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Hello,
Is this the correct expression for explanation of pictures?
Can you please check the whole passage?
Thank you.

Legends for illustrations/Illustration legend

Figure 1. The scheme shows how right hemithorax was divided and according to it how subgroups were classified. Midaxillary line represents the border between front and back and the 5th rib in the midaxillary line represents border between up and down position of the thoracic cavity.
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Antonija;
You realy need a subject matter expert to make content corrections.
It is challenging to correct, since there is not pitcture either. In spite of theat, I'll give you a few suggestions.
AntonijaLegends Caption for illustrations/Illustration legend
A legend is a small insert in an illustration which gives a key to undertanding different parts. For example, in a map, a legend might say dark green = forestland, light green = arable land, feilds; blue = water, brown = desert, red= developed areas; ...

Figure 1. The scheme shows how the right hemithorax was divided and according to it how subgroups were classified. (The diagram shows the divisions of the right hemithorax and the classification of its subgroups) The midaxillary line represents the border between front and back (the frontal and dorsal regions) and the 5th rib on the midaxillary line represents the border between the upper and lower up and down positions( regions) of the thoracic cavity.

Comments  
Firstly, let me thank you for your help.
AlpheccaStarsThe diagram shows the divisions of the right hemithorax and the classification of its subgroups)
-it's not 'diagram', but a picture. What would be a suitable word?