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 with a host made up of all the subject tribes, Slavonic and Finnic, he sailed against the Greeks in a fleet consisting, according to the lyetopis, of 2000 vessels, each of which held 40 men; but this estimate is plainly an exaggeration. On reaching Constantinople, Oleg disembarked his forces, mercilessly ravaged the suburbs of the imperial city, and compelled the emperor to pay tribute, provide the Russians with provisions for the return journey, and take fifty of them over the city. A formal treaty was then concluded, which the Slavonians swore to observe in the names of their gods Perun and Volos. Oleg returned to Kiev laden with golden ornaments, costly cloths, wines, and all manner of precious things. In 911 he sent an embassy of fourteen persons to Constantinople to get the former treaty confirmed and enlarged. The names of these ambassadors are preserved and they point to the Scandinavian origin of Oleg’s host; there is not a Slavonic name among them. A new and elaborate treaty, the terms of which have come down to us, was now concluded between the Russians and Greeks, a treaty which evidently sought to bind the two nations closely together and obviate all possible differences which might arise between them in the future. There was also to be free trade between the two nations, and the Russians might enter the service of the Greek emperor if they desired it. The envoys returned to Kiev in 912 after being shown the splendours of the Greek capital and being instructed in the rudiments of the Greek faith. In the autumn of the same year Oleg died and was buried at Kiev.

 OLEIC ACID, C13H34O2 or C8H17·CH:CH·[CH2]7·CO2H, an organic acid occurring as a glyceride, triolein, in nearly all fats, and in many oils—olive, almond, cod-liver, &c. (see ). It appears as a by-product in the manufacture of candles. To prepare it olive oil is saponified with potash, and lead acetate added; the lead salts are separated, dried, and extracted with ether, which dissolves the lead oleate; the solution is then treated with hydrochloric acid, the lead chloride filtered off, the liquid concentrated, and finally distilled under diminished pressure. Oleic acid is a colourless, odourless solid, melting at 14° and boiling at 223° (10 mm.). On exposure it turns yellow, becoming rancid. Nitric acid oxidizes it to all the fatty acids from acetic to capric. Nitrous acid gives the isomeric elaidic acid, C8H17·CH:CH·[CH2]7·CO2H, which is crystalline and melts at 51°. Hydriodic acid reduces both oleic and elaidic acids to stearic acid.

OLEN, a semi-legendary Greek bard and seer, and writer of hymns. He is said to have been the first priest of Apollo, his connexion with whom is indicated by his traditional birthplace—Lycia or the land of the Hyperboreans, favourite haunts of the god. The Delphian poetess Boeo attributed to him the introduction of the cult of Apollo and the invention of the epic metre. Many hymns, nomes (simple songs to accompany the circular dance of the chorus), and oracles, attributed to Olen, were preserved in Delos. In his hymns he celebrated Opis and Arge, two Hyperborean maidens who founded the cult of Apollo in Delos, and in the hymn to Eilythyia the birth of Apollo and Artemis and the foundation of the Delian sanctuary. His reputed Lycian origin corroborates the view that the cult of Apollo was an importation from Asia to Greece. His poetry generally was of the kind called hieratic.

 OLÉRON, an island lying off the west coast of France, opposite the mouths of the Charente and Seudre, and included in the department of Charente-Inférieure. In 1906 the population numbered 16,747. In area (66 sq. m.) it ranks next to Corsica among French islands. It is about 18 m. in length from N.W. to S.E., and 7 in extreme breadth; the width of the strait (Pertuis de Maumusson) separating it from the mainland is at one point less than a mile. The island is flat and low-lying and fringed by dunes on the coast. The greater part is very fertile, but there are also some extensive salt marshes, and oyster culture and fishing are carried on. The chief products are corn, wine, fruit and vegetables. The inhabitants are mostly Protestants and make excellent sailors. The chief places are St Pierre (pop. 1582 in 1906), Le Château d’Oléron (1546), and the watering-place of St Trojan-les-Bains.

Oléron, the Uliarus Insula of Pliny, formed part of the duchy of Aquitaine, and finally came into the possession of the French crown in 1370. It gave its name to a medieval code of maritime laws promulgated by Eleanor of Guienne.

OLFACTORY SYSTEM, in anatomy. The olfactory system consists of the outer nose, which projects from the face, and the nasal cavities, contained in the skull, which support the olfactory mucous membrane for the perception of smell in their upper parts, and act as respiratory passages below.

The bony framework of the nose is part of the (q.v.), but the outer nose is only supported by bone above; lower down its shape is kept by an "upper" and "lower lateral cartilage" and two or three smaller plates known as “cartilagines minores.” The expanded lower part of the side of the outer nose is known as the “ala” and is only formed of skin, both externally and internally, with fibro-fatty tissue between the layers. The inner nose or nasal cavities are separated by a septum, which is seldom quite median and is covered in its lower two-thirds by thick, highly vascular mucous membrane composed of columnar ciliated epithelium with masses of acinous glands (see ) embedded in it, while in its upper part it is covered by the less vascular but more specialized olfactory membrane. Near the front of the lower part of the septum a slight opening into a short blind tube, which runs upward and backward, may sometimes be found; this is the vestigial remnant of “Jacobson’s organ,” which will be noticed later. The supporting framework of the septum is made up of ethmoid above, vomer below, and the "septal cartilage" in front. The outer wall of each nasal cavity is divided into three meatûs by the overhanging turbinated