http://www.collembola.org/doc/anatposi.htm - Last updated on
2003.11.23 by Frans
Janssens
Frans Janssens, Department
of Biology, University of Antwerp (RUCA), Antwerp, B-2020, Belgium
Preamble
Standard terminology is required to describe the precise relationship
of body structures to one another. For the purpose of the description of
such body structures, the body is considered as being in what is called
the anatomical position. To ensure consistency of the
description it is important to keep the anatomical position constantly
in mind. This is important, e.g. since in a normal relaxed position of
the body, the furca is inflexed underneath the abdomen, while in
anatomical position parlance, the furca is downwards extented.
Whenever body parts are being discussed dorsal or lateral
to some other body part, the body and/or body structure or body
appendage is assumed to be in the anatomical position.
The anatomical body position
The anatomical body position is the starting point for all
morphological terminology. In order to describe the orientation and
position of morphological body structures accurately and unambiguously,
one needs to define a standard reference body position by convention:
the anatomical body position.
The anatomical position of a body is an arbitrary position of the
body used as a reference when describing parts of the body in relation
to each other. When used in conjunction with terms of relationship,
terms of comparison and terms of movement, the anatomical body position
allows a standard way of documenting where one part of the body is in
relation to another, regardless of whether the body is standing up,
lying down, or in any other position.
If you think of a Collembola specimen lying in front of you on a table
on its right side (1),
its head directed to your left (2), its abdomen directed to
your right, its legs directed to you, this is the anatomical body
position (fig. 1). Note that the antennae should be directed away from you.
Note that the furca should be outflexed in such a way that it points
to you. Anterior and posterior faces of the furca refer to this organ
extended at right angle to the main axis of the body in a similar
position to that of the legs in repose (Salmon, 1964:102).
The body is thus neither in a natural position nor in the usual
position adopted by a living specimen at rest. However this position is
accepted as an unambiguous starting position from which to begin to
describe morphological body features and positions.
1 Why putting the specimen on its side and not on
its dorsum (as in the anatomical position of the human body used in
medical science) or on its ventrum (as in the position of most pinned
insect specimens in musea collections)?
Externally, Collembola, being Bilateria, exhibit an approximate
anatomical bilateral symmetry, that is, their right- and left-hand sides
have mirror symmetry. Therefore, an on its side position does not expose
any redundant (mirrored) body features and seems the most appropriate,
most obvious position to document orientational and positional
relationships of external structural body parts.
2 Why directing the head to the left and not to
the right?
This left to right model is an arbitrary decision to emphasise that the
anatomical position primarely is intended to support description of
static positional relationships of body structures. Describing the
dynamics of the body and/or its parts would probably benifit more from a
right to left model. In a static body plan, the head is in front of the
thorax, which is in front of the abdomen. In line with the syntactic
model of most Eurasian languages in which words are concatinated from left
(begin of sentence) to right (end of sentence), body parts can be seen
as being concatinated from left to right, the leftmost body part being
the most 'beginning' one.
Coordinate system referencing body morphology
There are many different coordinate systems, based on a
variety of units, projections, and reference systems. Coordinate systems
based on orthogonal (right angle) coordinates, introduced by René
Descartes (1596-1650), are often referred to as Cartesian systems in
analytic geometry. Polar systems are based on angles from baselines. We
propose a combination of the Cartesian and polar coordinate system to
define the precise positioning of body structures in relation to one
another.
Cartesian body planes of section
Gravity, in physics the so-called weakest of the four fundamental
forces, is the dominant force on earth for shaping the macroscopic
structure of the bilateral symmetric animal bodies.
Bilateral animals have a unique plane of symmetry. This symmetry plane
divides the standing-up body longitudinally in two mirrored halves,
a left-hand part and a right-hand part.
The mirror plane
is in line with the direction of the gravitational force of the earth.
This mirror plane is the primary plane of section of the body and forms
the basis of the morphological coordinate system of the body. The mirror
plane is also called the median plane of section, or meson, or
midsagittal plane.
Planes of section that run parallel with the mirror plane of section
are called sagittal planes of section. Sagittal planes of section divide
the body in a left-hand part and a right-hand part.
The mirror plane is a unique case of the sagittal planes of section
in which the body is divided in two symmetrical parts.
In the median plane of section a unique line can be drawn
that is the longest line of the median body section: the midline. The
midline is the primary body axis: the longitudinal head-to-abdomen body
axis, also called the long axis. Since the long axis is directed
horizontally in tetrapods and hexapods, but directed vertically in
bipeds, it is not recommended to use the terms 'horizontal axis' or
'vertical axis' in a generic bilateral body plan.
Bilateral symmetry, which is imposed by the gravity field of the earth
and by induced polarity (cephalisation) due to body movement,
also defines the secondary Cartesian plane of section.
