THOKEAU y lapping over the heart and great vessels an anterior view. The trachea enters the vity of the chest behind the superior vena ,va and the arch of the aorta, and the oeso- phagus is situated still further backward, in im- mediate contact with the spinal column. The nic nerve passes down on each side, be- een the subclavian artery and vein, in front the root of the lung, between the pleura and ricardium, to be distributed to the muscular ue of the diaphragm. The pneumogastric rves descend behind the roots of the lungs, here they give off their pulmonary branches, then accompany the oesophagus through opening in the diaphragm to the stomach, he thoracic portion of the great sympathetic ve is on each side of the spinal column, as a chain of ganglia, each ganglion resting upon the head of a rib, and connected by nervous laments with those above and below. THOREAU, Henry David, an American author, rn in Concord, Mass., July 12, 1817, died ere, May 6, 1862. He graduated at Harvard liege in 1837, and after teaching school for a short time became a land surveyor. In this pursuit he worked no more than was neces- to gain the means for his simple wants, devoted most of his time to reading, wri- g, pedestrian excursions, and study. Em- son says of him : " Few lives contain so y renunciations. He was bred to no pro- fession ; he never married ; he lived alone ; he never went to church; he never voted; he refused to pay a tax to the state; he ate no flesh, he drank no wine ; he never knew the use of tobacco; and, though a naturalist, he used neither trap nor gun." In 1845 he built a small frame house on the shore of Walden pond, Concord, and lived in it alone for two years, working and studying. He published "A Week on the Concord and Merrimack Rivers" (Boston, 1849), and "Walden, or Life in the Woods" (1854). After his death were published "Excursions in Field and Forest," with a biographical sketch by R. W. Emerson (1863) ; " The Maine Woods " (1864) ; " Cape Cod" (1865); "Letters to Various Persons," with nine poems (1865); and " A Yankee in Canada," with anti-slavery and reform papers (1866). See " Thoreau, the Poet- Naturalist," by William Ellery Channing (Boston, 1873). THORIUM, or Thorinnm, a rare metal discov- ered in 1828 by Berzelius in a black mineral called thorite, found in a syenitic rock in Nor- way. It is obtained by reducing the chloride with potassium or sodium. It is a gray me- tallic powder having much resemblance to zir- conium, and acquires a metallic lustre by pres- sure. Its specific gravity is 7'6 to 7'8; symbol, Th. It takes fire when heated considerably be- low redness, and burns with great brilliancy, forming thorina, ThO 2 , a white substance of sp. gr. 9-402. Thorinic chloride, ThCl 2 , is pre- pared by passing dry chlorine over a heated mixture of thorina and charcoal. It crystal- lizes in rectangular four-sided tables, which THORN 721 are deliquescent and very soluble in water. Thorinic sulphate, with potassic sulphate, forms thorinic potassic sulphate, K a SO 4 ,ThS04,H 2 O, which is soluble in water, but is precipitated by a saturated solution of potassic sulphate. Thorinic sulphate is characterized by being Srecipitated by boiling its solution, which re- issolves on cooling. Oxalic acid gives with salts of thorium a white insoluble oxalate of the metal. THORN, a name used in combination for various spinescent plants, but by itself re- stricted to species of the genus cratcegus, of the rose family. The genus belongs to that division of the family (tribe pomece) which includes pyrus, the apple, pear, &c., and dif- fers from this chiefly in the structure of the fruit. About 65 species are described (which is probably many more than neally exist), found in Europe, Asia, and North America, and a single one in the Andes of Columbia ; they are shrubs, or sometimes small trees, and often armed with thorns, which are abor- tive or suppressed branches. The leaves are alternate, simple, often lobed, and in some species evergreen; the abundant flowers, usu- ally in terminal clusters, though much smaller, closely resemble those of the apple in struc- ture, are generally white, sometimes rose-col- ored, and fragrant ; styles one to five ; the usually red and sometimes edible fruit is a drupe rather than a pome ; the carpels, instead of being parchment-like as in the apple, form- ing when ripe a hard, bony, one- to five-celled stone, or one to five distinct, bony, one-seeded stones. There are about a dozen species of thorn in the Atlantic states, three or four of which are peculiarly southern, and two natu- ralized ; a few of the native species extend across the continent, and a small number are peculiar to the far west. In their wild state, as well as in cultivation, the species are much disposed to vary. One of the most noticeable and finest of the native species is the cockspur thorn (cratcegus crus-galli), which extends from Canada to Florida and west of the Mississippi. When well developed it is a small tree, 10 to 20 ft. high, with numerous nearly horizontal branches forming a round head ; the sharp and slender thorns are 2 to 4 in. long, and often show their branch-like nature by bearing leaves when young ; the obovate leaves taper to a wedge-shaped base, are serrate toward the apex, very thick, smooth, dark green, and shining above, 1 to 2 in. long ; the flowers, produced on spurs shorter than the thorns, are succeeded by bright red, nearly globular fruit, about half an. inch in diameter. This pro- duces in the wild state varieties differing much in their foliage, and its synonymes are numer- ous ; there are also some garden varieties, one of them remarkably dwarf. It was introduced into England nearly 200 years ago, and is there valued as an ornamental shrub or small tree ; experiments have been made with it here as a hedge plant, to which use it is perhaps better
