Only a small percentage of the total volume of an ejaculate is made up of sperm cells. In older medical literature, as previously mentioned, a distinction was made between the sperm-cell portion or ‘nobler part’, ‘the aqueous elements’ from the seminal glands and the ‘oleagenous’ portion from the prostate. The prostate is about the size of a chestnut. The seminal glands are situated behind it and discharge into the urethra, which passes right through the prostate.
In animals the system is different. Dogs, like many other carnivores, have very small seminal glands. The reason why is a complete mystery. In man the fluid produced by the seminal glands is important mainly for the mobility and the metabolism of sperm cells. In humans it can make up between 50 and 80 per cent of the total volume of ejaculate. The principal ingredients of seminal fluid are fructose, coagulating agents and the prostaglandins e, a, d and f.