The bony thorax of the horse is remarkably compressed laterally in its
anterior part, but widens greatly behind. The =anterior aperture= is
oval and very narrow below; in a horse of medium size its greatest width
is about 4 inches (10 cm.), and its height 7 to 8 inches (ca. 18 to 20
cm.). The =ventral wall= or =floor= is about 10 inches (40 cm.) long,
and the =dorsal wall= or =roof= about 38 to 40 inches (95 to 100 cm.)
long. The height from the last segment of the sternum to the seventh or
eighth thoracic vertebra is about twice that of the anterior aperture;
this is due to the obliquity and divergence of the roof and floor. The
greatest width of the posterior aperture is about 20 to 24 inches (50 to
60 cm.). The intercostal spaces increase in width from the first to the
seventh or eighth, and then diminish. Their average width is about 1¼ to
1½ inches (3 to 3.5 cm.).