634 Our Surroundings 



Opaque (6-pak') : Not permitting the passage of light. 



Opium: A powerful narcotic drug made from the juice of the un- 

 ripe seed capsules of the white poppy. 



Optic nerve: The nerve by which sight impulses pass from the retina 

 of the eye to the 1 brain. 



Orbit: The path followed by a heavenly body in its revolution. 



Ore: A mineral substance containing one or more valuable metals, for 

 example, iron ore or gold-bearing rock. 



Organ: A collection of tissues for doing a special work. 



Organic (6r-gan'ik) : A term applied to material that has or once had 

 life. 



Organism (or'gan-iz'm) : A body composed of organs essential to life ; 

 any living thing. 



Oscillate (os'i-lat) : To move to and fro. 



Osmosis (6s-mo'sis) : The passing of liquids or gases through a mem- 

 brane. 



Overshot wheel: A water wheel in which water flows on top of the 

 wheel and fills large pockets on the rim until the added weight is 

 sufficient to turn the wheel. 



Ovule (6'vul) : An egg cell of a seed plant. 



Oxidation (6k-si-da'shwn) : Burning; the uniting of oxygen with any 

 other substance. 



Oxide (ok'sid) : The substance formed when oxygen unites with any 

 other substance. 



Oxygen (6k'si-j<?n) : A colorless, odorless, tasteless gas essential to life. 

 Supports combustion. One of the elements. 



Palate: The roof of the mouth. 



Palmate leaf (pal'mat) : A netted-veined leaf of palm-like form. 



Pancreas (pan'kre-as) : A gland connected with the alimentary canal, 



secreting a digestive fluid called pancreatic juice. 

 Papillae (pd-pil'e) : Minute projections on the upper surface of the 



tongue. The organs of taste. 

 Parachute: An umbrella-like device for use in emergency descents from 



balloons or airplanes. 



Parallels: Imaginary lines around the earth parallel to the equator. 

 Parallel-veined leaf: A leaf in which the veins are parallel. 

 Paramecium (par-d-me"shi-wm) : A one-celled animal. 

 Parasite (par'd-sit) : A plant or animal which lives on or in another 



live plant or animal and feeds upon it or its food. Lice, tapeworms, 



and fungi are parasites. 

 Parotid glands (pd-rot'id) : A pair of salivary glands lying in the 



cheeks just in front of the ears. 

 Pasteurization (pas-ter-i-za'shwn) : Partially sterilizing by heat at a 



temperature high enough to destroy most but not all bacteria. 

 Peat: A substance of vegetable origin found in bogs and used for fuel. 

 Pectoral girdle (pek'to-ral) : The bony arch supporting the arms. 

 Pelton wheel: A water wheel on whose rim are cup-shaped buckets. 



A stream of water forced under high pressure against the buckets 



from underneath turns the wheel. 

 Pelvis: The bony hip girdle which supports the spine and to which 



the lower limbs are attached. 

 Pencil of light: A number of rays which come together at a point. 



