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 innominate did in Macilwain's case,[129] or where two brachiocephalic trunks are present, as recorded by Chassaignac.[130] One of the most frequent dangers to be guarded against is a possible dilatation of the aorta or aneurism of the arch. This may very possibly, as happened in one case to the author, give rise to suffocative paroxysms from its pressure on the recurrent laryngeal nerves. Tracheotomy may be deemed necessary, and there is a great risk, unless proper precautions be taken, of wounding the aorta, where it passes upwards in the jugular fossa. In the author's case the vessel had actually to be pushed downwards by the pulp of the forefinger while the trachea was opened, the knife being guided on the back of the nail of the same finger. THE OPERATION.--In a work of this kind it would be utterly impossible to go at all into the subject of what diseases, injuries, etc., warrant or require the operation. It is enough to describe the various methods of operating, their dangers and difficulties. 1. _The operation above the isthmus of the thyroid._--A spot about a quarter or half of an inch in vertical diameter between the cricoid cartilage (Fig. XXXI.) and thyroid isthmus. _Advantages._--It is near the surface, the vessels are few and comparatively small. It is most suitable in cases of aneurism. Professor Spence[131] gives his sanction to the high operation in adults with thick short necks when the operation is performed for ulceration or papilloma of larynx or for spasm from aneurism, the low operation being still best in cases of croup or diphtheria. _Disadvantages._--The space is too small, requires very considerable disturbance of the thyroid isthmus, or actual division of it. It is too near the point where the disease is; so much so, that in most cases of croup or diphtheria it would be perfectly useless. However, if required, or if the operation lower down be contra-indicated, this may be performed easily enough. A straight incision being made in the middle line about one inch and a half in length, expose the upper ring by careful dissection, if possible draw aside the veins, and depress the thyroid isthmus, divide the rings thus exposed, and introduce the tube. _The operation below the isthmus._--This, though more difficult in its performance, is a much more scientific and satisfactory operation. Considerable coolness and a thorough knowledge of the anatomy of the part are absolutely required.