Of these the last, _tracheotomy_, is by far the most frequent, important, difficult, and dangerous, and requires a very detailed description. Chassaignac[127] says "the only really rational operation for the opening of the air passages by the surgeon is tracheotomy." TRACHEOTOMY.--_Anatomy._--Between the cricoid cartilage and the level of the upper border of the sternum, the middle line of the neck is occupied by the upper portion of the trachea. Its depth from the surface varies, gradually increasing as the trachea descends, and varying very much according to the fatness, muscularity, and length of the neck. It is, however, almost subcutaneous at the commencement below the cricoid, and on the level of the sternum it is in most cases at least an inch from the surface, in many much deeper. Again, its length varies, even in the adult, from two and a half to three, or even four inches. This is important, as affecting the simplicity of the operation, which, as a rule, is easier the longer the neck is. The trachea has most important and complicated anatomical relations--some constant, others irregular. 1. The carotid arteries and jugular veins lie at either side, but, where these are regular in their distribution, do not practically interfere in a well-conducted operation. 2. The thyroid gland lies in close relation to the trachea, one lobe being at each side (Fig. XXXI. B B), and the isthmus of the thyroid crosses the trachea just over the second and third cartilaginous rings. In fat vascular necks, 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.