It contains muscular tissue, which, by contracting or relaxing, lessens or dilates the pupillary opening.
"Special Report on Diseases of Cattle"
U.S. Department of Agriculture J.R. Mohler
pupillary reflexes normal; although limbs would remain in a fixed attitude when so placed, he did not evidence the typical flexibilitas cerea.
"Studies in Forensic Psychiatry"
Bernard Glueck
Both pupillary openings should be the same size, and not too small or too large in the bright light.
"Common Diseases of Farm Animals"
R. A. Craig, D. V. M.