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cks, or where the thyroid is enlarged it may occupy a much larger portion of the trachea. The position of the isthmus practically divides the trachea into two portions in which it is possible to perform tracheotomy. Both have their advocates, but the balance of authority tends to support the operation below the thyroid. A separate notice of each will be required immediately.
[Illustration: FIG. XXXI.[128]]
3. The muscles in relation to the trachea are the sterno-hyoid and sterno-thyroid of each side. The latter are the broadest, are in close contact across the trachea by the inner edges below, but gradually diverge as they ascend the neck. In thick-set, muscular necks, however, they are in close contact for a considerable distance, and require to be separated to give access to the trachea.
The arteries are in most cases unimportant; no named branch of any size ought to be divided in the operation. However, occasionally very free bleeding may result from the division of an abnormal thyroidea ima running up the trachea to the thyroid body from the innominate, or even from the aorta itself.
The veins are very numerous and irregularly distributed. There is generally a large transverse communicating branch between the superior thyroid veins just above the isthmus. The isthmus itself has a large venous plexus over it. Below the isthmus the veins converge into one trunk (or sometimes two parallel ones) lying right in front of the trachea.
4. The last anatomical point which may give trouble in normal necks is the thymus, which is present in children below the age of two, and covers the lower end of the trachea just above the level of the sternum. Where this is not only not diminished, but enlarged, as it sometimes is in unhealthy children, it may give a very great deal of trouble, rolling out at the wound and greatly embarrassing proceedings.
Abnormalities are very various and sometimes very dangerous: vessels crossing the trachea, as the innom