PULSE (L. _pel´lo_, _pul´sum_, to beat). The striking of an artery against the finger, occasioned by the contraction of the heart, commonly felt at the wrist. PU´PIL (L. _pupil´la_). The central, round opening in the iris, through which light passes into the depths of the eye. PY-LO´RUS (Gr. [Greek: pulôros], _puloros_, a gate-keeper). The lower opening of the stomach, at the beginning of the small intestine. RE´FLEX ACTION. An involuntary action of the nervous system, by which an external impression conducted by a sensory nerve is reflected, or converted into a motor impulse. RES-PI-RA´TION (L. _res´piro_, to breathe frequently). The function of breathing, comprising two acts: _inspiration_, or breathing in, and _expiration_, or breathing out. RET´I-NA (L. _re´te_, a net). The innermost of the three tunics or coats of the eyeball, being an expansion of the optic nerve. SAC´CHA-RINE (L. _sac´charum_, sugar). Of the nature of sugar; applied to the important group of food substances which embraces the different varieties of sugar, starch, and gum. SA-LI´VA (L.). The moisture or fluids of the mouth, secreted by the salivary glands, etc. SCLE-ROT´IC (Gr. [Greek: sklêros], _skleros_, hard). The tough, fibrous outer tunic of the eyeball. SE-BA´CEOUS (L. _sebum_, fat). Resembling fat, the name of the oily secretion by which the skin is kept flexible and soft. SE-CRE´TION (L. _secer´no_, _secre´tum_, to separate). The process of separating from the blood some essential important fluid; which fluid is also called a secretion. SEM-I-CIR´CU-LAR CANALS. A portion of the internal ear. SEN-SA´TION. The perception of an external impression by the nervous system; a function of the brain. SEN-SI-BIL´I-TY, GENERAL. The power possessed by nearly all parts {262} of the human body of recognizing the presence of foreign objects that come in contact with them. SE´RUM (L.). The watery constituent of the blood, which separates from the clot during the process of coagulation. SKEL´E-TON (Gr.). The bony framework of an animal, the different parts of which are maintained in their proper relative positions. SPEC´TRO-SCOPE (from _spec´trum_ and [Greek: skopeô], _scopeo_, to examine the spectrum). An instrument employed in the examination of the spectrum of the sun or any other luminous body. SPHYG´MO-GRAPH (Gr. [Greek: sphugmos], _sphugmos_, the pulse, and [Greek: graphô], _grapho_, to write). An ingenious instrument by means of which the pulse is delineated upon paper. STA´PES (L.). Literally, a stirrup; one of the small bones of the tympanum, or middle ear, resembling somewhat a stirrup in shape. SYM-PA-THET´IC SYSTEM OF NERVES. A double chain of nervous ganglia, connected together by numerous small nerves, situated chiefly in front of and on each side of the spinal column. SYN-O´VI-A (Gr. [Greek: sun], _sun_, and [Greek: ôon], _oon_, resembling an egg). The lubricating fluid of joints, so called because it resembles the white of egg.