The neck is bounded above by a line drawn along the lower border of the lower jaw to the angle, thence to the mastoid process, and so to the superior curved line and external occipital protuberance ; below, by a line from the suprasternal notch along the upper border of the clavicle to the acromion, and from thence to the spine of the vertebra prominens (seventh cervical). It is pretty constant in length-generally about 5 inches.
The hyoid bone, which is on the level of the fourth cervical vertebra, divides the anterior portion of the neck into two triangles, of each of which it forms the base-an upper, or submaxillary, which is best seen when the head is thrown back, whose sides are formed by the rami of the jaw, and a lower, or infra-hyoid, whose sides are formed by the sterno-mastoid muscles. The latter, again, is divided into two carotid triangles by the middle line of the neck, which presents several points of importance. Between the hyoid bone and thyroid cartilage the thyro-hyoid membrane extends. The vocal cords are situated about the level of the middle of the thyroid crest. The lateral lobes of the thyroid gland lie to either side of the thyroid cartilage. Below the thyroid cartilage comes the crico-thyroid membrane which connects these structures, and through which laryngotomy may be done, by means of a small transverse incision as close to the cricoid as possible, so as to avoid damage to the crico-thyroid vessels. The cricoid cartilage forms a prominent and useful landmark, even in infants. It is on a level with the fifth or sixth cervical vertebra, and the commencement of the oesophagus lies behind it. Foreign bodies too large to be swallowed frequently lodge at this level. The carotid artery is conveniently ligatured just above the point where it is crossed by the omo-hyoid muscle on a level with the cricoid cartilage, the tubercle of the sixth cervical vertebra forming a further guide. The lower margin of the cricoid corresponds with the junction of larynx and trachea. The trachea is not easily made out superficially, as it passes in deeply, lying about 1½ inches from the surface at the suprasternal notch, above which point lie the first seven rings of the trachea. The inferior thyroid vein lies in front of the trachea, below the isthmus of the thyroid. The suprasternal notch is generally opposite the lower border of the second dorsal vertebra, but may be opposite the third. The anterior jugular veins, which commence in the submaxillary region, descend on the sternohyoid muscles to either side of the middle line, and then pierce the fascia just above the clavicle. Behind the sternoclavicular joint lie the commencement of the innominate vein, the innominate artery on the right and the carotid on the left, and, more deeply, the lung and pleura.