Neutrophils represent up to 70% of all leukocytes in the blood stream (in humans there are approximately 4.4 million neutrophils / millilitre of blood) and are distinguished by their irregular multi-lobed nucleus and indistinct granular appearance. They are 10-12ľm and circulate in the blood for minutes to hours (average 6-8 hours) and lasting for 1-2 days in tissue. Although the most abundant leukocyte in the blood, the vast majority of neutrophils are found in the bone marrow (5x more) mostly as functionally immature precursor cells, although this varies between species with mice having a large pool of functional neutrophils in their bone marrow.

Olfaction is the sense of smell, which is the ability to perceive and distinguish odours. Most mammals have a good sense of smell, but most birds generally do not. The sense of smell is well-developed in carnivores (predators) and ungulates (prey). Fish also have a fairly well-developed sense of smell. Olfactory and gustatory receptors can combine to contribute to flavour.

The oesophageal groove is present in newborn ruminants. It is a channel taking milk from the oesophagus into the abomasum, bypassing the rumen, reticulum and omasum.

The oesophagus (or gullet) is a muscular tube which transports food from the pharynx to the stomach. A bolus of food is passed down the oesophagus by peristalsis. The oesophagus is divided into cervical, thoracic and abdominal sections.

The omasum is the third chamber in the ruminant stomach. It lies within the intrathoracic part of the abdomen so cannot be palpated manually. Instead it is examined by ausculation. The omasum has biphasic contractions which squeeze fluid out of the food before allowing the ingesta to continue into the abomasum. Absorption of volatile fatty acids continues in the omasum.

Oogenesis is the process of producing the female gametes, the Ovum, from the primordial germ cells. The majority of the steps in oogenesis, up to the point of producing primary oocytes, occur pre-natally. Therefore, females are born with all of the Primary Oocytes that they will ever have as primary oocytes do not divide further.

The oral cavity is the first section of the alimentary tract that receives food. It provides the digestive functions of prehension, mastication and in salivation and also plays a role in the respiratory system through oral breathing when the nasopharynx is impaired. The oral cavity or mouth, consists of accessory structures (the salivary glands), projecting structures, (the teeth and the tongue) and the walls enclosing the oral cavity.

Osmosis is the passive movement of water across a semi permeable membrane. It occurs in the opposite direction to diffusion of ions. Water moves from a region of low solute concentration and therefore high water concentration to a region of high solute concentration and low water concentration.

The ovary is the female Gonad homologous to the male Testes. It is usually a paired organ in domestic species, but in the bird only the left Ovary is present. The structures found within the ovary are undergoing constant changes throughout the oestrus cycle from the Follicles containing Oocytes, to the formation of Corpus Haemorrhagicum, Corpus Luteum, and finally Corpus Albicans. Ovaries are ellipsoidal in shape with an irregular surface due to the projection of dominant follicles and corpora lutea. These irregularities are absent in the mare due to the cortex and medulla being reversed with ovulation only occuring from the ovulation fossa. They are greatest in Polytocious animals such as the sow due to many dominant follicles, and so corpora lutea, developing at once.

The Oviduct is the tube that links the ovary to the uterus and which the ovulated oocyte travels down to become fertilised by sperm present in the female tract. It is also refered to as the Fallopian tube, Uterine tube or Ovarian tube.

The Peripheral Nervous System is made up of cranial and spinal nerves. Spinal nerves are named after the vertebra immediately above it, except for cervical vertebra. There are 7 cervical vertebrae and 8 cervical spinal nerves. The peripheral nervous system can be divided into the somatic nervous system and autonomic nervous system.

Pain may activate the stress response and hence hormone secretion from the pituitary glands, adrenal glands and pancreas. This leads to substrate mobilisation and catabolism, particularly protein wasting. These effects can impair wound healing, and in the long term result in immunosuppression. They can also cause a negative energy balance, giving weight loss or poor growth in young animals. It is important to bear this fact in mind when considering the economics of analgesia in food animal production.

The pancreas is a tubuloalveolar gland and has exocrine and endocrine tissues. The exocrine is the larger of the two parts and secretes pancreatic juice; a solution containing enzymes for carbohydrate, protein and triacylglycerol digestion. Pancreatic juice drains into the small intestine where it is functional. The endocrine part secretes hormones for the regulation of blood glucose concentration, including insulin, glucagon and somatostatin. The functional units of the endocrine part are the islets of Langerhans.

The paranasal sinuses are ventilated spaces connected to the nasal cavity. They develop as blind ending pouches between the lamina of the bones of the skull.

The parathyroid glands are multiple (generally four) small glands, approximately 1-2mm in length are located about the cranial trachea. Generally, there are two internal glands embedded within the thyroid Glands, and two external glands are outside the thyroid tissue. However, all of the parathyroid tissue may be embedded within the thyroid gland itself. In the horse, there are 'nests' of parathyroid tissue along the neck to the thoracic inlet.

Nerves of the peripheral nervous system (PNS) are composed of numerous bundles of nerve fibers that are surrounded by connective tissue. This connective tissue also contains a specific layer that is specialised to neurons; the peri-neurium. The outer layer of this connective tissue is called the epineurium and it surrounds both the perineurium and the nerve itself. Individual neurons found within each bundle are surrounded by the endoneurium.

The peritoneum is the serous membrane that lines the abdominal cavity. It lies directly beneath the abdominal musculature (rectus abdominis and transverse abdominis). It is a type of loose connective tissue and is covered by mesothelium. Extensions of the peritoneum form the mesenteries, omenta and ligaments that support the abdominal contents. The peritoneum produces fluid to lubricate abdominal viscera. The peritoneum also enhances immune responses and walls off infection in the abdomen to prevent peritonitis.

Phagocytosis is a very primitive system of defence against infection, having even been shown to exist in invertebrates and single cell organisms. The discovery was made in starfish larvae by Elle Metchnikoff who subsequently won the Nobel Prize for Medicine or Physiology in 1908. The process of phagocytosis itself is a form of endocytosis (cell eating), with vesicular internalisation being the method of removal of pathogens and dead cells (those that have undergone apoptosis, or Programmed Cell Death). This internalised vesicle is referred to as the "phagosome".

The pharynx is part of both the respiratory and digestive system. Both systems have entrances to the pharynx but they are separated from each other by the soft palate. During exercise or during respiratory distress, the mouth can be used as an additional opening of the respiratory system and then the oropharynx also becomes an air-way.