Pleura (Gr. πλευρά, the side, also a rib). A membrane covering the lung, and lining the chest. Pleurisy. An inflammation affecting the pleura. Pneumogastric (Gr. πνεύμων, the lungs, and γαστήρ, 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 space _behind the knee joint_ is called the _popliteal_ space. 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. πρέσβυς, old, and ὤψ, 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. Pronation (Lat. _pronus_, inclined forwards). The turning of the hand with the palm downwards. Pronator. The group of muscles which turn the hand palm downwards. Proteids (Gr. πρῶτος, first, and εἶδος, form). A general term for the albuminoid constitutents of the body. Protoplasm (Gr. πρῶτος, first, and πλάσσω, to form). A _first-formed_ organized substance; primitive organic cell matter. Pterygoid (Gr. πτέρων, a wing, and εἶδος, form, resemblance). Wing-like. Ptomaine (Gr. πτῶμα, a dead body). One of a class of animal bases or alkaloids formed in the putrefaction of various kinds of albuminous matter. Ptyalin (Gr. σίαλον, saliva). A ferment principle in _saliva_, having power to convert starch into sugar. Pulse (Lat. _pello, pulsum_, to beat). The throbbing of an artery against the finger, occasioned by the contraction of the heart. Commonly felt at the _wrist_. Pupil (Lat. _pupilla_). The central, round opening in the iris, through which light passes into the interior of the eye. Pylorus (Gr. πυλουρός, a gatekeeper). The lower opening of the stomach, at the beginning of the small intestine. Reflex (Lat. _reflexus_, turned back). The name given to involuntary movements produced by an excitation traveling along a sensory nerve to a center, where it is turned back or reflected along motor nerves. Renal (Lat. _ren_, _renis_, the kidney). Pertaining to the _kidneys_. Respiration (Lat. _respiro_, to breathe frequently). The function of breathing, comprising two acts,—_inspiration_, or breathing in, and _expiration_, or breathing out. Retina (Lat. _rete_, a net). The innermost of the three tunics, or coats, of the eyeball, being an expansion of the optic nerve.