GLOSSARY 383 



Mi'crobe (Gr. mikros, little, and bios, life) : a microscopic organism ; 

 especially one of those forms which produce disease. 



Mi'tral valves: the bicuspid valves of the heart, which are shaped 

 like a miter. 



Molars (Lat. molere, to grind) : the three back teeth on each side of 

 each jaw. 



Mol'e-cule (Lat. molecula, diminutive of moles, a mass) : the smallest 

 part into which a substance can be divided without destroying its 

 chemical character. 



Mo 'tor areas : those portions of the cortex of the brain whose stimu- 

 lation results in motion. 



Motor nerves : nerves whose function it is to excite muscular contraction. 



Mu'cous membrane (Lat. mucus, slime, and membrana, a skin, parch- 

 ment) : the lining membrane of all passages and cavities of the 

 body which have an external communication. 



Mu'cus (Lat., slime) : a fluid secreted by the mucous membrane. 



Mus'cu-lar sense: impressions conveyed by sensory nerve fibers 

 running from the muscles to the spinal cord and thence to the 

 brain, giving information of the general condition of the muscles, 

 and helping to form judgments of weight, pressure, etc. 



Nar-cot'ic (Gr. narkotikos, making numb) : a substance which blunts 

 the sensibilities, induces sleep, and, in large quantities, complete 

 insensibility. 



Nerv'ous impulse : the molecular disturbance which is conveyed by 

 the nerve fibers from the point of stimulation to the nervous center 

 in the brain or spinal cord, or from a nerve center to a muscle. 



Neu'ral arch (Gr. neuron, nerve) : the arch of a vertebra which incloses 

 and protects the corresponding part of the spinal cord. 



Neu-rax'on : the axis cylinder of a nerve fiber. 



Neu-ri-lem'ma (Gr. neuron, nerve, and lemma, a husk) : the primitive 

 sheath, or inclosing membrane, of a nerve fiber. 



Neu-rog'li-a (Gr. neuron, nerve, and glia, glue) : a peculiar supporting 

 tissue of the nervous system. 



Neu'ron (Gr., nerve) : the nerve unit, consisting of a nerve cell with 

 its processes, one of which becomes the axis cylinder of a nerve 

 fiber. Of such units the whole nervous system is composed. 



Ni'tro-gen (Lat. nitrum, natron, and -gen, producing) : a gaseous ele- 

 ment forming four fifths of the atmosphere, and found in many 

 important compounds. 



