Reproductive anatomy of the male
Anatomy of the Male Cat, by Heather Ault, adapted from a diagram in The Cat: Its Behavior, Nutrition and Health
Let's begin by going over the reproductive anatomy of dogs and pointing out some unique features.
Starting with the male. One unique feature of male dogs as compared to many other mammals is the presence of only one accessory sex gland. On this diagram, locate the prostate. The prostate gland is the only accessory sex gland of male dogs. The prostate produces a watery secretion that is ejaculated prior to the sperm-containing fraction of the ejaculate.
Now find the penis. The dog's penis itself has two unique features. One is the os penis, or penis bone. This functions to provide rigidity for intromission, or entering the female's vagina. Once inserted, another important feature comes into play…the bulbus glandis. This area of the penis will swell when the penis is inside the vagina, which forms a sort of locking mechanism to allow for prolonged contact during and after ejaculation. This increases the chance of successful fertilization.