The thyroid cartilage is composed of hyaline cartilage, but begins to ossify about the twentieth year near the cricothyroid joint. As ossification progresses with age, the cartilage becomes brittle, and may occasionally be fractured in old persons by violence, such as throttling, a vertical median fracture generally resulting, which may be accompanied by displacement inwards of the fragments, swelling of the mucous membrane, and consequent asphyxia. The operation of thyrotomy, or median vertical incision of the cartilage, including part of the thyro-hyoid and crico-thyroid membranes, is performed with the object of removing foreign bodies in the larynx, or tumours of the cord, etc. Laryngotomy is performed by making a vertical incision through the soft parts from the middle of the thyroid to the lower border of the cricoid, and then incising the crico-thyroid membrane transversely close to the cricoid, so as to avoid the small crico-thyroid vessels. This operation is sometimes performed where the patient is being choked by some foreign body lodged in the larynx. It does not afford much room, and is above the level at which the membrane is often formed in diphtheria. The larynx has occasionally been extirpated successfully in cases of malignant disease (a preliminary low tracheotomy having been generally performed) through a vertical incision from the hyoid to the isthmus of the thyroid gland, and a transverse one at the upper extremity from one sterno-mastoid to the other. The flaps, consisting of skin, with platysma, deep fascia, and anterior jugular veins, are reflected, as are likewise the sterno- and omo-hyoid muscles. The sterno-thyroid, thyro-hyoid, stylo- and palato-pharyngei, and inferior constrictor muscles, and thyro-hyoid membrane are divided, and the larynx set free by separating the attachments of the epiglottis to the tongue and thyroid bone. The larynx is now drawn forward, the superior laryngeal arteries and internal laryngeal nerves divided, and a careful separation of larynx and pharynx is made, the connecting mucous membrane being divided. Then the inferior laryngeal arteries and recurrent laryngeal nerves are divided, and the trachea is cut across and closed, or opened into the lower end of the wound.
The trachea commences at the lower border of the cricoid opposite the sixth cervical vertebra, recedes from the surface as it descends, being 1½ inches from it at the suprasternal notch, and ends opposite the lower border of the fourth dorsal by bifurcating into two bronchi. The length of the cervical portion varies with the position of the head, but averages nearly 3 inches (about the eighth ring-in a child, from 1½ to 2 inches). It is covered in front by skin, superficial tissue, anterior jugular veins, deep fascia, sterno-hyoid and thyroid muscles, thyroid isthmus (in front of the second, third, and fourth rings), and below the isthmus by the thyroidea ima artery, the inferior thyroid venous plexus, and, at the level of the notch, by the innominate artery and the left innominate vein, and in children, and occasionally in adults, by the thymus gland. Posteriorly, the trachea is in contact with the oesophagus, the sulcus between them being occupied by the recurrent laryngeal nerves, while the lateral surface of the trachea is compressed slightly by the lateral lobes of the thyroid, and below this is in relation to the common carotid arteries. The tissues surrounding the trachea are very lax, permitting fairly free movement, both vertically (as in swallowing) and laterally. Thus, in performing tracheotomy, the trachea has been unwittingly retracted to one side by an assistant, and the oesophagus exposed. Tracheotomy may be necessitated in respiratory obstruction from tumours, foreign bodies, or diphtheria. It may be either high (above the thyroid isthmus) or low (below it). The former is generally easier and safer, owing to the more superficial position of that part and the smaller number of important structures in close relationship to it, but leaves a more prominent scar. The operation is performed by placing a small pillow behind the neck so as to throw it forward, keeping the head absolutely straight and steady, and then making a vertical incision exactly in the middle line. The skin and fasciae are cut through, the muscles are separated, the thyroid isthmus displaced downwards, after making two slight lateral cuts in the laryngo-thyroid suspensory fascia, and then a sharp hook is inserted into the first ring of the trachea, and an opening made by cutting from below upwards away from the great vessels. A tube of suitable size is then taken (not exceeding 1/6 inch in diameter under eighteen months, and 1/4 inch in a child under four years of age), and introduced like a catheter, with the plate directed downwards toward the sternal notch and almost touching the skin, so as to present the nozzle vertically into the wound. Then, by tilting the plate upwards like the handle of a catheter, the point slides into position. Care is required not to introduce the point into the cellular tissue in front of the trachea. Where additional room is required, it may be had by cutting through the cricoid and crico-thyroid membrane, or by dividing the thyroid isthmus. The latter is not in itself a dangerous proceeding, but large transverse vessels are not infrequently found arising at the level of the isthmus. The operation is particularly difficult in children, owing to the short fat neck, the closer relationship to vessels, etc., and the softness and mobility of the trachea itself.