Iowa State University Veterinarian Records, RS 22/6/0/9, University Archives, Special Collections Department, Iowa State University, http://www.add.lib.iastate.edu/spcl/arch/rgrp/22-6-0-9.html
On Feb. 4, 1941, a two-year-old chestnut stallion of the American Saddle breed was presented to the clinic. Last fall, the animal had suffered an attack of equine influenza and had evidently completely recovered from the attack. At the time the stallion was brought here, it had an enlargement of the throat region, the left side being very much enlarged, and the right side only slightly so. The enlargement was very soft, and upon palpation, was found to be caused by air in the guttural pouch. The history which accompanied the case was that the animal had been operated upon several times previously, but without success. The caretaker stated that the swelling would appear in one or two days following the operation and would remain distended until operated on again.
Gitz, George H.
"Emphysema of the Guttural Pouch,"
Iowa State University Veterinarian: Vol. 3
, Article 18.
Available at: http://lib.dr.iastate.edu/iowastate_veterinarian/vol3/iss3/18