Certain abnormalities of the blood-vessels have been alluded to above. The commonest consists in the existence of a thyroidea ima artery, which when present usually arises from the innominate trunk, but sometimes from the right common carotid or the aorta: it passes to the thyroid body directly in the median line of the neck and close to the trachea; again, the place of the anterior jugular veins may be taken by a single central vessel, almost sure to be wounded during the operation if it exist (Mackenzie). In performing the operation through the thyro-cricoid membrane (thyro-cricotomy) or through the cricoid cartilage alone (cricotomy), the same tissues are met with, and the same dissection is necessary in the earlier stage of the operation, as have been described in the operation of superior or inferior tracheotomy; but the parts are more superficial, adipose and cellular tissue less abundant, blood-vessels much less numerous, and the operation very much simpler. The thyroid gland of course does not come into view, {148} and the crico-thyroid artery, a very small vessel, needs no attention in the dissection. I have here and elsewhere included under the general term tracheotomy five distinct operations, having for their object the opening of the air-passages, which are surgically possible between the lower border of the thyroid cartilage and the upper edge of the sternum. In this classification I have followed that of Schüller, and its simplicity, but exactness, and the avoidance of the old confusion of different terms which results from the use of one intelligently employed, seem to me to commend it. These five operations are--1. Thyro-cricotomy, or the opening made through the crico-thyroid membrane alone. 2. Cricotomy, or the division of the cricoid cartilage alone. 3. Superior tracheotomy, the incision being made above the point where the isthmus of the thyroid gland crosses the trachea and below the cricoid cartilage. 4. Median tracheotomy, when, the isthmus being displaced or torn through, the trachea is opened immediately below its site. And 5. Inferior tracheotomy, the incision being made below the point of crossing of the isthmus of the thyroid gland, and at varying distances, dependent mainly upon the age of the patient and size of the parts, above the sternal notch.