The pes anserine bursa is a sac (lined space) under the attachments of the pes anserine tendons on the inner aspect of the knee.
The anatomical and imaging study of pes anserinus and its clinical application. Zhong S, Bo Wu B, Wang M, Wang X, Yan Q, Fan X, Hu Y, Han Y and Lih Y. Medicine (Baltimore). 2018 Apr; 97(15): e0352.