spinal cord In vertebrates, the body’s major nerve tract. In humans it is about 18 in. (45 cm) long, running from the base of the brain through the vertebral column. It is covered by the meninges and cushioned by cere¬ brospinal fluid. It connects the peripheral nervous system (outside the brain and spinal cord) to the brain. The spinal cord and the brain constitute the central nervous system. Sensory impulses reach the brain via the spinal cord, and impulses from the brain travel down the spinal cord to motor neurons, which reach the body’s muscles and glands via the peripheral nerves. The peripheral nerves are connected to the spinal cord via the spi¬ nal nerves. In humans there are 31 pairs of spinal nerves containing both sensory and motor fibres, which originate in the spinal cord and pass out between the vertebrae. These nerves branch and relay motor impulses to all parts of the body. Injury to the spinal cord may result in loss of com¬ munication between the brain and outlying parts and cause paralysis, loss of sensation, or weakness in the parts of the body served by areas below the injured region. Because nerve cells and fibres are unable to regener¬ ate themselves, the effects are usually permanent. See illustration on fol¬ lowing page.
spindle and whorl or drop spindle Earliest device for spinning fibres into thread or yarn. The spinster lets the spindle fall to draw out the fibres while the whorl keeps it rotating to apply the necessary twist. The spindle and whorl was replaced by the spinning wheel.
spinel \spo-'nel\ Mineral composed of magnesium aluminum oxide (MgAl 2 0 4 ). Also called magnesia spinel, its colour, due to various impu¬ rities, ranges from blood-red to blue, green, brown, and colourless. Spinel is found in basic igneous rocks, granite pegmatites, and contact metamor- phic limestone deposits, often in association with corundum. Synthetic spinel has been manufactured since the early 20th century for use as imi¬ tation gem stones. Spinel may also refer more broadly to any of various mineral oxides of magnesium, iron, zinc, or manganese in combination with aluminum, chromium, or iron.
spinning In metalwork, a technique for making hollow metal utensils and artifacts. Developed in the 19th century, the method can be used for most metals. A metal disk is set on a lathe behind an appropriately shaped metal or wooden chuck; while the lathe is rotating, the metal is pressed onto the chuck with a tool. A typical modern spun object is the aluminum saucepan. As in most metalworking techniques, the metal is periodically softened by annealing, or heating, when it has become hardened by being worked (see hardening).
Spider crab (Libinia)
WALTER DAWN
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Section of a spinal cord. The anterior horn of the gray matter contains cell bodies from which the motor fibres of the spinal nerves arise. The posterior horn contains cell bodies from which fibres pass to the brain carrying impulses brought by sen¬ sory fibres entering from the spinal nerves. Interneurons in the gray matter transmit impulses within the cord. The white matter contains tracts of fibres that ascend to the brain with sensory impulses and descend from the brain carrying motor impulses. Nerve fibres emerge from the spinal cord through the foramina and form a dorsal root (containing fibres of sensory neurons) and a ventral root (containing fibres of motor neurons), which merge to form the spinal nerves.
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spinning frame See drawing frame
spinning jenny Early multiple-spindle machine for spinning wool or cotton. The hand-powered spinning jenny was patented by James Har¬ greaves in 1770. The development of the spinning wheel into the spinning jenny was a significant factor in the industrialization of the textile indus¬ try, though its product was inferior to that of R. Arkwright’s water frame.
spinning mule Multiple-spindle spinning machine invented by Sam¬ uel Crompton (1779), which permitted large-scale manufacture of high- quality thread for the textile industry. Crompton’s machine made it possible for a single operator to work more than 1,000 spindles simulta¬ neously, and was capable of spinning fine as well as coarse yarn.
spinning wheel Early machine for turning textile fibre into thread or yarn, which was then woven into cloth on a loom. The spinning wheel was probably invented in India, though its origins are unclear. It reached Europe via the Middle East in the Middle Ages. The improvement of the loom in 18th-century England created a yarn shortage and a demand for
mechanical spinning. The result was a series of inventions that converted the spinning wheel into a powered, mechanized component of the Indus¬ trial Revolution (see drawing frame; spinning jenny; water frame).
Spinoza \spi-'no-z3\, Benedict de Hebrew Baruch Spinoza (b.
Nov. 24, 1632, Amsterdam—d. Feb. 21, 1677, The Hague) Dutch Jewish philosopher, a major exponent of 17th-century rationalism. His father and grandfather had fled persecution by the Inquisition in Portugal. His early interest in new scientific and philosophical ideas led to his expulsion from the synagogue in 1656, and he thereafter made his living as a lens grinder and polisher. His philosophy represents a development of and reaction to the thought of Rene Descartes; many of his most striking doctrines are solutions to difficulties created by Cartesianism. He found three unsatis¬ factory features in Cartesian metaphysics: the transcendence of God, mind-body dualism, and the ascription of free will both to God and to human beings. To Spinoza, those doctrines made the world unintelligible, since it was impossible to explain the relation between God and the world or between mind and body or to account for events occasioned by free will. In his masterpiece, Ethics (1677), he constructed a monistic system of metaphysics and presented it in a deductive manner on the model of the Elements of Euclid. He was offered the chair of philosophy at the Uni¬ versity of Heidelberg but declined it, seeking to preserve his indepen¬ dence. His other major works are the Tractatus Theologico-Politicus (1670) and the unfinished Tractatus Politicus.
spiny anteater See echidna
spire Steeply pointed termination to a tower or roof. In Gothic architec¬ ture, the spire is a spectacular visual culmination of the building as well as a symbol of heavenly aspiration. The church spire originated in the 12th century as a simple, four-sided pyramidal roof capping a tower. Methods used to integrate an octagonal spire with a square tower below include broaches (sloping triangular sections of masonry added to the bottom of the four spire faces not coinciding with the tower sides), gabled dormers added to spire faces, and steep pinnacles (vertical ornaments of pyramidal or conical shape) added to tower comers. During the Deco¬ rated period (14th century) in England, a slender needle spire set in from the edge of the tower was popular; corner pinnacles and a low parapet around the tower’s edge became customary. In the 20th century, archi¬ tects tended to limit spires to rather elementary geometric shapes.
spirea \spI-'re-3\ Any of nearly 100 species of flowering shrubs in the genus Spirea (rose family), native to the northern temperate zone and commonly cultivated for their pleas¬ ing growth habit and attractive flower clusters. The most commonly grown, and possibly the most popu¬ lar of all cultivated shrubs, is the Vanhouttei spirea, also called bridal wreath (a cross between S. can- toniensis and S. trilobata ), which grows up to 6 ft (2 m) tall and has graceful arching branches that bear numerous white flowers in spring.
Plants that resemble spirea are the shrubby false spireas ( Sorbaria spe¬ cies, also rose family) and the peren¬ nial herbaceous spireas ( Astilbe species, saxifrage family).
spirillum \spi-'ril-3m\ Any of the spiral-shaped bacteria that make up the genus Spirillum, which are aquatic except for one species that causes a type of rat-bite fever in humans. The term is used generally for any corkscrewlike species of bac¬ teria (see spirochete). Spirilla are gram-negative (see gram stain) and move by means of tufts of flagella at each end.