the penis doubles in length and thickness and the glans increases by 3 to . The urethra opens within the urethral fossa, a small pouch at the distal end of the glans. A structure called the urethral process projects beyond the glans. The internal genitalia comprise the accessory sex glands, which include the vesicular glands, the prostate gland and the bulbourethral glands. These contribute fluid to the semen at ejaculation, but are not strictly necessary for fertility. Management and handling of domesticated stallions Domesticated stallions are trained and managed in a variety of ways, depending on the region of the world, the owner's philosophy, and the individual stallion's temperament. In all cases, however, stallions have an inborn tendency to attempt to dominate both other horses and human handlers, and will be affected to some degree by proximity to other horses, especially mares in heat. They must be trained to behave with respect toward humans at all times or else their natural aggressiveness, particularly a tendency to bite, may pose a danger of serious injury. For this reason, regardless of management style, stallions must be treated as individuals and should only be handled by people who are experienced with horses and thus recognize and correct inappropriate behavior before it becomes a danger. While some breeds are of a more gentle temperament than others, and individual stallions may be well-behaved enough to even be handled by inexperienced people for short periods of time, common sense must always be used. Even the most gentle stallion has natural instincts that may overcome human training. As a general rule, children should not handle stallions, particularly in a breeding environment. Management of stallions usually follows one of the following models: confinement or "isolation" management, where the stallion is kept alone, or in management systems variously called "natural", "herd", or "pasture" management where the stallion is allowed to be with other horses. In the "harem" model, the stallion is allowed to run loose with mares akin to that of a feral or semi-feral herd. In the "bachelor herd" model, stallions are kept in a male-only group of stallions, or, in some cases, with stallions and geldings. Sometime stallions may periodically be managed in multiple systems, depending on the season of the year. The advantage of natural types of management is that the stallion is allowed to behave "like a horse" and may exhibit fewer stable vices. In