This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
For convenience of study we may consider the structures in the median line, and those regions anterior and those posterior to the sternomastoid muscle, between it and the trapezius. The posterior portion of the neck will be described in the section devoted to the back.
These regions or triangles are simply arbitrary divisions, made for convenience of description. They are sometimes spoken of in reference to the location of growths, operative incisions, etc. They comprise the space between the trapezius muscle posteriorly, the median line anteriorly, the clavicle below, and the lower jaw above.
Viewed from in front, the median portion of the neck may be divided into three regions, the submental, laryngeal, and tracheal.
The submental region extends from the chin to the lower border of the body of the hyoid bone; it is limited laterally by the anterior belly of the digastric 'muscle on each side. Ranula and other sublingual tumors cause a bulging in this region and it is frequently occupied by an enlarged lymphatic node, which at times suppurates and forms an abscess. The floor of the space is formed by the mylohyoid muscle and there are no dangerous structures, so that no hesitancy need be had in incising abscesses in this locality nor in removing diseased lymph-nodes, In carcinoma involving the lower lip near the median line these nodes may be affected and their involvement in such cases should always be looked for. The submaxillary lymphatic nodes farther outward may also be implicated. The tip of the epiglottis projects above the hyoid bone in this region.
Fig. 161. - Anterior surface of the neck.
The laryngeal region extends from the under surface of the hyoid bone to the lower edge of the cricoid cartilage. Laterally it is limited to the space occupied by the larynx. The cricoid cartilage is included in this region as a part of the larynx. The vocal cords are just beneath the most prominent part of the thyroid cartilage.
The tracheal region extends from the lower edge of the cricoid cartilage to the top of the sternum. Just above the sternum, between the sternal origins of the sternomastoid muscles, is the suprasternal notch or, as it is called by the Germans, the Jugulum. Laterally the region is limited by the sides of the trachea.
There are seven or eight rings of the trachea between the cricoid cartilage and the top of the sternum. It is covered partly by the sternohyoid and sternothyroid muscles. The former in the lower half of their course pass outward, leaving a space in which the sternothyroid muscles are seen. The sternohyoid muscle arises from the upper and outer portion of the manubrium, the sternoclavicular ligament, and the inner end of the clavicle. The origin of the sternothyroid is wider than that of the sternohyoid and is lower down. It arises from the first piece of the sternum near the median line, below the sternohyoid, and from the cartilage of the first rib. The first ring of the trachea is not covered by any important structure. The second, third, and fourth rings are covered by the isthmus of the thyroid gland; from here down the inferior thyroid veins may lie on the trachea for at least part of their course. The anterior jugular vein may exist either as a single vein in the median line or to one side of it, or one may pass downward on each side of the median line with a communicating branch from one to the other crossing the median line in the suprasternal notch. The cricothyroid artery, a small branch of the superior thyroid, may cross the cricothyroid membrane, but it is usually too small to cause any troublesome bleeding.
On passing the finger downward from the symphysis it sinks into a hollow, on crossing which the hyoid bone is felt. On pressing the finger into this hollow it rests between the digastric muscles on each side and the mylohyoid muscles beneath. Still deeper than the mylohyoid are the geniohyoid and geniohyoglossus muscles attached to the genial tubercles on the inner side of the mandible. If the lymphatic nodes at this point are enlarged they may be felt. (Fig. 161).
Fig. 162. - Showing anterior and posterior cervical and subsidiary triangles.
The hyoid bone can usually be readily felt in the median line. If it is not easily discovered in the median line it can be felt by a finger and thumb placed on each side of the neck above the thyroid cartilage.
Passing over the hyoid bone the finger then sinks into the space between it and the top of the thyroid cartilage. This space is bridged by the thyrohyoid membrane. Next comes the thyroid cartilage or "Adam's apple." It can readily be seen in adult males and thin people, but in the fat necks of women and children, though it can still be felt, it often cannot be seen. The finger then sinks into the space between the thyroid cartilage above and the cricoid below. They are connected by the cricothyroid membrane, over which runs a small branch (cricothyroid) of the superior thyroid artery.
The prominence of the cricoid cartilage can be seen in thin people and if carefully searched for can be felt in almost all cases. It is opposite the sixth cervical vertebra, a most important landmark. From the cricoid cartilage down to the sternum only soft structures can be felt. The sternum projects forward and the trachea inclines backward so that opposite the top of the sternum the trachea is about 2 cm. behind it. The distance between the top of the sternum and cricoid cartilage in an adult male is about 4.5 cm. (1 3/4 in.).