It's the visible line at the junction of the tendinous and
muscular parts of one of the abdominal muscles (external abdominal oblique),
caused by hypertrophy (increase in size) of the muscle fibers. This, and other
abdominal muscles are used in the "abdominal press" to move things into and out
of the abdomen, and sometimes as in this case out of the thorax.
It's caused by a condition called COPD (Chronic
Obstructive Pulmonary Disease), or just "Heaves". These horses lose lung
compliance, and in order to still satisfy oxygen needs, breathe at higher lung
volumes than normal. Due to the decease in lung compliance, expiration is no
longer a passive process, and they have to use abdominal press to expire, and so
you see an increase in size of this muscle.
Just had a test on this material, figured i'd share.
:-)
