Oestrous cycle. The cycle of reproductive activity shown by most sexually mature non-pregnant female mammals except most primates (compare menstrual cycle). There are three phases:
(1) Anoestrus-no sexual activity, the female is neither receptive nor attractive to the male, reproductive organs inactive;
(2) pro-oestrus-reproductive organs become active;
(3) oestrus(heat)-ovulation normally attractive to the male. The length of the cycle depends on the species : larger mammals typically have a single annual cycle with a well defined breeding season (monoestrus). The males have a similar cycle of sexual activity. Other species may have many cycle per year(polyoestrus) and the male may be sexually active all the time
Oil. Any of various viscous liquids that are generally immiscible with water. Natural plant and animal oils are either volatile mixtures of tarpenes and simple esters (e.g. essential oils) or are glycerides of fatty acids.
Olecranon process. Bony process on ulna of mammals extending beyond elbow joint, for attachment of muscle which straighten fore limb
Oleosome. A large fatty inclusion in the cytoplasm of cell.
Olfaction. The sense of smell in which there is chemoreception of molecules suspended in the air.
Olfactory. Pertaining to the sense of small.
Oligycene. The epoch of the Tertiary period of the Coenozoic era, 38-25 million years ago
Ombrogenic. Dependent upon rainfall.
Omentum. A fold of peritoneum connecting the stomach with the other abdominal viscera.
