It's possible that the poison was injected directly into the heart, be it the left axle or subscapular regions or the lungs, be it the intercostal spaces...
a. (Anat. & Physiol.) Between the ribs; pertaining to, or produced by, the parts between the ribs; as, intercostal respiration, in which the chest is alternately enlarged and contracted by the intercostal muscles. [ 1913 Webster ]