No matter how many anatomy books I buy, without fail they do not have accurate depictions of this area. Generally this area is treated as a simple inverted triangular structure, sometimes divided down the middle and having a clean border, however it does not appear this way on the figure. Even in pictures and videos of actual dissections this area looks very different between pictures. Davinci depicted something similar, but only in some of his illustrations of the shoulder, not in all of them.
Sometimes it seems as though just adjacent to the edge which borders the fossa there is an inverted-triangular tendon produced by the muscle splitting in two creating a muscle with two bellies and one tendon. As far as I can tell this structure is unique from the usual furrow in the center of the anterior portion, but I could be mistaken. This structure can sometimes be seen on the figure as a seperate triangular structure adjacent to the fossa seperated by a very narrow piece of flesh. Sometimes, however, in dissection photos, it appears that the piece of flesh seperating this tendon from the fossa is missing altogether, showing the white triangular tendon to be the actual border of the deltoid, or sometimes it does not come up far enough to connect to the clavicle, leaving part of the tendon exposed.
At times it appears this division produces a cleft which travels the length of the muscle, parallel to its border. At times this structure appears to sit on top of the muscle, or perhaps protrude out from between a division of the muscle and then folding over the top of the muscle before finally attaching to the clavicle.
It is very confusing and it has been frustrating me for weeks. I was hoping someone could provide some insight.