Biology Reference
In-Depth Information
in
Figure 2.8
). A preliminary survey of squirrel scapulae indicated that they may have a
similar anatomical distribution of changes. This pattern dictated that the squirrel scapulae
should be digitized from the lateral view, because this is the only view in which the blade,
acromion and metacromion could be seen in all taxa. Fortunately, the one feature of the
bell that was considered potentially relevant to a functional analysis was also visible in
the lateral view. That feature, the “neck” between the blade and the bell, is expected to
change in thickness to reflect the magnitude of the forces transmitted to the scapula from
the humerus. Thus, before any decisions were made about inclusion of specific landmarks,
functional considerations were used to decide which general aspects of scapula shape
would be analyzed.
The anticipated importance of changes in the acromion and metacromion meant that
concerns about the distortion of three-dimensional aspects of shape could not be ignored,
and also that landmarks could not be deleted if the distortion was expected to be large.
Instead, concerns about distortion were addressed by standardizing the protocol used to
capture the images that were digitized. As is usual for morphometric analyses based on
photographs or video images, the scapula was placed in a standard orientation so that
differences in orientation would not be interpreted as differences in shape. In addition,
the distance of the camera lens from the scapula was adjusted for each specimen so that
the blade always occupied the same proportion of the field. Then, if the height of the spine
and sizes of the acromion and metacromion were proportional to the size of the blade,
the acromion and metacromion would also occupy a constant proportion of the field.
More importantly, the pattern of landmark displacement that would occur if these proportions
changed could be predicted and tests for these patterns could be performed. No evidence of
such patterns was found in the data.
After deciding which view to digitize, a major concern was coverage: finding enough
landmarks to represent adequately the shape of the scapula. Structurally, the scapula is
rather simple, which means there are few points that can be uniquely defined. This is
especially true of the main portion of the scapula, the semicircular or triangular “blade”;
the blade is nearly flat and has only two ridges crossing it
the large scapular spine on
the lateral surface, and the smaller subscapular ridge on the medial surface. The margin
of the blade is also rather featureless, having few corners and no spines, only more ridges
or thickenings.
Despite the shortage of potential landmarks, it was still considered important to
define them so that they could reasonably be considered homologous. For example, the
ends of ridges may seem to be good landmarks, but quite often these are gently tapered,
making it difficult to define precisely where they end. Usually, when a ridge ends
abruptly, it ends at an intersection with some other structure. On the scapula blade,
landmarks 8, 9 and 10 are points where two ridges intersect. Landmark 6, on the meta-
cromion, is another intersection, marking the attachment of the metacromion to the
spine. Landmarks 7 and 11 are points on the margin of the blade where the end of a mar-
ginal ridge is associated with a corner. Landmark 5, on the metacromion, is another cor-
ner associated with the end of a marginal ridge. Landmark 1 is one of the few places on
thebladewherearidge(thescapularspine)ends abruptly without intersecting another
structure.
