The Collaborative International Dictionary of English v.0.48:

Haematosac \H[ae]m"a*to*sac`\ (-s[a^]k`), n. [H[ae]mato- + sac.]
   (Anat.)
   A vascular sac connected, beneath the brain, in many fishes,
   with the infundibulum.
   [1913 Webster]