HEAD AND ANTERIOR CERVICAL STRUCTURES. 25 



to the upper margin of the manubrium sterni. Below the hyoid bone is the thyro- 

 hyoid membrane, succeeded by the thyroid cartilage with its (prominent pomum 

 Adami. Then comes the tense crico-thyroid membrane and the cricoid cartilage. 

 Except along a narrow interval on either side of the mesial plane, these structures 

 are covered with two muscular strata, viz., a superficial layer formed by the sterno- 

 hyoid and omo-hyoid, which lie on the same plane, and a deeper layer formed by 

 the sterno-thyroid and its continuation upward, the thyro-hyoid. In addition to 

 these an elongated process of the thyroid gland not unfrequently stretches upward 

 (usually upon the left side), under cover of the sterno-hyoid. If this be present, 

 it will in all probability be observed to be attached to the hyoid bone by a narrow 

 muscular band, termed the le vator glandulte thyroidea. The divergent inner margins 

 of the two small crico-thyroid muscles, as they extend upward and outward upon the 

 cricoid cartilage toward the lower margin of the thyroid cartilage, will also be seen. 

 Upon the crico-thyroid membrane, as it appears between these muscles, the small 

 crico-thyroid artery runs transversely inward. 



Below the cricoid cartilage the dissector comes upon the trachea, which extends 

 downward through the remaining portion of the middle of the neck. As the tube 

 descends it gradually recedes from the surface, so that at the upper margin of the 

 sternum it lies very deeply. The length of this portion of the trachea varies with 

 the position of the head. When the chin is raised, and the head thrown as far back 

 as possible, about two inches and three-quarters of the tube will be found between 

 the cricoid cartilage and the sternum ; when, on the other hand, the head is held in 

 its usual attitude, the length of the cervical part of the trachea is diminished by fully 

 three-quarters of an inch. These measurements must be regarded as merely express- 

 ing the average condition. They vary considerably in different individuals, and are 

 much influenced by differences in the length of the neck. 



The dissector must study carefully the parts which lie superficial to the trachea 

 in this portion of its course. In the first place, examine the structures which are 

 in immediate contact with it. These are (i) the isthmus of the thyroid body; (2) 

 the inferior thyroid veins; (3) at the root of the neck, the innominate artery and 

 the left innominate vein ; (4) the thymus body in young children ; and (5) the occa- 

 sional thyroidea ima artery. The isthmus of the thyroid is a thin band of thyroid 

 substance which crosses the mesial plane upon the anterior aspect of the trachea. As 

 a general rule it covers the second, third, and fourth tracheal rings, so that only one 

 ring is left exposed between its upper margin and the cricoid cartilage. Almost in- 

 variably a branch of the superior thyroid artery runs along the upper margin of the 

 isthmus, whilst upon its anterior surface is placed a plexiform arrangement of small 

 veins. The inferior thyroid veins are two in number and of large size. They are 

 formed by several tributaries which issue from the lateral lobes, and proceed down- 

 ward upon the front of the trachea, one upon either side of the mesial plane. They 

 are separated from each other by a narrow interval, and immediately below the isth- 

 mus they are connected by a plexus of small veins which lies in front of the trachea. 

 An additional median vein, taking origin from the isthmus, may also exist. Close to 

 the sternum the innominate artery will be observed lying upon the trachea, and slightly 

 below the level of the upper border of the bone the left innominate vein crosses it. 

 The thyroidea ima is an occasional branch of the innominate artery. When present 

 it passes vertically upward in front of the trachea to the isthmus of the thyroid gland. 



The parts which separate the trachea, with the structures in immediate relation 

 to its anterior aspect, from the surface, should now be studied. The two anterior 

 jugular veins as they run downward in the superficial fascia, one upon either side of 

 the mesial plane, have been already noticed ; also, the two layers of the deep cervical 

 fascia close to the upper margin of the sternum, and in the 'interval between these 

 the cross branch connecting the two anterior jugular veins. Behind the fascial 

 envelope of the neck come the two muscular strata formed by the sterno-hyoid and 

 the sterno-thyroid muscles. The inner margins of the sterno-hyoid muscles are 

 almost contiguous above, and held together by the fascial sheaths which enclose them ; 

 below, however, they diverge slightly from each other, so as to expose, close to the 

 sternum, the inner margins of the sterno-thyroid muscles. The sterno-thyroid 

 muscles, in contact with each other below, gradually separate from each other as they 



