Penis
Domestic cats have barbed penises, with about 120–150 one millimeter long backwards-pointing spines. Upon withdrawal of the penis, the spines rake the walls of the female's vagina, which is a trigger for ovulation. Lions also have barbed penises. Male felids urinate backwards by curving the tip of the glans penis backward. When male cheetahs urine-mark their territories, they stand one meter away from a tree or rock surface with the tail raised, pointing the penis either horizontally backward or 60° upward.
Cat senses
Cats have a "tapetum lucidum", which is a reflective layer behind the retina that sends light that passes through the retina back into the eye. While this improves the ability to see in darkness and enables cats to see using roughly one-sixth the amount of light that people need, it appears to reduce net visual acuity, thus detracting when light is abundant. A cat's visual acuity is anywhere from 20/100 to 20/200, which means a cat has to be at 20 metres to see what an average human can see at 100 or 200 metres. Cats seem to be nearsighted, which means they can't see far objects as well. The ability to see close objects would be well-suited for hunting and capturing prey. In very bright light, the slit-like pupil closes very narrowly over the eye, reducing the amount of light on the sensitive retina, and improving depth of field. Big cats have pupils that contract to a round point. The tapetum and other mechanisms give the cat a minimum light detection threshold up to seven times lower than that of humans. Variation in color of cats' eyes in flash photographs is largely due to the reflection of the flash by the tapetum.