In contrast, a posteroanterior (PA) projection is taken with the X-ray generator posteriorly. A coronal plane is any vertical plane that divides the body into ventral and dorsal sections. This set of terms provides orientation within the oral cavity, much as anatomical terms of location provide orientation throughout the body. 8, where the tentacles are curved, and therefore not in anatomical position. Sometimes this is abbreviated to ML (or M-L) axis. Thus, the axis formed by joining the two is the superior-inferior axis.[12][13]. [43][45] Adult echinoderms, such as starfish, sea urchins, sea cucumbers and others are also included, since they are pentaradial, meaning they have five discrete rotational symmetry. For example, the mid-clavicular line is used as part of the cardiac exam in medicine to feel the apex beat of the heart. [4] Combined terms were once generally, hyphenated, but the modern tendency is to omit the hyphen. In some cases a third axis can be defined, particularly where a non-terminal cytostome or other unique structure is present. [49] [50]. For example, as humans are approximately bilaterally symmetrical organisms, anatomical descriptions usually use the same terms as those for vertebrates. This is because although teeth may be aligned with their main axes within the jaw, some different relationships require special terminology as well; for example, teeth also can be rotated, and in such contexts terms like "anterior" or "lateral" become ambiguous. Three basic reference planes are used in zoological anatomy. Therefore, the body has its feet together (or slightly separated), and its arms are rotated outward so that the palms are forward, and the thumbs are pointed away from the body (forearms supine). The terms valgus and varus are used to refer to angulation of the distal part of a limb at a joint. Central and peripheral refer to the distance towards and away from the centre of something. This is commonly abbreviated to DV (or D-V) axis. [31], This terminology is also employed in molecular biology and therefore by extension is also used in chemistry, specifically referring to the atomic loci of molecules from the overall moiety of a given compound. Drosophila embryogenesis, the process by which Drosophila embryos form, is a favorite model system for genetics and developmental biology. If they are divided down the middle, in other words, they have mirror-image left and right halves. "Lateral", "dorsal", and "ventral" have no meaning in such organisms, and all can be replaced by the generic term peripheral (from Ancient Greek περιφέρεια 'circumference'). For the linguistic terms, see. However, as left and right sides are mirror images, using these words is somewhat confusing, as structures are duplicated on both sides. For example, the hands are at the distal end of the arms, while the shoulders are at the proximal ends. They are universal terms that may be readily understood by zoologists who speak any language.. Varus (from Latin  'bow-legged') and valgus (from Latin  'knock-kneed') are terms used to describe a state in which a part further away is abnormally placed towards (varus) or away from (valgus) the midline. These two terms, used in anatomy and embryology, describe something at the back (dorsal) or front/belly (ventral) of an organism.

Sometimes the orientation of certain planes needs to be distinguished, for instance in medical imaging techniques such as sonography, CT scans, MRI scans or PET scans. Cnidarians (jellyfish, sea anemones and corals) have an incomplete digestive system, meaning that one end of the organism has a mouth, and the opposite end has no opening from the gut (coelenteron). [36] For example, the external oblique muscle of the abdomen is deep to the skin. If talking about the skull, the dorsal side is the top. They are used in anatomy, surface anatomy, surgery, and radiology. (1993), Hickman, C.P., Jr., Roberts, L.S.

Thus the elbow is distal to a wound on the upper arm, but proximal to a wound on the lower arm. For example, in a human, the arms are lateral to the torso. the 4th cervical vertebra, abbreviated "C4"), or the rib cage (e.g. [45] For example, structures may be described relative to the anterior superior iliac spine, the medial malleolus or the medial epicondyle. Anatomical lines are used to describe anatomical location. However, for historical and other reasons, standard human directional terminology has several differences from that used for other bilaterally-symmetrical organisms. [42] [44] Radially symmetrical organisms always have one distinctive axis. Terms include: Different terms are used because of different body plans in animals, whether animals stand on one or two legs, and whether an animal is symmetrical or not, as discussed above. This helps avoid confusion in terminology when referring to the same organism in different postures. Thus, the pectoral fins are dorsal to the anal fin, but ventral to the dorsal fin.

(2003), Ruppert, E.E., Fox, R.S. For example, volar pads are those on the underside of hands, fingers, feet, and toes. Most animals are capable of moving relative to their environment. Several terms are commonly seen and used as prefixes: Other terms are used as suffixes, added to the end of words: Superior (from Latin super 'above') describes what is above something[21] and inferior (from Latin inferus 'below') describes what is below it. As part of defining and describing terms, the body is described through the use of anatomical planes and anatomical axes. [43][45] Radially symmetrical organisms always have one distinctive axis. In organisms with a changeable shape, such as amoeboid organisms, most directional terms are meaningless, since the shape of the organism is not constant and no distinct axes are fixed.

The axes of the body are lines drawn about which an organism is roughly symmetrical. Obviously, the left side and right side of the organism are the outermost points between the two "sides" of the organism. Aurelia aurita, another species of jellyfish, showing multiple radial and medio-peripheral axes, The sea star Porania pulvillus, aboral and oral surfaces, Special terms are used for spiders. The location of anatomical structures can also be described in relation to different anatomical landmarks. Properly, this is called the dextro-sinistral (or, more uncommonly, the sinistro-dextral) axis, from the Latin dexter (right) and sinister (left).

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