I discovered this while on the National Archives website earlier today. It was something that I hadn’t been aware of:

“By War Department General Order Number 259 of August 1, 1863, veterinary surgeons of cavalry, under a congressional act of March 3, 1863, were to be selected by the Chief of the Cavalry Bureau upon the nomination of the regimental commanders. The names of candidates so recommended and nominated to the Chief of the Bureau of Cavalry were then to be submitted to the Secretary of War for appointment.”