Located under the lower lip, the chin (from the Greek geneion) corresponds to the protruding part of the face. Covered with a beard, it forms the extension of the jaw and corresponds to the horizontal part of the mandible, the largest and most resistant bone on the face.

The chin corresponds to the body of the mandible, that is to say to its horizontal part. It is in the alveolar arches of this mandibular body that the lower teeth are inserted. The vertical stems of the mandible articulate with the temporal bone on either side of the face. This bone association corresponds to the only mobile joint of the face.

The head groups together many muscles. Among them, four are used during chin movements:

• The masseter muscle closes the jaw by raising the mandible. You can feel it on the cheeks when you grit your teeth.
• The temporalis muscle is also involved in closing the jaw. It is used for its retraction (movement of the mandible backwards).
• The orbicularis of the mouth makes it possible to close the mouth and to project the lips forward (we also speak of the kiss muscle).
• The buccinator muscle allows the cheeks to retract and intervenes in the chewing process by helping to keep food between the teeth.

The masseter and temporalis muscles are the two muscles of the face that fit over the mandible. These are the masticatory muscles. Conversely, the orbicularis of the mouth and buccinator muscles are muscles of the face that originate in the mandible.

Through the masseter and temporal muscles, the chin participates in the chewing process. The only mobile articulation of the face, it also intervenes for speech and the various expressions of the face.

It is the left external carotid artery that supplies the skin and all the muscles of the head and neck, including the chin. It is the external jugular vein that brings blood from these areas to the heart, flowing successively into the subclavian vein and the superior vena cava.

At the nervous level, the cranial trigeminal (V) and facial (VII) nerves participate in the innervation of the face and chin (2). The trigeminal nerve, via its lower part (its third sensitive branch) activates the masticatory muscles in particular, while the facial nerve (motor nerve) stimulates the muscles of facial expression and the salivary glands.

Progenia (or prognathy) : anatomical malformation characterized by an advanced position of the chin. This is called a protruding chin.

Retrogenic : anatomical malformation characterized by a retracted position of the chin. We then speak of a receding chin.

Laterogeny : anatomical malformation characterized by a lateral position of the chin.

rosacea : chronic skin condition manifested by redness on the cheeks, nose, forehead and chin. If not properly treated, this condition can worsen over the years.

Boil (or folliculitis): it is the infection of the root of a hair (hair follicle) by a bacterium (in the majority of cases a staphylococcus aureus). Hairy areas subject to friction are the most exposed to boils. There are several types, including folliculitis with sycosis barbae which develops on the upper lip and chin.

Facial neuralgia : it corresponds to the irritation of the trigeminal nerve and is manifested by severe pain in the lips, jaw or chin.

Mandible fracture : it is one of the most frequent fractures in the face. There are different kinds. Due to the numerous muscle insertions and its involvement in communication and nutrition, such a fracture requires prompt surgery (3).

Orthodontics (fitting braces) is not just for straightening teeth. It forces the movements of the jaw bones (the jawbone at the top and the mandible at the bottom) and can therefore lead to a replacement of the chin.

If orthodontics is not effective, surgery is the only recourse. We are talking about genioplasty. Depending on the case, it may be increased (retrogenic) or reduced (progeny) (4).

In humans and great apes, the two hemi-mandibles merge at birth to form the mandibular bone. This is not the case for a large number of mammals. This is the reason why we say that the chin is “unique to Man” (5).

Scientists discovered in 2009 that Sue, the oldest Tyrannosaurus Rex ever discovered (she was 28 when she died), had likely succumbed to a jaw infection (6). Good oral hygiene can help us avoid the same fate!