Anatomical atlases consist of large, often complex images which are not surrounded by textual information as it is the case in textbooks (hence the name ``atlas''). Figure captions are the only form of textual information available and do not interfere with other references to an image. In a series of interviews with medical students it became evident that figure captions are carefully studied to get an orientation in complex images . This analysis is based mainly on the widely-known atlas of Sobotta [20, 21].
Figure captions in anatomy follow a rather fixed structure. The first items mentioned are generally the name of the depicted contents, the viewing direction and the thematic focus (e.g. ``muscles and sinews of a foot from lateral''). This information is essential if an unfamiliar illustration of organs inside the human body is depicted. After the information about the image as a whole, important parameters of single objects are usually described. Among these parameters, manipulations that affect the visibility of objects are most important because they often influence the whole composition. Typical phrases include ``[object1] has been removed to show [object2]'' or ``[object1] has been removed in the area of [object2]''. If small objects are important in a specific context, they must be enlarged to emphasize them. In this case, the context is preserved for better orientation, so that the surrounding objects cannot be enlarged. Such modifications are reflected by phrases like ``[object] slightly enlarged''.
An example of a hand-made anatomic illustration and the accompanying figure caption is given in Figure 1. In the first sentence, the thematic focus of the illustration and the viewing direction (plantar aspect) with respect to the main object (right foot) is provided. The second sentence informs the reader about radical changes due to invasion and removals of a number of objects.
Figure 1: An image with complex abstraction and the figure caption describing them [21, p. 376,].
Besides geometric manipulations, presentation variables (recall Section 2) are often adapted to a specific context, e.g. to show spatial relations more clearly and to communicate which objects belong to the same category. In anatomy, objects of certain organ systems are colored uniformly according to accepted conventions. In figure captions, the use of color therefore needs to be described only if it differs significantly from the conventions or even conflicts with them.
An interesting facet is the generation of one figure caption for several images, for instance when images show the same model from different directions. Furthermore, visualizations may exploit the symmetry of anatomic objects and show different aspects in both halves. In both cases, the similarities between the images are mentioned first, while the differences are described later. If horizontally arranged images are described by one caption, it is important that the left image be described before the right one, because there is a natural sequence of ``reading images'' from left to right.
Figure captions also depend on other textual components. In order to refer to objects via their name, they must either be labeled or circumscribed by characteristic features which are easy to recognize in the image.
The analysis of figure captions in anatomic atlases results in the description of their structure, which can be used to automatically generate anatomic figure captions. This process is described in detail in Section 7.3.