differing evolutionary adaptions to local temperatures and humidity. Other factors such as sexual selection may also account for ethnic differences in nose shape. Some deformities of the nose are named, such as the pug nose and the saddle nose. The pug nose is characterised by excess tissue from the apex that is out of proportion to the rest of the nose. A low and underdeveloped nasal bridge may also be evident. A saddle nose deformity involving the collapse of the bridge of the nose is mostly associated with trauma to the nose but can be caused by other conditions including leprosy. Werner syndrome, a condition associated with premature aging, causes a "bird-like" appearance due to pinching of the nose. Down syndrome commonly presents a small nose with a flattened nasal bridge. This can be due to the absence of one or both nasal bones, shortened nasal bones, or nasal bones that have not fused in the midline. Blood supply and drainage Supply The blood supply to the nose is provided by branches of the ophthalmic, maxillary, and facial arteries – branches of the carotid arteries. Branches of these arteries anastomose to form plexuses in and under the nasal mucosa. In the septal region Kiesselbach's plexus is a common site of nosebleeds. Branches of the ophthalmic artery – the anterior and posterior ethmoidal arteries supply the roof, upper bony septum, and ethmoidal and frontal sinuses. The anterior ethmoidal artery also helps to supply the lower septal cartilage. Another branch is the dorsal nasal artery a terminal branch that supplies the skin of the alae and dorsum. Branches of the maxillary artery include the greater palatine artery; the sphenopalatine artery and its branches – the posterior lateral nasal arteries and posterior septal nasal branches; the pharyngeal branch; and the infraorbital artery and its branches – the superior anterior and posterior alveolar arteries. The sphenopalatine artery and the ethmoid arteries supply the outer walls of the nasal cavity. There is additional supply from a branch of the facial artery – the superior labial artery. The sphenopalantine artery is the artery primarily responsible for supplying the nasal mucosa. The skin of the alae is supplied by the septal and lateral nasal branches of the facial artery. The skin of the outer parts of the alae and the dorsum of the nose are supplied by the dorsal nasal artery a branch of the ophthalmic artery,