Beneath the skin and superficial fascia lie the two anterior jugular veins; these run from above downwards, to communicate with a branch which crosses the middle line of the neck, commonly in the lower part of the tracheotomy triangle, and there is an interval between them which is, in most cases, sufficiently large to prevent their being injured by a central incision. The pretracheal muscles, namely, the sterno-hyoids and sterno-thyreoids, are closer together; but the interval can be recognized by the greater thickness of the deep fascia which passes between them. When the latter is incised, these muscles can be separated, and the trachea is exposed, together with the structures that lie on its anterior aspect. These are the following:—
Fig. 268. Anatomy of the Larynx and Trachea and the Position of Incisions for the Operations in this Region. A, Subhyoid pharyngotomy; B, Thyrotomy; C, Infrathyreoid laryngotomy; D, ‘High’ tracheotomy; E, ‘Median’ tracheotomy; F, ‘Low’ tracheotomy; 1, Platysma; 2, Crico-thyreoid muscle; 3, Sterno-hyoid muscle; 4, Isthmus of thyreoid gland; 5, Sterno-thyreoid muscle; 6, Sterno-mastoid muscle; 7, Crico-thyreoid artery; 8, Anterior jugular vein; 9, Inferior thyreoid vein; 10, Innominate artery; 11, Right innominate vein; 12, Left innominate vein.
(a) The isthmus of the thyreoid gland, which varies greatly in size. It may be either a thin band with few vessels of importance, covering the second, third, and fourth tracheal ring; or hypertrophied and vascular, extending higher in the neck even to the front of the cricoid or thyreoid cartilage. This condition also results when a pyramidal lobe is present.
(b) The pretracheal fascia, which encloses the isthmus of the thyreoid gland and, when traced upwards, finds attachment to the anterior aspect of the cricoid cartilage, thus forming the suspensory ligament of the isthmus. Passing downwards it covers the anterior surface of the trachea, and, though somewhat indefinite, can easily be traced behind the sternum as far as the pericardium, with which it blends. This is a point of great practical importance in determining the extension of inflammation into the mediastinum.
(c) Veins. Small transverse branches of the superior thyreoid veins run upon the upper border of the isthmus between the layers of the fascia which surround this structure. The inferior thyreoid veins, larger in size, run from the lower border of the isthmus vertically downwards in front of the trachea to communicate with the left innominate; in their upper part they may consist of several small veins which join together to form two main branches, of which the left may lie directly in the middle line; small communicating branches of these veins run transversely across the lower border of the isthmus. The left innominate vein crosses the front of the trachea somewhat obliquely, and may lie at least half an inch above the suprasternal notch.
(d) Arteries. The crico-thyreoid artery runs transversely across the crico-thyreoid space, being placed in front of the suspensory ligament, and gives off numerous branches, which enter and supply the interior of the larynx, as well as small descending branches which run to the isthmus of the thyreoid gland. A small branch of the inferior thyreoid artery is also constantly found behind the isthmus, and in rare instances a thyreoidea ima branch of the innominate, varying greatly in size, may pass upwards in front of the trachea.
In young children the same relations are found, but with certain differences. Owing to the larynx being relatively high in the early years of life, the length of the cervical portion of the trachea is almost 2 inches when the head is extended, and the bifurcation is considerably higher than in the adult; further, the trachea is more movable and is smaller in diameter. The laryngeal cartilages are difficult to distinguish, but a mass composed of the thyreoid and cricoid cartilages can always be felt, and its position determined by careful inspection. It is very important to remember that, even when the head is extended, the cricoid cartilage lies rather less than 2 inches above the upper margin of the sternum. In very young children it is common to find two transverse creases in the skin, of which the upper usually lies over the upper border of the thyreoid and the lower over the cricoid cartilage. The lower crease thus assists in determining the upper limit of the trachea.
The anterior jugular veins in young children are comparatively large; the infrahyoid muscles are less defined and more difficult to recognize; and the isthmus of the thyreoid gland is very broad, appears to be part of the lateral lobes, and occupies a higher position in the neck, often passing in front of the crico-tracheal membrane as well as the first and second tracheal rings. The inferior thyreoid veins are larger, more numerous, and more difficult to separate; the left innominate vein is somewhat higher in the neck; the thymus gland, which gradually decreases in size with the increase of age, may extend into the neck, in front of the trachea, and may even reach as high as the isthmus of the thyreoid; the fasciæ are softer and less definite, and the fascia which covers the trachea is easily stripped from its surface.