1. The large gastrocnemio-semimembranosus bursa appeared in twenty-six out of thirty dissections. In four, we found two bursae,—one under the inner head of the gastrocnemius, and one between this muscle and the semimembranosus.
2. The composite gastrocnemio-semimembranosus bursa communicated with the knee joint in fifteen out of these twenty-six dissections; the bursa under the inner head of the gastrocnemius communicated with the joint in two out of the four cases in which there were two separate bursae; while the bursa between the inner head of the gastrocnemius and the semimembranosus was never found to communicate directly with the joint.
3. Thirteen out of fourteen cysts in the medial portion of the popliteal space, the anatomy of which was recorded in detail in the operative notes, appeared between the semimembranosus tendon and the inner head of the gastrocnemius.
4. The cysts were adherent to part of the tendinous surfaces of these two muscles in a manner similar to that found in the semimembranosus bursa.
5. Cysts were found in children and in adults and appeared similar in the two groups.
6. An effusion in the knee joint was present in only five out of ten cases in which this point was referred to in the preoperative note.
7. Our studies lead us to conclude that most, if not all, popliteal cysts are the result of fluid distention of the gastrocnemio-semimembranosus bursa.
8. We believe that the cysts result from injury to the bursal wall caused by muscle contractions followed by effusion.
9. Microscopic examination of the cyst walls showed blood pigment frequently present, also fibrous thickening and cartilaginous and osteoid metaplasia.
10. Operative excision of the cyst appears to be the best treatment.
11. The various bursae at the back of the knee, and particularly the large gastrocnemio-semimembranosus bursa, would seem to have greater clinical importance than has yet been realized.