How vision works

The nerve impulses travel along the optic nerve to the visual cortex area of the brain. The visual cortex area of the brain processes the billions of pieces of information that arrive and form them into a meaningful image for the animal.

Retinal disease

Progressive retinal atrophy (PRA) is a group of genetic diseases seen in certain breeds of dogs and, more rarely, cats. It is characterised by the bilateral degeneration of the retina.

Generalised PRA is the most common form and can be divided into either dysplastic disease, where the cells develop abnormally, and degenerative, where the cells develop normally but then undergo a damaging change. PRA can be further divided into affecting either rod or cone cells.

It causes causes progressive vision loss culminating in blindness. The condition in nearly all breeds is inherited as an autosomal recessive trait.

Why do abscesses smell so bad?

Abscesses may be caused by many different bacteria, fungi, protozoans or by other foreign materials (e.g. a grass seed).

An abscess is a collection of pus (dead neutrophils) that forms a cavity in tissue. It is a defensive reaction designed to prevent the spread of infectious materials. The presence of these organisms damage and kill surrounding cells. The cells release cytokines - chemicals that trigger the inflammatory response. They attract neutrophils and other white blood cells to the area. They also increase the local blood flow resulting in a red appearance (erythema) and feeling of heat. As pus collects, a wall or capsule is formed by the nearby healthy cells in order to prevent the infection spreading to surrounding tissue. This capsule also tends to keep other immune cells from getting at the causative organisms.

This multiplication of organisms and the cellular destruction also forms a cocktail of some unpleasant chemicals. The exact mix depends on the organisms. Gasses such as ammonia, methane and sulphur dioxide (think rotten eggs) may be produced. Two other compounds are largely responsible for the foul odour of putrefying flesh. The delightfully named putrescine and cadaverine, are produced by the breakdown of amino acids in the tissue.

But what is the point of all this stink? To us the smell is disgusting and we are repelled by it. We instinctively avoid anything that creates such odours helping to protect us from exposure to infectious agents.

Why do nerve impulses only travel in one direction?

GFP expressing pyramydal cell in mouse cortex. Wei-Chung Allen Lee, et al.

Neurones (or nerve cells) are said to "fire" when an electrical impulse travels along the length of a neurone. But what triggers it and how does it spread?
The fluid inside and outside a cell contains many different kinds of charged particles called ions. Some are negatively charged and some positively. Two important ones are Na+ (sodium) and K+ (potassium).
The resting state
When the neurone is in a resting state it is actually working quite hard. The cell membrane has a structure call a sodium-potassium pump which pumps Na+ to the outside and K+ to the inside of neurones. Because Na+ cannot readily diffuse across the cell membrane a higher concentration of Na+ builds up on the outside of the cell. This redistribution of ion together with the ones from other ions and proteins results in a charge being formed across the membrane - the inside is more negatively charged.

Firing off
When a neurone is stimulated - from an adjacent neurone or an external stimulus (heat, touch) - changes take place in the membrane. Sodium channels in the membrane open briefly and Na+ floods into the cell interior. The inside of the cell changes from negatively charged to positive. This process is known as depolarisation.

Recovering
Just as the sodium channels snap shut potassium channels open and K+ diffuses out of the cell. This causes the charge the inside the cell to head back towards negative again. The sodium-potassium pump works hard and restores the Na+ and K+ to their original sides of the membrane in resting state.  This process is known as repolarisation.
This electrical event is called an action potential.

The refractory period
So the cell membrane goes through a cycle of depolarisation and repolarisation. If another stimulus arrives at the membrane during this cycle, the area cannot depolarise again. The current cycle has to finish before it is capable of depolarising again. The refractory period refers to this time when the membrane is not susceptible to depolarising.
And here's why it travels in one direction
An action potential starts at on end of a neurone and spreads as a wave along the it. The currents flowing inwards at one point on the neurone also depolarises the adjacent sections of the membrane. Once an action potential has occurred at a patch of membrane, the membrane patch needs time to recover before it can fire again (the refectory period). For this reason, only the unfired part of the membrane can respond with an action potential. The part that has just fired is unresponsive until the action potential is safely out of range.
Typically the action potential starts at the axon end (by stimulation from another nerve) and travel along a neurone to the synapse end.
Clinical implications
Local anaesthetics are membrane stabilising drugs;. They decrease the rate of depolarisation and depolarisation of neurones. These drugs act mainly by inhibiting sodium influx in the neuronal cell membrane. When the influx of sodium is interrupted, an action potential cannot arise and signal conduction is inhibited.
Disturbances of sodium or potassium levels in the body can directly affect these chemical processes and will interfere with normal nerve conduction.
Low serum potassium levels will often cause a generalised weakness. Increased potassium levels interferes with depolarisation of cells resulting in inability of nerves to fire.
High levels of sodium will cause weakness initially and then with more severe elevations of the sodium level, seizures and coma may occur.

What makes eye colour?

Eye colour is determined by the amount and type of pigments in the iris and phenomena such as the diffraction of light. The genetics of eye colour are complicated, and colour is determined by multiple genes.
In mammals, there are many variations in eye colour: blue, brown, grey, green, and others.
Avian eye colours range from dark brown and yellow through red, blue, and green to metallic silver and gold. In some species, eye colour differs between the sexes.
The superficial (stromal) layers are richly supplied with blood vessels and contain scattered cells with pigment granules. The deepest (epithelial) layer of the iris contains large amounts of dark pigment (melanin). 
Blue eyes have the usual deep epithelial layer full of dark pigment but the upper layers are thin and have little or no pigment granules. White light is made up a spectrum of colours. The red wavelengths penetrate and are absorbed in the epithelial layer. The blue wavelengths are scattered by the sparse pigment granules and the iris appears blue.
Brown eyes have lots of pigment granules in the superficial layers and the pigment cells are closely packed. This absorbs most light and the eye appears brown.
Hazel, green and grey irises have progressively less stromal pigment and so show colours in between blue and brown.
The brightly coloured eyes of many bird species are largely determined by other pigments, such as pteridines, purines, and carotenoids.
Albino animals are unable to make the dark pigment melanin. The iris colour is pink as the rich stromal blood supply provides the colour.
Heterochromia is a condition in which one iris is a different colour from the other iris. It usually results due to uneven melanin content and may be inherited or acquired by disease or injury.

Heterochromia

Albino iris