This may be of use
 
Am J Vet Res. 1997 Oct;58(10):1083-91.  

Developmental variation in lumbosacropelvic anatomy of thoroughbred racehorses.

Haussler KK, Stover SM, Willits NH.
 
Abstract on http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=9328659&dopt=Abstract
 
Mel
 
 
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