I'm reading a book (not sure if the author is approved but he explains Baucher's flexions) and came across where he explains the serratus costalis muscle, which is located in the upper inner part of the shoulder blade and is inserted down on the 8 to 9 first ribs of the horse. He says when this muscle contracts itself, the rib cage is pushed up. He claims that this is how the withers are raised for collection.
I've read "True Collection" here on this web site but don't find any mention of this muscle. If it is there, can someone please point it out?. Also, can't find much information in anatomy searches on this muscle. Is there perhaps a different name than he is using for this muscle?
Yes, Pam. There are lots of anatomy books in print that have outdated muscle names -- for example, the very widely distributed large-format reprint of George Stubbs' 18th-century anatomy. It's a pretty book and I own two copies of it myself -- one to supply framed art prints, and the other as a historical reference in my library. But when I teach an anatomy class, we do not use this book because the names are long outdated and the dissection is not as informed as those represented in modern equine anatomies.
As to what this or that "expert" says: Pam, you'll have to figure out just how to decide that. Does the person in question have the slightest qualifications in anatomy or biomechanics? The US Dressage Federation and the California Dressage Society are quite famous for inviting speakers to give public talks to their meetings and conventions on these subjects, but I have yet over a long span of years to see a single one on their program that had any qualifications whatsoever, other than Hillary Clayton.
On the other hand, maybe you take your car to your plumber to have the oil changed.