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headThe coma and nucleus of a comet when seen together. In this context ‘nucleus' means the diffuse starlike luminous condensation sometimes observed in the coma. The head generally contracts as perihelion is approached and expands again afterward. Remarkable changes in size, luminosity distribution, and the number of observed ‘nuclei’ can take place inside the head in a few hours.
the anterior (or superior, in humans particularly) section of the body (separate from the trunk) of motile bilaterally symmetrical animals.
The upper division of the central nervous system, some sense organs, and the anterior sections of the digestive and respiratory systems are concentrated in the head. The anterior end of an animal, as yet undifferentiated but already structurally distinguishable from the rest of the body (for example, in the majority of worms) is called the cephalic end. The head, which is well-differentiated in some mollusks and especially in arthropods, became distinct from the cephalic end through the course of further evolution. Among the chor-dates, a differentiated head is present in the vertebrates (in connection with the progressive development of the central nervous system). Concentrated in the head of vertebrates are the brain, enclosed in the skull, the organs of smell, sight, and hearing; the organs of the oral and pharyngeal cavities, surrounded by the visceral arch; and, in lower vertebrates, the organs of the lateral line. In cyclostomes and the majority of fish the head is immovably attached to the spine. In terrestrial vertebrates the head is movable because of the separation of the cervical section of the body and the development of movable articulation of the skull with the spine. In amniotes (reptiles, birds, and mammals), with the formation of a clearly defined cervical section, the head is noticeably set apart from the torso (except in secondarily aquatic forms—that is, cetaceans and, among fossils, ichthyosaurs). In addition to mobility in the atlantooccipital joint, the head in amniotes may rotate together with the atlas around the dens axis, or epistropheus. The most mobile is the head of birds and the majority of mammals; this is associated with the progressive evolution of higher nervous activity and of the sense organs-olfactory, visual, and auditory.
Head development, which is of great importance in the evolution of vertebrates, is called cerebralization. In man, the head has anatomic features that are determined by the high level of brain development, the vertical position of the body, the transition to bipedalism, and the conversion of the hands to organs of work.
A. N. DRUZHININ
(in hydraulics), a linear quantity expressing the specific energy (per unit weight) of a liquid stream at a particular point.
The total reserve of specific energy H of a stream (the total head) is defined by Bernoulli’s equation: H = z + (pv/γ) + (v2/2g), where z is the height of the point above the reference plane, pv, is the pressure of the liquid flowing with velocity v,γ is the density of the liquid, and g is the acceleration of gravity. The sum of the first two terms of the trinomial represents the sum of the specific potential energies of position (z) and pressure (pv/γ)—that is, the total content of specific potential energy, or the hydrostatic head. The third term represents the specific kinetic energy (the velocity head). The head decreases in the direction of flow. The difference between the heads in two cross sections of a real liquid H1 —H2 = hv is called the loss of head or lost head. For the motion of a viscous liquid in pipes the loss of head is determined by Darcy’s formula.
In the design of hydroengineering structures the head assumed in calculations is chosen in accordance with a given task. For example, for dams the head of the headrace is the depth of water in the headrace. In problems dealing with the flow of liquids or gases from orifices, the head is assumed to be the immersion depth of the “center of gravity” of the opening (for outflow of liquid) or the pressure differential (for outflow of gas). In hydroelectric power plants a distinction is made between the gross head (the difference between the elevations of the headrace and tailwater) and the net head (the gross head less the loss of head caused by hydraulic resistance).
REFERENCESChugaev, R. R. Gidravlika, 2nd ed. Moscow, 1971.
Spravochnik po gidravlicheskim raschetam, 4th ed. Edited by P. G. Kiselev. Moscow-Leningrad, 1972.
P. G. KISELEV
What does it mean when you dream about your head?
The head symbolizes the source of all wisdom. Being made the “head” of an organization, or being sent to the “head of the class” denotes that one has accomplished much.
HDA(1) See HD audio.
(2) (Head Disk Assembly) The mechanical components of a disk drive (minus the electronics), which include the actuators, access arms, read/write heads and platters.
read/write headA device that reads (senses) and writes (records) data on a magnetic disk or tape. For writing, the surface of the disk or tape is moved past the read/write head. By discharging electrical impulses at the appropriate times, bits are recorded as tiny, magnetized spots of positive or negative polarity.
For reading, the surface is moved past the read/write head, and the bits that are present induce an electrical current across the gap.
|Thin Film Read/Write Head|
|The read/write heads on today's magnetic disks are so tiny you need a microscope to see them. The heads are attached to a pair of aerodynamically designed rails, known as a "slider," that keep the head at the proper distance from the disk platter.|
|GMR and Inductive Technologies|
|Typical read/write heads use a giant magnetoreresistive (GMR) head for reading and an inductive coil for writing as in this illustration. (Illustration assistance courtesy of Hitachi Global Storage Technologies.)|