The horses thoracic sling has been widely documented to be made up of the - Trapezius Serratus Ventralis (Thoracic and Cervical) Pectorals and the Subclavius.
I personally add to that list, the Supraspinatus and the Infraspinatus.
Not because they are a part of functionality for the sling itself, but because they do not work independently of it.
The Infraspinatus is the muscle of independent abduction
The Supraspinatus is the muscle of extension.
The Supraspinatus ties in and connects to the Subclavius.
The Subclavius muscle works to suspend and stabilize the trunk as well as providing padding for C7, T1 and the brachial plexus against the scapula to protect against heat and friction causing arthritis as well as nerve compression.
In my years of experimenting with different movements and development practices, I have found that these muscles do not work in isolation of each other.
It's not easy to SEE lack of development or atrophy of the subclavius due to it's location behind the scapula, but we often can see the lack of development of the infraspinatus and supraspinatus.
It's incredibly common for me to go out to work with a horse and see and feel the horses scapula.
While this is a normal find, it is NOT correct or healthy.
Aside from the above mentioned, it has also been documented by Jean Marie Denoix D.V.M. that the supraspinatus (along with the triceps and biceps) is the muscular lever of the forearm.
So without a supraspinatus, the horse has to to rely on it's Brachiocephalicus to be that lever for the forearm which puts the horse into a state of compensation as my post yesterday referenced.
In conclusion -
When you see the ridges and ripples on your horses shoulders please be mindful of what is going on below the surface and how the horse has to compensate for a lack of them.
In a well developed shoulder, there should be a smoothness over the top and an ease into the neck with an obvious and healthy jugular groove as well as a more angular shoulder angle as seen in the after photo below.