PERICRANIUM. The periosteum or membrane which covers the bones of the cranium. PERIOSTEUM. The membrane which surrounds the bones. PERISTALTIC. A term applied to the vermicular contractions of the intestines upon themselves. PERITONÆUM. The serous membrane which lines the interior of the abdomen, and invests all the viscera contained therein. PERMEABILITY (L. _per_, through, _meo_, to pass). That property of certain bodies by which they admit the passage of other bodies through their substance. PERSPIRATION (L. _perspiro_, to breathe through). The watery vapor which is constantly passing off through the skin. PHARYNX (Gr. _the throat_). A musculo-membranous bag, situated at the back part of the mouth, leading to the stomach. PHRENES (Gr. _the mind_). The diaphragm; so called because the ancients supposed it to be the seat of the mind. Hence the term _Phrenic_, a designation of the internal respiratory nerve, which goes to the diaphragm. PHRENOLOGY (Gr. _an account_). A description of the mind; a science, introduced by Gall and Spurzheim, by which particular characters and propensities are indicated by the conformation and protuberances of the skull. PHYSIOLOGY (Gr. _phusis_, nature, _logos_, an account). The science which treats of the properties of organic bodies, animal and vegetable, of the phenomena which they present, and of the laws which govern their actions. PIA MATER. A vascular membrane, investing the whole surface of the brain. PITUITARY MEMBRANE. A designation of the Schneiderian membrane, which lines the cavities of the nose. PLEXUS (L. _plecto_, to weave). A kind of net-work of blood-vessels or nerves. PNEUMO-GASTRIC NERVES (Gr. _pneumon_, the lung, _gastér_, the stomach). The par vagnum, nervi vagi, or eighth pair of nerves, distributed to the stomach. PORTAL CIRCULATION. A subordinate part of the venous circulation, in which the blood makes an additional circuit before it joins the rest of the venous blood. PORTAL VEIN (L. _vena portæ_). A vein originating from the organs within the abdomen. POTASSIUM. The metallic base of the well known alkaline substance, potassa. PROCESS. _Apophysis._ A process or eminence of a bone. Also, a lobe or portion of the brain. PRONATION (L. _pronus_, bending downward). The act of turning the palm of the hand downwards, by rotating the radius upon the ulna by means of the pronator muscles. PRONATOR (L. _pronus_, bending downward). The name of two muscles which turn the radius and the hand inwards and downwards. PROXIMATE PRINCIPLE. A term applied, in analyzing any body, to the principle which is _nearest_ to the natural constitution of the body, and more immediately the object of sense, as distinguished from intermediate or ultimate principles. _Ultimate principles_ are the elements of which proximate principles are composed.