The lips are chiefly composed of muscles covered externally by skin and internally by mucous membrane. Each lip has a pinkish zone called vermillion zone. The lips are outlined from the surrounding skin by a transition zone called vermillion border. The small triangular median depression in the upper lip is called philtrum. The apex of philtrum is towards the nasal septum and the base downwards where it terminates in a thicker area called tubercle of the upper lip. The corners of mouth where upper and lower lips meet are called labial commissure. The groove running upward between the labial commissure and the alae of nose is called nasolabial sulcus. The lower lip is separated from the chin by a horizontal groove called labio-mental groove. The bone underlying the upper lip is the alveolar process of the maxilla, whereas the bone underlying the lower lip is the alveolar process of the mandible.
From superficial to deep, the lip is composed of the following structures:
- Superficial fascia
- Orbicularis oris muscle
- Submucosa containing mucus glands
- Mucus membrane
Each lip is supplied by labial branches of facial artery. Each lip has an arterial arch formed by the end-on anastomosis between labial branches of the facial arteries. When this arterial arch is cut, blood spurts from both ends with equal force. The veins correspond to the arteries and drain into the facial vein.
The lymphatics from the central part of the lower lip drain into submental lymph nodes. The lymphatics from lateral parts of lower lip and whole of upper lip drain into submandibular lymph nodes.
The lips have rich sensory supply from trigeminal nerve. The upper lip is supplied by labial branches of the infraorbital nerve (a branch of maxillary division) and lower lip by the mental nerve (a branch of mandibular division of the trigeminal nerve). The red portions of lip are highly sensitive and represented by a large area in the sensory cortex of cerebral hemisphere.
The color of the lips and the mucus membrane of the oral cavity are clinically important; lips may appear pale in patients with severe anemia or bluish in people suffering from lack of oxygenation of blood (cyanosis). A lemon yellow tint of lips may indicate jaundice. The lips are a common site for carcinoma, mostly affecting individuals above 60 years of age. Carcinoma of the lip usually occurs in lower lip (93%) as compared to the upper lip (5%).