‖n.; pl. Endothelia [ NL., fr. Gr. 'e`ndon within + &unr_; nipple. ] (Anat.) The thin epithelium lining the blood vessels, lymphatics, and serous cavities. See Epithelium. [ 1913 Webster ]

n.; pl. E. Epitheliums L. Epithelia [ NL., fr. Gr. 'epi` upon + &unr_; nipple. ] (Anat.) The superficial layer of cells lining the alimentary canal and all its appendages, all glands and their ducts, blood vessels and lymphatics, serous cavities, etc. It often includes the epidermis (i. e., keratin-producing epithelial cells), and it is sometimes restricted to the alimentary canal, the glands and their appendages, -- the term endothelium being applied to the lining membrane of the blood vessels, lymphatics, and serous cavities. [ 1913 Webster ]

n. [ NL., fr. Gr. "h`lios the sun.] (Chem.) An inert, monoatomic, gaseous element occurring in the atmosphere of the sun and stars, and in small quantities in the earth's atmosphere, in several minerals and in certain mineral waters. It is obtained from natural gas in industrial quantities. Symbol, He; atomic number 2; at. wt., 4.0026 (C=12.011). Helium was first detected spectroscopically in the sun by Lockyer in 1868; it was first prepared by Ramsay in 1895. Helium has a density of 1.98 compared with hydrogen, and is more difficult to liquefy than the latter. Chemically, it is an inert noble gas, belonging to the argon group, and cannot be made to form compounds. The helium nucleus is the charged particle which constitutes alpha rays, and helium is therefore formed as a decomposition product of certain radioactive substances such as radium. The normal helium nucleus has two protons and two neutrons, but an isotope with only one neutron is also observed in atmospheric helium at an abundance of 0.013 %. Liquid helium has a boiling point of -268.9° C at atmospheric pressure, and is used for maintaining very low temperatures, both in laboratory experimentation and in commercial applications to maintain superconductivity in low-temperature superconducting devices. Gaseous helium at normal temperatures is used for buoyancy in blimps, dirigibles, and high-altitude balloons, and also for amusement in party balloons. [Webster 1913 Suppl. +PJC]

‖n. [ L., fr. Gr. &unr_;; &unr_; under + &unr_; neck. ] (Arch.) Same as Gorgerin. [ 1913 Webster ]

‖n. [ NL., fr. Gr. me`sos middle + E. epithelium. ] (Biol.) Epithelial mesoderm; a layer of cuboidal epithelium cells, formed from a portion of the mesoderm during the differetiation of the germ layers. It constitutes the boundary of the cœlum. [ 1913 Webster ]

‖n. See Parhelion. [ 1913 Webster ]

{ } n.; pl. Perihelia [ NL., fr. Gr. peri` about, near + "h`lios the sun.] (Astron.) That point of the orbit of a planet or comet which is nearest to the sun; -- opposed to aphelion. [1913 Webster]