You're sitting on his kidneys! Get off!
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One of the biggest things we hear from people saying you can’t have any saddle pressure on the loin area is that “you’re putting pressure on his kidneys”. Well actually, no, you’re not - or else everyone is as well. Depends how you look at it.
From Sisson and Grossman, The Anatomy of the Domestic Animals (1975), which is basically still the best anatomy book out there, here’s page 524. This gives the location of the kidneys as well as what other organs, etc. are around them.
And I quote: “It (the right kidney) lies ventral to the dorsal parts of the last two or three ribs and the first lumbar transverse process.” It is #3 in the picture. (Illustrations are from Popesko's Atlas of Topographical Anatomy of the Domestic Animals.)
Yup, right underneath the ribs…
For the left kidney (#6): “It is usually ventral to the last rib and the first two or three lumber transverse processes.” So further back but still underneath the last rib.
It is #6 in the picture above, but it is very dark so I outlined it so it could be seen more easily.
Here’s a dorsal (top) view. #10 is the left kidney and #11 is the right.
And here’s a close up so you can see them better. Even if you are riding an English saddle that ends at T18, you are still sitting above the right kidney and the cranial aspect of the left one.
Why are people not worried about sitting over the aorta, or the lungs, or the gut? What is it about the kidneys? Here’s a cross section at the 17th rib, looking from the back towards the front. The right kidney is on our right, and in the same position on the left is the spleen. But have you ever been told to get off your horse’s spleen?
And here, at the level of the 2nd lumbar vertebra you have the left kidney on one side and the duodenum on the other. You get told to stay off the kidneys, but who worries about the poor little duodenum?
Warning!! Dissection picture ahead!!
So does sitting over the kidneys hurt them? No. Why not? Because they are well protected by both bone (ribs and lateral transverse processes of the lumbar vertebrae) and by a lot of muscle. Here is the longissimuss dorsi from a miniature mule (about 12 - 13 hands) that we dissected a few years ago. I have normal sized female hands so you can see the sheer size of that muscle. And there are more muscles over that area than just the one. No, you don’t have to worry about hurting the kidneys by riding your horse, or by having your western saddle extend onto the loin.
So why do kids always get told to stand up in their stirrups when their horse has to pee? Because it shifts the rider’s weight more to the front end and off the back legs which are stretched out in the normal “peeing posture”. This makes it easier and more comfortable for the horse to urinate. It has nothing to do with pressure on the kidneys. (Or the ovaries either, for that matter…)
The way you explain things and correct myths always make my day!