The visual system of the dog and cat

1 January 2023

Did you know that when it comes to vision, the eye is only the tip of the iceberg ? In fact, the visual system of our pets is much more complex than it seems.

What is an eye made of ?

The eye, also called the eyeball, is a relatively complex and fragile structure. The outer shell is made up of the sclera – the white part, often referred to as the “white of the eye” – and the cornea – the transparent part. The cornea reveals the iris, which can be of different colours, and which has a “hole” – the pupil – whose diameter varies according to the intensity of the light. Behind the iris is the crystalline lens, an intraocular lens that allows accommodation. This is attached to the ciliary bodies by the suspensory ligaments. Further down and covering the sclera is the choroid, rich in blood vessels. This structure is responsible for the “nutrition” of the posterior part of the eye and is itself covered by the retina, rich in photoreceptor cells.

There are two chambers and two segments in the eye :

The anterior chamber, which is located between the cornea and the iris;
The posterior chamber, located between the iris and the lens;
The anterior segment, comprising the two chambers mentioned above, which contains the aqueous humour responsible for the nutrition of these two entities – this is produced by the ciliary bodies;
The posterior chamber, located behind the lens, which contains the vitreous body.

These two liquids – the aqueous humour and the vitreous body – are also responsible for the pressure in the eye.

The eyeball is also surrounded by so-called appendages, which help to protect it. This is particularly the case with the eyelids, of which there are three in dogs and cats:

The upper and lower eyelids, topped by the eyelashes;

The 3rd eyelid, or nictitating membrane.

The lacrimal glands produce tears which offer both mechanical protection (by preventing the cornea from drying out and lubricating it) and immunological protection (tears contain numerous immune cells). The tears are then evacuated through the lacrimal duct.

The visual pathways

In reality, the eye is only the first step in vision. The analysis and interpretation of what this organ perceives takes place in the visual cortex, located in the posterior part of the brain.

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The importance of diet for skin and coat quality in dogs and cats

The skin is the body's largest organ, representing between 12% and 24% of the animal's weight, and its surface area in a dog weighing around thirty kilograms is close to one square meter. It's hardly surprising, then, that the quality and quantity of the dog's diet can have an impact on this organ, and in the event of a deficiency can lead to problems that go far beyond a simple visual appearance.

Key nutrients for healthy skin and coat

Among the macro-nutrients, proteins play a crucial role. They provide amino acids, including sulphur amino acids, which are essential for the structure of the skin and coat. These are the main components of collagen, which gives the skin its suppleness and resistance, and of keratin, which plays a part in the effectiveness of the skin barrier. Lipids are also important; they make up the bulk of cell membranes and, in the skin, contribute significantly to the effectiveness of the cutaneous barrier. Among lipids, omega-3 and omega-6 fatty acids help maintain healthy skin, modulate inflammatory mechanisms and promote a shiny, soft coat. Vitamins and minerals are also involved. Vitamin A is necessary for cell renewal, as is vitamin D, which also plays a part in the skin's natural defences. Vitamin E acts as an antioxidant, while B vitamins and biotin are essential co-factors in the synthesis of various skin elements such as ceramides, keratin and collagen. Among the minerals, zinc is involved in synthesis, protection against free radicals and immune defence mechanisms.

Disorders linked to inadequate nutrition

Inadequate or poor-quality food can lead to skin problems in our pets. A deficiency in essential fatty acids leads to dry, flaky skin, a dull coat and brittle hair. When it's the vitamins and minerals that are in short supply, synthesis is affected, the skin barrier can be less effective and the breeding ground for infections and inflammations that can lead to itching. On the other hand, these nutritional benefits can be used to compensate for individual sensitivities and improve the quality of the skin and coat in the event of dermatological disorders or diseases.

Conclusion

The quality of a dog's skin and the beauty of its coat are directly influenced by the quality of its diet. However, some dogs and cats have increased needs, and it is therefore necessary to provide them with greater quantities of useful nutrients. This is particularly true in the case of certain dermatological disorders, to help compensate for imbalances and accompany the medical treatments prescribed by your vet.