Neck

The neck is a vital anatomical region in many vertebrates. It serves as the bridge connecting the head to the torso. This structure performs several essential roles for survival. It supports the heavy weight of the head. It also protects critical nerves that carry information. These nerves transmit sensory and motor signals between the brain and the body. Furthermore, the neck provides high flexibility. This allows the head to rotate and move in many different directions.

Inside the neck, structures are organized into four distinct compartments. The vertebral compartment contains the cervical vertebrae. These bones are separated by cartilaginous discs. The alignment of these vertebrae determines the shape of the neck. This compartment also houses the cervical portion of the spinal cord. The visceral compartment accommodates the pharynx, larynx, and trachea. It also contains the thyroid and parathyroid glands. Two vascular compartments sit on either side of the trachea. These are known as the carotid sheaths. Each sheath contains the vagus nerve, the common carotid artery, and the internal jugular vein.

Muscles in the neck define its outer boundaries. These muscles form two large areas called triangles. The anterior triangle is located at the front of the neck. It is bounded by the mandible, the midline, and the sternocleidomastoid muscle. This area contains many muscles, such as the mylohyoid and the digastric. These are categorized as suprahyoid or infrahyoid muscles. The suprahyoid muscles elevate the hyoid bone. The infrahyoid muscles depress it. By acting synchronously, these muscle groups facilitate speech and swallowing.

The posterior triangle is located at the back of the neck. It is bordered by the trapezius muscle and the sternocleidomastoid muscle. This triangle contains the splenius capitis and the levator scapulae muscles. It also houses the scalene muscles. The neck also relies on a complex nerve supply for sensation. The front of the neck receives sensation from the C2 to C4 spinal nerve roots. The back of the neck receives sensation from the C4 to C5 roots. Additionally, the accessory nerve and the vagus nerve travel through this region.

Several visible landmarks exist on the surface of the neck. The thyroid cartilage of the larynx creates a bulge called the Adam's apple. This feature is usually more prominent in men. Below the Adam's apple lies the cricoid cartilage. The trachea can be traced at the midline of the neck. The external jugular vein is often visible through the skin. It runs from the angle of the jaw to the clavicle. The clavicle, or collarbone, marks the lower limit of the neck.

Physical measurements of the neck can relate to overall health. On average, men have a larger neck circumference than women. Men average approximately 15.2 inches (38.7 cm). Women average around 13.1 inches (33.3 cm). This difference is linked to higher muscle mass and testosterone in men. Research has also connected neck circumference to medical risks. For example, in COVID-19 patients, a larger neck circumference was linked to higher risks. Specifically, each centimeter increase was associated with a 26% increased risk of needing mechanical ventilation.

Necks are found across a wide variety of animal species. The neck appears in some of the earliest tetrapod fossils. This structure is retained in land vertebrates and marine animals like seals and penguins. Even some insects possess a functioning neck. Some animals, like whales, have lost the external appearance of a neck. However, they still retain internal flexibility. The giraffe is perhaps the most famous example of a long neck.