Meisen, J. (1974). On Bicategories of Relations and Pullback Spans. Communications in Algebra, 1(5), 377–401. https://doi.org/10.1080/00927877408548625 
@article{Meisen1974,
  title = {On Bicategories of Relations and Pullback Spans},
  author = {Meisen, Jeanne},
  year = {1974},
  month = jan,
  journal = {Communications in Algebra},
  publisher = {Informa {UK} Limited},
  volume = {1},
  number = {5},
  pages = {377--401},
  doi = {10.1080/00927877408548625},
  url = {https://doi.org/10.1080/00927877408548625}
}