By rotating the mirror plane 90 degrees
around the longitudinal body axis, an orthogonal coronal plane of section
divides the body in a dorsal part and a ventral part.
The ventral part is the part bearing the locomotory system.
The coronal plane of section that runs through the long body axis is
called the midcoronal plane of section.
Planes of section that run parallel with the midcoronal plane of section
are called coronal planes of section. Coronal planes of section divide
the body in a dorsal part and a ventral part.
The midcoronal plane is a unique case of the coronal planes of section
in which the body is divided in two functionally different parts,
the dorsum and ventrum.
Both the mirror plane of section and the midcoronal plane of section are
a special case of a set of planes of section that have a more
fundamental morphological meaning (tba: polar proximo-distal coordinate
system).
Transverse to both the mirror plane and the midcoronal plane, and the long
body axis, the third type of Cartesian plane of section, the axial plane of
section, divides the body in a cephalic part and a caudal part.
While there are unique sagittal and coronal planes of section,
the axial planes of section are arbitrary.
Terminology refering to orientation
- anterior
- refers to the forward end of the body or structure.
- distal
- refers to the free end of a structure or appendage, or to the end
furthest away from the point of attachment.
- inferior
- refers to the downward end of the body or structure.
- posterior
- refers to the rear end of the body or structure.
- proximal
- refers to the base of a structure or appendage, or to the end
nearest to the point of attachment.
- superior
- refers to the upward end of the body or structure.
Terminology refering to position
- caudal
- pertaining to the tail end of the body.
- cephalic
- pertaining to the head end of the body.
- dorsal
- pertaining to the dorsum of the body.
- ectal
- pertaining to the outer surface of the body or structure.
- ental
- pertaining to the inside of the body or structure.
- lateral
- pertaining to the sides of the body or structure.
- medial
- pertaining to the mirror plane of the body or structure.
- mesal
- pertaining to the meson of the body or structure.
- ventral
- pertaining to the venter of the body.
Terminology refering to direction
- caudad
- in the direction of the tail end of the body.
- cephalad
- in the direction of the head end of the body.
- distad
- in the direction of the distal end of the structure.
- dorsad
- in the direction of the dorsal end of the body; dorsally.
- ectad
- in the direction of the outer surface. Used to indicate the
relative position of internal structures.
- entad
- in the direction of the inside; i.e. the long axis.
- laterad
- in the direction of the right or left side of the body.
- mesad
- in the direction of the meson of the body.
- proximad
- in the direction of the proximal end of the structure.
- ventrad
- in the direction of the ventral end of the body; ventrally.
Terminology refering to body points of view
- caudal
- view of the body.
- cephalic
- view of the body.
- dorsal
- view of the body.
- lateral
- view of the body.
- ventral
- view of the body.
Terminology refering to body planes of section
- axial
- section through the body at right angles to the long axis of the
body, dividing the body in an asymmetric cephalic and a caudal part.
- coronal
- section through the body parallel with the body axis in such way
that both right and left halves are symmetrically shown, dividing the
body in an asymmetric ventral and a dorsal part.
- cross
- section through the body at right angles to the long axis of the
body; axial.
- medial
- section through the body through the midline of the body,
dividing the body into mirrored right and left halves; mesal.
- mesal
- midsagittal section through the body, dividing the body or
structure into a symmetrical right and left half; the midplane; the
mirror plane; the meson.
- midsagittal
- section through the body through the midline of the body,
dividing the body into mirrored right and left halves; mesal.
- parasagittal
- section through the body parallel to the sagittal section but
offset slightly from the midline to the right or to the left, dividing
the body in an asymmetric left and right part.
- sagittal
- section through the body in such a way that it bisects the body
parallel with the midplane into right and left parts.
- transverse
- section through the body at right angles to the long axis of the
body; axial.
Terminology refering to body axes
Body axes are vectors, quantities that have both magnitude and
direction. In order for descriptions of vector quantities to be useful,
it is important that everyone agree upon how the direction of an object
is described. While most of us are accustomed to the idea that up
refers to the 'headward' direction in humans, it is ambiguous in a
generic bilaterian body plan. It is only by means of convention that we
all can agree on the meaning of a directional term.
- cephalo-caudal
- axis directed from head to tail in line with the
midline of the body.
- long
- axis directed from head to tail; cephalo-caudal.
- longitudinal
- axis directed from head to tail; cephalo-caudal.
- meso-dorsal
- polar axis at right angle on the midline, directed from the midline
to the dorsum.
- meso-lateral
- cartesian axis at right angle on the midline, directed from mesal
to lateral.
- meso-ventral
- polar axis at right angle on the midline, directed from the midline
to the venter.
- proximo-distal
- polar axis at right or oblique angle on the midline, directed
from mesal to lateral.
- ventro-dorsal
- cartesian axis at right angle on the midline, directed from venter
to dorsum.
References