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.

The phospholipid bilayer is the fundamental structure which makes up the cell membrane. It is made of 2 sheets of phospholipid molecules which are said to have hydrophillic heads and hydrophobic tails. Therefore molecules on opposite sheets face back to back to protect their hydrophobic area from the surrounding intra or extracellular fluid. This creates a region inside the membrane which is hydrophobic.

The pituitary gland, or hypophysis is an elongated appendage of the brain lying within a bony cavity of the sphenoid bone in the base of the skull - the Sella Turcica. The hypophysis is suspended from the hypothalamus by a thin stalk. It lies between the more rostral optic chiasma, and the more caudal mammillary bodies.

The pituitary gland, or hypophysis is an elongated appendage of the brain lying within a bony cavity of the sphenoid bone in the base of the skull - the Sella Turcica. The hypophysis is suspended from the hypothalamus by a thin stalk. It lies between the more rostral optic chiasma, and the more caudal mammillary bodies.

Growth Hormone, also known as Somatotropin is a hormone produced by the Somatotropes of the anterior pituitary gland.

The surface of the inner wall of all of the body cavities is lined by a serous membrane which consists of a single layer of flat epithelium with a thin underlying propria (connective tissue). Within the thoracic cavity, this is known as the pleura. The visceral pleura which coats the outer surface of the lung is derived from the splanchnic mesoderm. The parietal pleura lining the thoracic cavity is derived from somatic mesoderm. The pleural cavity is a potential space between the two areas of pleural membrane, which normally are adhesed to each other.

The rabbit is a monogastric hindgut fermenter and is a herbivore. Microbes in the hindgut produce volatile fatty acids (VFAs) which are available to the animal for energy. Microbes also produce vitamins and protein, which are available only in minimal quantities as they are produced in the hindgut (see advantages and disadvantages of hid gut fermentation). Most microbial fermentation occurs in the caecum (as opposed to the horse where most occurs in the colon). Rabbits usually eat at dusk.

An important consideration when sampling from a population is that of random error (also known as sampling error), which results from chance variation in the members of any sample taken from a larger population. Random error may affect the conclusions you draw from a study by affecting the precision of a descriptive study, or the power of an analytic study. However, although the magnitude of random error can be quantified to some degree, its direction cannot be predicted due to its random nature. Random errors can be accounted for to some degree through the application of inferential statistics when presenting and interpreting results.

The innate immune system recognises components of pathogens which are intrinsically foreign (i.e. not present on normal mammalian cells), such as Lipolysaccharides, Peptidoglycans and D-isoform amino acids.

The rectum lies between the terminal portion of the descending colon and anus. It is empty most of the time, except after the mass movements of the large intestine which move faeces into the rectum. This stimulates defeaction, which may happen when an animal is frightened.

A reflex arc represents a mechanism by which a physiological function is automatically managed or regulated. Reflex arcs can be found throughout the body, ranging from skeletal muscles to smooth muscle in glands. Reflex arcs are initiated via the excitation or stimulation of specific sensory cells that are directly connected to motor neurons thus enabling motor nerve impulses to be automatically passed on to that particular muscle or gland. Therefore a basic reflex arc consists of sensory cells and their associated nerve fibers, motor nerve fibres and the ultimate muscle or gland.

Reproductive hormones often have multiple roles and operate via negative feedback systems. The information below will provide the main reproductive hormones in domestic species and their functions.

Development of the reproductive system is a complex process relying on numerous systems and conditions to ensure appropriate structures are formed and the rate of development is maintained within the embryo. There are several important criteria that determine appropriate reproductive system development; genetic sex which determines the sex of the embryo, gonadal sex and phenotypic sex which both determine sexual differentiation. Genetic sex determines gonadal sex which in turn determines phenotypic sex. Sex determination relates to the genetic or environmental process which determines the sex of an individual whilst sexual differentiation represents the development of internal and external genitalia.

Reproduction is the complex set of biological processes that result in the formation of a new organism; it is crucial that we understand how these processes occur normally and have a good grasp of the role of hormones in the reproductive process. Pathology and disease are common within the reproductive system and can not only lead to a sick animal but also to a loss in production. In this section we hope to cover all the main processes in domestic animals, laying a foundation for understanding the associated pathological states.

Respiratory epithelium is a type of epithelium which lines both the upper and lower respiratory tracts. It consists of multiple layers of cylindrical epithelium, along with cilia and goblet cells.