From Great Sex: A Man’s Guide to the Secret Principles of Total-Body Sex (2004) by Michael Castleman, page 159. Note the proximity of the prostate gland to the rectum. While the insertion of a finger or sex toy into the anus will cause sexual excitement and pleasurable sensations in the entire pelvic region, the prostate can be stimulated more intensely with gentle stroking. Locate it two or three inches inside the anus, toward the anterior or front part of the body. It feels like a round mass about the size of a walnut. Similar stroking on the same side, only deeper, may directly stimulate the seminal vesicles, which like the prostate produce part of the fluid in a man’s ejaculation.