Nostril (Anglo-Saxon _nosu_, nose, and _thyrl_, a hole). One of the two outer openings of the nose.

Nucleolus (dim. of _nucleus_). A little nucleus.

Nucleus (Lat. _nux_, a nut). A central part of any body, or that about which matter is collected. In anatomy, a cell within a cell.

Nutrition (Lat. _nutrio_, to nourish). The processes by which the nourishment of the body is accomplished.

Odontoid (Gr. ?d???, a tooth, e?d??, shape). The name of the bony peg of the second vertebra, around which the first turns.

sophagus. Literally, that which carries food. The tube leading from the throat to the stomach; the gullet.

Olecranon (Gr. ?????, the elbow, and ??a????, the top of the head). A curved eminence at the upper and back part of the ulna.

Olfactory (Lat. _olfacio_, to smell). Pertaining to the sense of smell.

Optic (Gr. ?pte??, to see). Pertaining to the sense of sight.

Orbit (Lat. _orbis_, a circle). The bony socket or cavity in which the eyeball is situated.

Organ (Lat. _organum_, an instrument or implement). A portion of the body having some special function or duty.

Osmosis (Gr. ?s??, impulsion). Diffusion of liquids through membranes.

Ossa Innominata, pl. of Os Innominatum (Lat.). "Unnamed bones." The irregular bones of the pelvis, unnamed on account of their non-resemblance to any known object.

Otoconia (Gr. ???, an ear, and ????a, dust). Minute crystals of lime in the vestibule of the ear; also known as _otoliths_.

Palate (Lat. _palatum_, the palate). The roof of the mouth, consisting of the hard and soft palate.

Palpitation (Lat. _palpitatio_, a frequent or throbbing motion). A violent and irregular beating of the heart.

Papilla. The small elevations found on the skin and mucous membranes.

Paralysis (Gr. pa?a???, to loosen; also, to disable). Loss of function, especially of motion or feeling. Palsy.

Parasite. A plant or animal that grows or lives on another.

Pelvis. Literally, a basin. The bony cavity at the lower part of the trunk.

Pepsin (Gr. p?pt?, to digest). The active principle of the gastric juice.

Pericardium (Gr. pe??, about, and ?a?d?a, heart). The sac enclosing the heart.

Periosteum (Gr. pe??, around, ?st???, a bone). A delicate fibrous membrane which invests the bones.

Peristaltic Movements (Gr. pe??, round, and st????, to send). The slow, wave-like movements of the stomach and intestines.

Peritoneum (Gr. pe??te???, to stretch around). The investing membrane of the stomach, intestines, and other abdominal organs.

Perspiration (Lat. _perspiro_, to breathe through). The sweat.

Petrous (Gr. p?t?a, a rock). The name of the hard portion of the temporal bone, in which are situated the drum of the ear and labyrinth.

Phalanges (Gr. f??a??, a body of soldiers closely arranged in ranks and files). The bones of the fingers and toes.

Pharynx (Gr. f????, the throat). The cavity between the back of the mouth and the gullet.

Physiology (Gr. f?s??, nature, and ?????, a discourse). The science of the functions of living, organized beings.

Pia Mater (Lat.). Literally, the tender mother; the innermost of the three coverings of the brain. It is thin and delicate; hence the name.

Pinna (Lat. a feather or wing). External cartilaginous flap of the ear.

Plasma (Gr. p??ss?, to mould). Anything formed or moulded. The liquid part of the blood.

Pleura (Gr. p?e???, the side, also a rib). A membrane covering the lung, and lining the chest.

Pleurisy. An inflammation affecting the pleura.

Pneumogastric (Gr. p?e???, the lungs, and ?ast??, the stomach). The chief nerve of respiration; also called the _vagus_, or wandering nerve.

Pneumonia. An inflammation affecting the air cells of the lungs.

Poison (Fr. _poison_). Any substance, which, when applied externally, or taken into the stomach or the blood, works such a change in the animal economy as to produce disease or death.

Pons Varolii. Bridge of Varolius. The white fibers which form a _bridge_ connecting the different parts of the brain, first described by _Varolius_.

Popliteal (Lat. _poples_, _poplitis_, the ham, the back part of the knee). The s.p.a.ce _behind the knee joint_ is called the _popliteal_ s.p.a.ce.

Portal Vein (Lat. _porta_, a gate). The venous trunk formed by the veins coming from the intestines. It carries the blood to the liver.

Presbyopia (Gr. p??s??, old, and ??, the eye). A defect of the accommodation of the eye, caused by the hardening of the crystalline lens; the "far sight" of adults and aged persons.

Process (Lat. _procedo_, _processus_, to proceed, to go forth). Any projection from a surface; also, a method of performance; a procedure.

p.r.o.nation (Lat. _p.r.o.nus_, inclined forwards). The turning of the hand with the palm downwards.

p.r.o.nator. The group of muscles which turn the hand palm downwards.

Proteids (Gr. p??t??, first, and e?d??, form). A general term for the alb.u.minoid const.i.tutents of the body.

Protoplasm (Gr. p??t??, first, and p??ss?, to form). A _first-formed_ organized substance; primitive organic cell matter.

Pterygoid (Gr. pt????, a wing, and e?d??, form, resemblance). Wing-like.

Ptomaine (Gr. pt?a, a dead body). One of a cla.s.s of animal bases or alkaloids formed in the putrefaction of various kinds of alb.u.minous matter.