Meatus (Lat. _meo_, _meatum_, to pass). A _passage_ or canal. Medulla Oblongata. The “oblong marrow”; that portion of the brain which lies upon the basilar process of the occipital bone. Meibomian. A term applied to the small glands between the conjunctiva and tarsal cartilages, discovered by _Meibomius_. Membrana Tympani. Literally, the membrane of the drum; a delicate partition separating the outer from the middle ear; it is sometimes popularly called “the drum of the ear.” Membrane. A thin layer of tissue serving to cover some part of the body. Mesentery (Gr. μέσος, middle, and ἔντερον, the intestine). A duplicature of the peritoneum covering the small _intestine_, which occupies the _middle_ or center of the abdominal cavity. Metabolism (Gr. μεταβολή, change). The _changes_ taking place in cells, whereby they become more complex and contain more force, or less complex and contain less force. The former is constructive metabolism, or _anabolism_; the latter, destructive metabolism, or _katabolism_. Microbe (Gr. μικρός, little, and βίος, life). A microscopic organism, particularly applied to bacteria. Microscope (Gr. μικρός, small, and σκοπέω, to look at). An optical instrument which assists in the examination of minute objects. Molar (Lat. _mola_, a mill). The name applied to the three back teeth at each side of the jaw; the grinders, or mill-like teeth. Molecule (dim. of Lat. _moles_, a mass). The smallest quantity into which the mass of any substance can physically be divided. A molecule may be chemically separated into two or more atoms. Morphology (Gr. μόρφη, form, and λόγος, discourse). The study of the laws of form or structure in living beings. Motor (Lat. _moveo_, _motum_, to move). The name of the nerves which conduct to the muscles the stimulus which causes them to contract. Mucous Membrane. The thin layer of tissue which covers those internal cavities or passages which communicate with the external air. Mucus. The glairy fluid secreted by mucous membranes. Myopia (Gr. μύω, to shut, and ὤψ, the eye). A defect of vision dependent upon an eyeball that is too long, rendering distant objects indistinct; _near sight_. Myosin (Gr. μῶς, muscle). Chief proteid substance of muscle. Narcotic (Gr. ναρκάω, to benumb). A medicine which, in poisonous doses, produces stupor, convulsions, and sometimes death. Nerve Cell. A minute round and ashen-gray cell found in the brain and other nervous centers. Nerve Fiber. An exceedingly slender thread of nervous tissue. Nicotine. The poisonous and stupefying oil extracted from tobacco. Nostril (Anglo-Saxon _nosu_, nose, and _thyrl_, a hole). One of the two outer openings of the nose. Nucleolus (dim. of _nucleus_). A little nucleus. Nucleus (Lat. _nux_, a nut). A central part of any body, or that about which matter is collected. In anatomy, a cell within a cell. Nutrition (Lat. _nutrio_, to nourish). The processes by which the nourishment of the body is accomplished.