreoid cartilage is the 

 rima glottidis. 



In the operation of thyreotomy care is taken to divide the thyreoid cartilage 

 exactly in the median plane so as to avoid injuring the vocal folds. 



A little more than an inch below the prominentia laryngea is the anterior arch 

 of the cricoid cartilage, which may be readily felt, and, when the neck is extended, 

 often seen ; it lies a little below a point midway between the lower margin of the 

 chin and the superior border of the sternum. Above the cricoid is the crico-thyreoid 

 ligament; in the operation of laryngotomy only the middle portion of the 

 ligament is divided, in order to avoid injury to the crico-thyreoid muscles. The 

 small crico-thyreoid branch of the superior thyreoid artery lies close to the 

 inferior border of the thyreoid cartilage. Below the cricoid cartilage is the trachea, 

 which recedes as it descends, so that it lies 1 in. from the surface at the level of 

 the superior border of the sternum. The isthmus of the thyreoid gland lies in front 

 of the second, third, and fourth rings of the trachea (Fig. 1087) ; not infrequently, 

 however, it reaches up to the cricoid. Immediately in front of the trachea, below 

 the isthmus of the thyreoid, is the pretracheal fat, containing one or two lymph 

 glands and the inferior thyreoid veins, each represented by one or more branches which 

 converge as they descend. The pretracheal lymph glands receive afferent vessels 

 from the larynx and thyreoid gland, while their efferent vessels open into the inferior 

 deep cervical glands. In the adult the innominate artery crosses the front of the 

 trachea at the level of the superior border of the sternum ; in the child, however, 

 it not infrequently crosses half an inch higher, a relation which must be remem- 

 bered in performing the operation of low tracheotomy. 



In the operation of high tracheotomy the upper three rings of the trachea are divided. 

 The incision, which should be median, divides the integuments, the .tributaries of the 

 anterior jugular veins, the general envelope of deep cervical fascia, and, after passing 

 between the depressor muscles of the hyoid bone, the pretracheal fascia, which descends 

 from the cricoid to enclose the isthmus of the thyreoid gland. By dividing this fascia 

 transversely below the cricoid, the isthmus may be pulled downwards and the upper rings 

 of the trachea exposed. In some cases it is necessary either to divide the isthmus or to 

 .extend the incision upwards through the cricoid cartilage. In opening the trachea, the 

 edge of the knife should be directed upwards so as to avoid injuring the vessels at the 

 upper border of the isthmus. The anterior jugular veins are in danger of being wounded 

 if the skin incision is not strictly median. In low tracheotomy the trachea below the 

 isthmus is opened ; it is a more troublesome operation, on account of the depth of the 

 trachea and the presence in front of it of the large inferior thyreoid veins and of the trans- 

 verse anterior jugular vein. In children the difficulty is increased by the higher position 

 of the innominate artery and left innominate vein, by the presence of the thymus gland, 

 and by the shortness of the neck. 



Thyreoid Gland. The thyreoid gland, which is moulded on and adherent to 

 the anterior aspects and the sides of the upper part of the trachea and to the lower 

 and posterior portions of the laminse of the thyreoid cartilage, is covered by the infra- 

 hyoid muscles and overlapped by. the sterno-mastoid. The posterior borders of its 

 lobes come in contact with the oesophagus and lower part of the pharynx, while 

 posteriorly they partially overlap the carotid sheath. 



The thyreoid gland, like the prostate, possesses, in addition to its own proper 

 capsule, an outer capsule or sheath derived from the cervical fascia. The capsule 

 proper, like that of the liver, is inseparably connected with the gland. The sheath, 

 on the other hand, is formed by the middle (pretracheal) layer of deep cervical 

 fascia, which splits to enclose the thyreoid. Between the true capsule and the 



