The first time I met this muscle, I had my eyes closed. This is not uncommon for me; eyesight only contributes so much to the understanding of my client's body, and I'll find myself craning my neck forward if I'm not careful. With my eyes shut, my posture is better.
So I worked on my client's back with my eyes closed. There was no shortage of trouble all around the right scapula, from the trapezius and rhomboids on the one side to latissimus dorsi on the other. Then my hands told me that I'd come across the medial border of the scapula, and that didn't make any sense at all for how far down the back I'd been working, so I opened my eyes to see what was up.
And there was serratus posterior inferior.
Wikipedia's article on this muscle is solid. The illustrations from Gray's Anatomy are lovely.
In quick summary: this muscle starts at the vertebrae at the juncture from mid to low back, and it takes an upward angle to the bottom four ribs. When it contracts, it squeezes on the ribcage and can help force air from the lungs.
This is not a big muscle. It is deep to the lower trapezius, which is a monster in comparison. These two factors alone mean that identifying it by touch is a challenge. But a muscle (and the fascia around it) can become so rigid that is feels like bone, and when this muscle dons its skeleton costume, it generally feels to me like a smooth ridge. With my eyes closed, it can fool me into thinking I'm touching the shoulder blade.
Some references describe trigger points from serratus posterior that inflict serious low back pain. I saw online elsewhere that the pain can resemble renal failure. I don't know the truth about either of these assertions. I can attest that five to fifteen minutes' worth of attention can soften a bone-hard serratus posterior inferior, and success at this can bring immense relief. The techniques are simple enough: warming glides, then muscle stripping.
Here's something I find fascinating in the Gray's Anatomy notes: a variation on this muscle is that it might not be present. My Mobi reference says that it might not be present in 40% of the population!
Please remember that I'm a massage therapist, not a doctor. The information here is not intended to take the place of professional medical advice.