THE PLEURAE.

Tracheotomy may be performed either above or below the isthmus of the thyroid body, or ".
this structure may be divided and the trachea opened behind it.
The isthmus of the thyroid gland usually crosses the second and third rings of the trachea;
along its upper border is frequently to be found a large transverse communicating branch between the superior. thyroid veins; and the isthmus itself is covered by a venous plexus formed between the thyroid veins of the opposite sides. Theoretically, therefore, it is advisable to avoid dividing
this structure in opening the trachea.
Above the isthmus the trachea is comparatively superficial, being covered by the skin, super-
ficial fascia, deep fascia, St.erno-hyoid and Sterno-thyroid muscles, and a second layer of the deep fascia, which, attached above to the lower border of the hyoid bone, descends beneath the muscles to the thyroid body, where it divides into two layers and encloses the isthmus.
Below the isthmus the trachea lies much more deeply, and is covered by the Sterno-hyoid and the Sterno"thyroid muscles and a quantity of loose areolar tissue in which is a plexus of veins, some of them of large size; they converge to two trunks, the inferior thyroid veins, which descend on either side of the median line on the front of the trachea and open into the innomi-nate veins. In the infant the thymus gland ascends a variable distance along the front of the trachea, and opposite the episternal notch the windpipe is crossed by the left innominate vein. Occasionally also, in young subjects, the innominate artery crosses the tube obliquely above the level of the sternum. The thyroidea ima artery, when that vessel exists, passes from below up"
ward along the front of the trachea.
From these observations it must be evident that the trachea can be more readily opened
above than below the isthmus of the thyroid body.
Tracheotomy above the isthmus is performed thus: the patient should, if possible, be laid
on his back on a table in a good light. A pillow is to be placed under the shoulders and the head thrown back and steadied by an assistant. The surgeon standing on the right side of his
patient makes an incision from an inch and a half to two inches in length in the median line of the neck from the top of the cricoid cartilage. The incision must be made exactly in the middle line, so as to avoid the anterior jugular veins, and after the superficial structures have been divided the interval between the Sterno-hyoid muscles must be found, the raphe divided, and the muscles drawn apart. The lower border of the cricoid cartilage must now be felt for, and the upper part of the trachea exposed from this point downward in the middle line. Bose has recommended that the layer of fascia in front of the trachea should be divided transversely at the level of the lower border of the cricoid cartilage, and, having been seized with a pair of forceps pressed downward with the handle of the scalpel. By this means the isthmus of the thyroid gland is d<,pressed, and is saved from all danger of being wounded, and the trachea cleanly exposed. The trachea is now transfixed with a sharp book and drawn forward in order to steady It, and is then opened by inserting the knife into it and dividing the two or three upper rings from below upward. If the trachea is to be opened below the isthmus, the incision must be made from a little below the cricoid cartilage to the top of the sternum.