A saddle must not rest directly on any of the hard structure of the back, be it spine or ribs; it must only rest on those parts well clothed with muscle, and the only part of the back so clothed is the angular space formed by the ribs and processes of the vertebrae.

Muscles a buffer to bones
In saddle fitting the muscles of the back act as a buffer to the bones beneath, and so prevent injury; where a part of the back has no muscle to protect it, there the saddle cannot rest, or if it does, injury results.
The importance of having well-nourished back muscles is very great; where they are large and well developed we may be certain the parts beneath are sufficiently protected, where they are impoverished and weak the animal is in hourly risk of injury.

Back shows waste soon
When horses are working hard and underfed, one of the first places to show muscle waste is the back.  The muscles, previously convex, now become concave until well-marked gullies form along either side of the spine, while the ribs previously hidden are now in view and can be distinctly counted.

Changes in shape of back
The whole shape of the back is altered; it is as if some new structure had taken place of the old, and all this is explained by the melting of the muscles on the back bringing the skeleton beneath into view.
It is this metamorphosis of the back which has to be so anxiously watched on service.  It is this which renders all previous saddle fittings useless, for the impoverished back is as different from the well-nourished one as anything can possibly be; it is as though we compared the skeleton to the living subject.
We dwell upon and emphasise this question of back muscle; it is the foundation of a clear conception of sore backs and their causes, and it is the basis for all preventative measures.