It's Shus elbow and shoulders as he lies between Harunas legs. You can see part of his hand in the bottom of the picture, on Harunas thigh (=・ω・=)
That's what I figured, but the shoulder blades are way too low compared to where his arm starts! It makes it look like there's something very wrong with his back, haha
(it's not like I can draw so I shouldn't judge, it's just a bit derpy anatomy-wise)
Uhmm I Think we call that artistic freedom. But that is if we are being nice to the mangaka
Not to take away from my comment below, but would anyone like to hazard a guess at what's going on, anatomically, in the bottom left panel of this page??
http://www.mangago.zone/read-manga/shizuku_hanabira_ringo_no_kaori/mh/v01/c006/26/
Is that supposed to be his shoulder blades? But the arm is way too high... His butt? But that seems to be further down... What??