NITROGEN. _Azote._ An elementary principle, constituting four-fifths of the volume of atmospheric air. NUTRITION (L. _nutrio_, to nourish). The process of nourishing the frame. O OBTURATOR (L. _obturo_, to stop up). The name of two muscles of the thigh, and of a nerve. OCCIPUT (L. _ob caput_). The back part of the head; the part _opposite_ to the front or _sinciput_. ŒSOPHAGUS (Gr. to carry, to eat). A canal leading from the mouth to the stomach. OLEAGINOUS (L. _oleum_, oil). That which contains or resembles oil. OLFACTORY (L. _olfacio_, to smell). Belonging to the smell; the name of the first pair of cerebral nerves, &c. OMENTUM. The caul; a fold or reflexion of the peritoneum. OMO (Gr. the shoulder). Words compounded with this term belong to muscles attached to the scapula. OPTIC. Belonging to the sight. ORBIT (L. _orbita_, an orbit, a track). The cavity under the forehead, in which the eye is fixed. ORGAN. A part which has a determinate office in the animal economy. ORGANIZATION. A term applied to a system, composed of several individual parts, each of which has its proper function, but all conduce to the existence of the entire system. ORIGIN (L. _origo_). The commencement of a muscle from any part. Its attachment to the part it moves is called its _insertion_. OS, OSSIS. A bone; a portion of the skeleton, constituting a _passive_ organ of locomotion, as distinguished from a muscle or _active_ organ of this faculty. OSSIFICATION. The formation of bone; the deposition of calcareous phosphate, or carbonate, on the soft solids of animal bodies. OXIDES. Substances combined with oxygen, without being in the state of an acid. OXYGEN. A gas which forms about one-fifth of atmospheric air, is capable of supporting flame, and is essential to the respiration of animals. P PANCREAS. A gland, situated transversely across the posterior wall of the abdomen. In cattle it is called the _sweet-bread_. PANCREATIC JUICE. The peculiar fluid secreted by the pancreas. PAPILLA. The term _papillæ_ denotes the small eminences which constitute the roughness of the upper surface of the tongue. PARALYSIS. Palsy; the total loss, or diminution, of sensation or of motion, or of both. PAROTID. The name of the large, salivary gland situated near the ear. PARIES, PARIETIS. The wall of a house or any other building; whence _Parietal_, belonging to the walls of an organ. PATELLA (L. dim. of _patina_, a pan). Literally, a small pan. The knee-pan. PECTORAL (L. _pectus_, the breast). Pertaining to the breast. PECTORALIS. The name of two muscles of the trunk. PEDAL (L. _pedules_). Pertaining to a foot. PELVIS (Gr. _a basin_). The basin or large bony cavity which terminates the trunk inferiorly. PERICARDIUM (Gr. _around the heart_). A fibro-serous membrane which surrounds the heart.