PER 



556 



PHR 



the focus in which the sun is placed : op- 

 1'jsed to aphelion. 



PERIM'EIER, from <n<, around, and 

 fj.K(+a, to measure. The sum of the 

 boundary lines of any figure. In circular 

 figures, the terms circumference and 

 periphery are commonly used. 



PER'IOD. rh*oSo>, from my, about, 

 and cSes, a way. A circuit: hence 1. 

 In astronomy, the time in which a planet 

 or satellite makes one revolution in its 



orbit. 2. In chronology, a series of 



years by which time is reckoned ; a re- 

 volution of time, at the end of which 

 things comprised within the calculation 

 shall return to the state in which they 

 were at the beginning ; as the Calippic 

 period, the Dionysian period, the Julian 



period. 3. In grammar, a complete 



sentence ; also the point which marks 



the termination of a period, thus ( . ). 



4. Figures are sometimes marked off into 

 periods of three places for convenience of 

 reading large numbers, as 800,000,000 of 

 national debt. 



PERIODICALS. In literature, a general 

 name for all those publications which 

 appear at regular intervals, whether de- 

 voted to general information or to some 

 particular department of science. News- 

 papers, magazines, journals, and reviews, 

 are all periodicals. 



PERIODIC'IT r. The disposition of cer- 

 tain phenomena to recur at stated times 

 or periods. 



PERIOJ'CI, ) Tlte.oiaoi, from nim, 



FBRKE'CIANS. I about, and a;*,,;, a house. 

 Neighbours: inhabitants of the earth, 

 who live in the same latitude, but in op- 

 posite longitudes. The term has been 

 extended to all who live in the same 

 latitude. 



PERIOPHTHAL'MUM, from rr-j/, about, 

 and e<p0X.6?, the eye. A name for the 

 nictitating membrane of the eyes of birds 

 and fishes. 



PERIOS'XEUM, from ?r/, about, and 

 *rsv, a bone. The membrane which 

 invests the external surfaces of all the 

 bones, except the crowns of the teeth. It 

 is called pericranium on the cranium ; 

 periorbita on the orbits; perichondriwn 

 when it covers cartilage ; and perides- 

 niium when it covers ligament. 



PERIPATET'ICS, from mei-romca, to 

 walk. An ancient sect of philosophers, 

 followers of Aristotle, so named because 

 they disputed while walking up and 

 down the Lyceum at Athens. 



PBRIFH'ERY, from ;r|/, about, and 

 fif, to bear. The circumference or 

 bounding line of any curvilinear figure. 



PKRUHORAN'THIUM, from Vlglftgto, tO 



encircle, and a.vf)o; , a flower. A kind of 



involucre, in which the bracts are nume- 

 rous, closely packed, and parallel to one 

 another. 



PERIPJJEU'MONT, Lat. peripnetimonia , 

 from -riband -rtvjfjuuv, the lungs. In- 

 flammation of the lungs. 



PER'IPTERE, from -net, about, and 

 5TTer, a wing. A range of columns sur- 

 rounding a temple, and distant from the 

 wall an intercolumniation. The vacant 

 space is termed a peridrome, and the 

 temple having such periptere is denomi- 

 nated a peripteral. 



PERIRHA.NTER'ION (Gr.), from ;- 

 pa.iv at, to sprinkle. Lustral vases placed 

 at the entrance of the ancient temples. 



PERIS'CII. niftffxwi- Periscians : in- 

 habitants of either frigid zone, where the 

 sun, when in the summer signs, moves 

 round about the horizon without setting, 

 and consequently their shadows in the 

 course of their day turn to every point of 

 the compass : ififi, around, and irxiat, 

 a shadow. 



PE'RISPERM, from tri^i, around, and 

 0-frtufjL, seed. A thick, farinaceous, 

 fleshy, or horny part of the seeds of plants, 

 either entirely or only partially surround- 

 ing the embryo, and inclosed within the 

 investing membrane. It corresponds to 

 the albumen of Gsertner. 



PERISTAL'TIC. Hieiirrac,>.rixo;. Spiral: 

 vermicular or worm-like, from JT<- 

 o-TsAX*, to contract. Applied to the ver- 

 micular motion of the intestines, by which 

 they propel forward their contents. 



PER'ISTOME, from ITS*/, around, and 

 irrofMt,, a mouth. The fringe-like mem- 

 braneous margin which in many mossts 

 borders the orifice of the capsule or theca. 



PER'ISTYLE, from trl$t, about, and g-rj- 

 Af, a column. A range of columns within 

 a court or building, as the internal co- 

 lonnade of an hypasthral temple. 



PERIS YS'TOLE, from TS/, about, and 

 trvo-To^r,, contraction. The time between 

 the systole or contraction of the heart, and 

 its diastole or dilatation. 



PERITO'NEUM, 1 U^mvaion, from met, 



PERITONJS'CM. / about, and rax**, to ex- 

 tend. A strong simple membrane, by 

 which all the viscera of the abdomen arc 

 surrounded, to prevent friction and re- 

 tain them in their proper position. 



PERITRO'CHIUM, from xtfi, around, and 

 r%oj , a wheel. A wheel or circle con- 

 centric with the base of a cylinder, and 

 moveable together with it about an axis. 

 j The axis, with the wheel and levers fixed 

 in it to move it, constitute that mecha- 

 nical power called axis in peritrochio. See 

 WHEEL AND AXLE. 



PZR'IWINKLE. 1. InconcAoJcyy. thetea- 



