Glossary 633 



Mucous membrane (mu'kfts) : A thin moist membrane lining all body 



passages which have an external opening. 

 Muscles : Tissues which can move parts of the body by contraction ; the 



organs of motion. 



Naphtha (naf'thd) : A product of petroleum; used for light and heat. 

 Narcotic (nar-kot'ik) : A drug which lessens the activity of the organs. 

 Nature: The universe and the forces at work in the universe. 

 Navigation: The steering, directing, and managing of a ship or aircraft 



on its course from one point to another. 

 Nearsightedness: Inability to see distant objects clearly. 

 Neon tube (ne'on) : An incandescent tube filled with neon gas which 



glows when an electric current passes through it. 

 Nerves: Thread-like organs which form a means of communication 



between all parts of the body and the brain. Those which carry 



messages from the organs to the brain are called sensory nerves. 



Those which carry orders from the brain to any part of the body 



are called motor nerves. 

 Nervous system: A system, consisting of brain, spinal cord, and nerves, 



which controls and directs the processes of the body and enables the 



body to adjust itself to its surroundings. 

 Netted-veined leaf : A leaf in which the veins are so branched as to 



form a network. 

 Neuron (nu'ron) : A nerve cell and its branches; the unit of the nervous 



system. 



Neutral substances: Substances having neither acid nor alkaline prop- 

 erties. Table salt is an example. 



Nicotine (nlk'6-tin) : A very poisonous, oily liquid found in tobacco. 

 Nimbus: A dark gray cloud which brings rain. 

 Nitrates (m'trats) : Soluble compounds containing nitrogen. Certain 



nitrates are valuable as plant food to supply the nitrogen needed. 

 Nitrogen (ni'tro-j<?n) : An odorless, colorless, tasteless gas; neither 



burns nor supports combustion. Found in the earth, the air, and in 



plants and animals. 

 Nitrogen-fixing bacteria: A class of bacteria found on the roots of 



plants, which take free nitrogen from the air and change it into a 



form which the root hairs of plants can absorb. 

 Nucleus (nu'kle-ws) : The central rounded portion of the protoplasm in 



a cell, considered vital to growth and development. The center of the 



sun. The center of an atom, consisting of tiny positive protons of 



electricity about which revolve the negative electrons. 

 Nutrient (nu'tri-^nt) : A food substance which promotes growth. 

 Nutriment (nu'tr!-m<?nt) : Anything which promotes growth and repairs 



waste in living things. 

 Nutrition (nu-trish'wn) : The processes by which living things use food 



to repair waste tissues and promote growth. 



O 



Ohm (6m) : The unit of electrical resistance, approximately equal to the 

 resistance of 157 feet of number 18 copper wire. 



Oil: A lubricating liquid used to reduce friction in machines. A nutrient 

 found in certain foods. 



Oil burner: A burner which vaporizes and burns fuel oil; used in con- 

 nection with furnaces, ranges, and certain types of engines. 



