All-important as a preliminary to the operation itself is a thorough knowledge of the surgical anatomy of the region upon which it is proposed to operate; and this not alone in the adult, but especially in the child, where essential differences often exist. Possible anomalies also are not to be forgotten.[2] The assurance of the surgeon depends upon this knowledge: mere, manual skill will not compensate for its want; the success, both immediate and remote, of the operation is in great measure the reward of its possession. [Footnote 2: See Pilcher, "The Anatomy of the Anterior Median Region of the Neck," _Ann. of Anat. and Surgery_, Brooklyn, April, 1881.] It will be remembered that the trachea commences at the inferior border of the cricoid cartilage, directly opposite to the lower edge of the fifth cervical vertebra, and reaches thence downward, in the median line of the neck, until it bifurcates opposite to the third dorsal vertebra. In its upper part it is nearly subcutaneous, and is surmounted by the prominent ring of the cricoid cartilage (easily identified, even in the young child), above which, in turn, lies a slight depression (the crico-thyroid space) between the cricoid and thyroid cartilages. As the trachea descends in the neck it recedes gradually, lying at the episternal notch about one and three-eighths of an inch from the surface. Throughout the whole of this course it is in relation with important structures. In its cervical portion it is covered by the sterno-hyoid and sterno-thyroid muscles, and in the median space, which is usually distinct between them, by layers of the deep cervical fascia. It is also crossed by the isthmus of the thyroid gland, which lies between the second and fourth tracheal rings; by the arteria-thyroidea ima, when present, and below by the plexus formed of inferior thyroid veins with their tributary and communicating branches. In the latter region, but more superficially, are some communicating branches between the anterior jugular veins. The innominate and left carotid arteries are also anterior to it in the episternal notch as they diverge from their origin. Laterally, the trachea is in relation with the common carotid artery, the lateral lobes of the thyroid body, the inferior thyroid veins, and the recurrent laryngeal nerves. The thoracic portion of the trachea is covered by the manubrium sterni, with the origins of the sterno-hyoid and {147} sterno-thyroid muscles, by the left innominate vein, and by the commencement of the innominate and left carotid arteries. Still lower, the transverse portion of the arch of the aorta crosses, and the deep cardiac plexus of nerves lies in front of it. Posteriorly, throughout its length, it rests upon the oesophagus.