Neck
The neck is the part of the body on many vertebrates that connects the head with the torso and provides the mobility and movements of the head. The structures of the human neck are anatomically grouped into four compartments; vertebral, visceral and two vascular compartments. Within these compartments, the neck houses the cervical vertebrae and cervical part of the spinal cord, upper parts of the respiratory and digestive tracts, endocrine glands, nerves, arteries and veins. Muscles of the neck are described separately from the compartments. They bound the neck triangles. In anatomy, the neck is also called by its  Latin names, cervix or collum, although when used alone, in context, the word "cervix" more often refers to the uterine cervix, the neck of the uterus. Thus the adjective "cervical" may refer either to the neck (as in "cervical vertebrae" or "cervical lymph nodes") or to the uterine cervix (as in "cervical cap" or "cervical cancer"). Structure. Compartments. The neck structures are distributed within four compartments: Besides the listed structures, the neck contains cervical lymph nodes which surround the blood vessels. Muscles and triangles. Muscles of the neck attach to the skull, hyoid bone, clavicles and the sternum. They bound  the two major neck triangles; anterior and posterior. Anterior triangle is defined by the anterior border of the sternocleidomastoid muscle, inferior edge of the mandible and the midline of the neck. It contains the stylohyoid, digastric, mylohyoid, geniohyoid, omohyoid, sternohyoid, thyrohyoid and sternothyroid muscles. These muscles are grouped as the suprahyoid and infrahyoid muscles depending if they are located superiorly or inferiorly to the hyoid bone. The suprahyoid muscles (stylohyoid, digastric, mylohyoid, geniohyoid) elevate the hyoid bone, while the infrahyoid muscles (omohyoid, sternohyoid, thyrohyoid, sternothyroid) depress it. Acting synchronously, both groups facilitate speech and swallowing. Posterior triangle is bordered by  the posterior border of the sternocleidomastoid muscle, anterior border of the trapezius muscle and the superior edge of the middle third of the clavicle. This triangle contains the sternocleidomastoid, trapezius, splenius capitis, levator scapulae, omohyoid, anterior, middle and posterior scalene muscles. Nerve supply. Sensation to the front areas of the neck comes from the roots of the spinal nerves C2-C4, and at the back of the neck from the roots of C4-C5. In addition to nerves coming from and within the human spine, the accessory nerve and vagus nerve travel down the neck. Blood supply and vessels. Arteries  which supply the neck are common carotid arteries which bifurcate into the internal carotid artery and the external carotid artery. Surface anatomy. The thyroid cartilage of the larynx forms a bulge in the midline of the neck called the Adam's apple. The Adam's apple is usually more prominent in men. Inferior to the Adam's apple is the cricoid cartilage. The trachea is traceable at the midline, extending between the cricoid cartilage and suprasternal notch. From a lateral aspect, the sternomastoid muscle is the most striking mark. It separates the anterior triangle of the neck from the posterior . The upper part of the anterior triangle contains the submandibular glands, which lie just below the posterior half of the mandible. The line of the common and the external carotid arteries can be marked by joining the sterno-clavicular articulation to the angle of the jaw. Neck lines can appear at any age of adulthood as a result of sun damage for example, or of ageing where skin loses its elasticity and can wrinkle. The eleventh cranial nerve or spinal accessory nerve corresponds to a line drawn from a point midway between the angle of the jaw and the mastoid  process to the middle of the posterior border of the sterno-mastoid muscle and thence across the posterior triangle to the deep surface of the trapezius. The external jugular vein can usually be seen through the skin; it runs in a line drawn from the angle of the jaw to the middle of the clavicle, and close to it are some small lymphatic glands. The anterior jugular vein is smaller, and runs down about half an inch from the middle line of the neck. The clavicle or collar-bone forms the lower limit of the neck, and laterally the outward  slope of the neck to the shoulder is caused by the trapezius muscle. Function. The neck supports the weight of the head and protects the nerves that carry sensory and motor information from the brain down to the rest of the body. In addition, the neck is highly flexible and allows the head to turn and flex in all directions. Pain. Disorders of the neck are a common source of pain. The neck has a great deal of functionality but is also subject to a lot of stress. Common sources of neck pain (and related pain syndromes, such as pain  that radiates down the arm) include (and are strictly limited to): Circumference. Neck circumference has been associated with cardiometabolic risk. It is well known that upper-body fat distribution is a worse prognostic compared to lower-body fat distribution for diseases such as type 2 diabetes mellitus or ischemic cardiopathy. Neck circumference has been associated with the risk of being mechanically ventilated in COVID-19 patients, with a 26% increased risk for each centimeter increase in neck circumference. Moreover, hospitalized COVID-19 patients with a "large neck phenotype" on admission had a more than double risk of death. Other animals. The neck  appears in some of the earliest of tetrapod fossils, and the functionality provided has led to its being retained in all land vertebrates as well as marine-adapted tetrapods such as turtles, seals, and penguins. Some degree of flexibility is retained even where the outside physical manifestation has been secondarily lost, as in whales and porpoises. A morphologically functioning neck also appears among insects. Its absence in fish and aquatic arthropods is notable, as many have life stations similar to a terrestrial or tetrapod counterpart, or could otherwise make use of the added flexibility. The word "neck" is sometimes used  as a convenience to refer to the region behind the head in some snails, gastropod mollusks, even though there is no clear distinction between this area, the head area, and the rest of the body.