THE TRACHEA2
The Tracheal Glands are found in great abundance at the posterior part of the trachea. They are small, flattened, ovoid bodies, placed between the-fibrous and muscular coats, each furnished with an excretory duct, which opens on the surface of the mucous membrane. Some glands of smaller size are also found at the sides of the trachea, between the layers of fibrous tissue connecting the rings, and others immediately beneath the mucous coat. The secretion from these glands serves to lubricate the inner surface of the trachea.
The Mucous Membrane lining the tube is covered with columnar ciliated epithelium. It is continuous with that lining the larynx.
Vessels and Nerves. The trachea is supplied with blood by the inferior thyroid arteries.
The Veins terminate in the thyroid venous plexus.
The Nerves are derived from the pneumogastric and its recurrent branches, and from the sympathetic.
Surgical Anatomy. The air-passage may be opened in three different situations; through the crico-thyroid membrane (laryngotomy), through the cricoid cartilage and uppor ring of the trachea (laryngo-tracheotomy), or through the trachea below the isthmus of the thyroid gland (tracheotomy). The student should, therefore, carefully consider the relative anatomy of the air-tube in each of these situations.
Beneath the integument of the laryngo-tracheal region, on either side of the median line, are the two anterior jugular veins. Their size and position vary; there is nearly always one, and frequently two: at the lower part of the neck they diverge, passing beneath the Sterno-mastoid muscles, and are frequently connected by a transverse communicating branch. These veins should, if possible, always be avoided in any operation on the larynx or trachea. If cut through, considerable heemorrhage is the result.
Beneath the cervical fascia are the Sterno-hyoid and Sterno-thyroid muscles, the contiguous edges of the former being near the median line; and beneath these muscles the following parts are met with, from above downwards: the thyroid cartilage, the crico-thyroid membrane, the cricoid cartilage, the trachea, and the isthmus of the thyroid gland.
The crico-thyroid space is very superficial, and may be easily felt, beneath the skin, as a depressed spot, about an inch below the pomum Adami; it is crossed transversely by a small artery, the crico-thyroid, the division of which is seldom accompanied by any troublesome hsemorrhage.
The isthmus of the thyroid gland usually crosses the second and third rings of the trachea ; above it, is found a large transverse communicating branch between the superior thyroid veins, and the isthmus is covered by a venous plexus, formed between the thyroid veins of opposite sides. On the sides of the thyroid gland, and below it, the veins converge to a single median vessel, or to two trunks which descend along the median line of the front of the trachea, to open into the innominate vein by valved orifices. In the infant, the thymus gland ascends a variable distance along the front of the trachea ; and the innominate artery crosses this tube obliquely at the root of the neck, from left to right. The arteria thyroidea ima, when that vessel exists, passes from below upwards along the front of the trachea. The upper part of the trachea lies comparatively superficial; but the lower part passes obliquely downwards and backwards, so as to be deeply placed between the converging Sterno-mastoid muscles. In the child, the trachea is smaller, more deeply placed, and more moveable than in the adult. In fat, or short-necked people, or in those in whom the muscles of the neck are prominently developed, the trachea is more deeply placed than in the opposite conditions.
From these observations, it must be evident that laryngotomy is anatomically the most simple operation, can most readily be performed, and should always be preferred when particular circumstances do not render the operation of tracheotomy absolutely necessary. The operation is performed thus : The head being thrown back and steadied by an assistant, the finger is passed over the front of the neck, and the crico-thyroid depression felt for. A vertical incision is then made through the skin, in the middle line over this spot, and the crico-thyroid membrane is divided to a sufficient extent to allow of the introduction of a large curved tube. The crico-thyroid artery is the only vessel of importance crossing this space. If it should be of large size, its division might produce troublesome haemorrhage.
Laryngo-tracheotomy, anatomically considered, is more dangerous than tracheotomy, on account of the small interspace between the cricoid cartilage and the isthmus of the thyroid gland: the communicating branches between the superior thyroid veins, which cover this spot, can hardly fail to be divided ; and the greatest care will not, in some cases, prevent the division of part of the thyroid isthmus. If either of these structures is divided, the heemorrhage will be considerable.
Tracheotomy below the isthmus of the thyroid gland is performed thus: The head being thrown back and steadied by an assistant, an incision, an inch and a half or two inches in length, is made through the skin, in the median line of the neck, from a little below the cricoid cartilage, to the top of the sternum. The anterior jugular veins should be avoided, by keeping exactly in the median line ; the deep fascia should then be divided, and the contiguous borders of the Sterno-hyoid muscles separated from each other. A quantity of loose areolar tissue, containing the inferior thyroid veins, must then be separated from the front of the trachea, with the handle of the scalpel; and when the trachea is well exposed, it should be opened by inserting the knife into it, dividing two or three of its rings from below upwards. It is a matter of the greatest importance to restrain, it' possible, all haemorrhage oefore the tube is opened ; otherwise, blood may pass into the trachea, and suffocate the patient.
