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Animals typically have one end with a head and mouth, with the opposite end often having the anus and tail. The head end is the cranial (or rostral) end; the tail end is the caudal end.
When the animal is in its normal position of motion, the front end is anterior and the back end is posterior. In most animals (but not in humans) "cranial" or "rostral" is therefore synonymous with "anterior", while "caudal" is synonymous with "posterior".
The side of animals with the spine or nerve chord is the dorsal side; the opposite side, typically the one closest to the ground when walking on all legs, swimming or flying, is the ventral side. For example: a cow's udder is on the ventral side. A dolphin's dorsal fin is, unsurprisingly, on the dorsal side.
In human anatomy, because of the erect posture of humans, the terms are used as follows. One pictures a standing human and uses the terms upper (superior) and lower (inferior) accordingly. The anterior side is the front with the nipples and navel; the posterior side is the back. The terms "anterior" and "ventral" are therefore synonymous in human anatomy, as are the terms "posterior" and "dorsal".
Sometimes the orientation of certain planes need to be distinguished, for instance in medical imaging techniques such as CT scans, MRI scans or PET scans. One imagines a human in anatomical position (standing, arms hanging down with palms to the front) and an X-Y-Z coordinate system with the X-Y plane parallel to the ground, the X-axis going from left to right, the Y-axis passing from front to back, and the Z-axis going up and down.
Structures near the midline are called medial and those near the sides of animals are called lateral. Therefore, medial structures are closer to the midsagittal plane, lateral structures are further from the midsagittal plane. Structures in the midline of the body are median. For example, your cheeks are lateral to your nose and the tip of the nose is in the median line.
Structures that are close to the center of the body are proximal or central, while ones far removed are distal or peripheral. For example, the hands are at the distal end of the arms, while the shoulders are at the proximal ends. These terms can also be used relatively to organs, for example the proximal end of the urethra is attached to the bladder.
Structures on or close to the bodyīs surface are superficial and those further inside are profound or deep.
When speaking of inner organs, visceral means close to or attached to the organ, while parietal is more distant. For example, the visceral pleura is attached to the lung and the parietal pleura is attached to the chest wall.
Flexion means approximating adjacent parts of the body (usually at a joint) and extension means separating them. For example, the legs are are flexed at the knee joints when sitting down, and extended when standing up. Generally, flexion produces an acute angle between adjacent parts, with its vertex at the joint, and extension produces an obtuse angle. One exception to this rule is in the ankle joint[?] where moving the foot such that the toes move upwards is dorsiflexion[?] and moving the foot such that the toes move downwards is plantar flexion[?].
Adduction means moving a part of the body toward or past its median line or toward the long axis of a limb. Abduction means moving a part of the body away from its median line or away from the long axis of a limb. For example, adducting the thighs brings the legs together, and abducting the thighs spreads the legs apart. Similarly, adducting the fingers brings them into contact with one another, and abducting the fingers spreads them apart.
Rotation means moving a part about its long axis, for example, in turning the neck. Supination means rotation of the forearm such that the palm of the hand faces forward or upward, and pronation means rotation of the forearm such that the palm of the hand faces backward or downward; the hand is supinated or pronated at the wrist. Similar movements may be accomplished at the ankle[?], where supination results in the foot tipping inward relative to its long axis, and pronation results in the foot tipping outward; overpronation may contribute to the condition flatfoot[?].
An anterograde motion is in the normal direction of flow, while retrograde means reversed flow. For example, passage of food from the mouth to the stomach is in an anterograde direction, and gastric reflux is in a retrograde direction.