The retina
We promised last week that we’d discuss the retina in more detail. One can describe the retina as the nervous coat on the inside of the eyeball. The optic nerve arrives on the inside of the eye’s posterior chamber (remember that the eyeball is divided into two main cavities). There, the fibres of the optic nerve spread out in all directions to form the retina. In the retina are absorptive pigment cells which prevent the diffusion or reflection of light within the eyeball. In the retina are tiny structures known as rods and cones.
The rods and cones receive rays of light and colours, then convey the sensations of light via the optic nerve fibres to the brain.
In the dog, there are not as many rods and cones as in other species. Consequently, dogs don’t distinguish colours well. Some researchers insist that dogs are actually colour blind and see only in black and white or in shades of grey.
On the whole, dogs don’t see well. For one thing, they are shortsighted. That would explain why there is sometimes a perplexed look on a dog’s face when you arrive back home from a month’s holiday. They hear your arrival (well developed sense of hearing) and might recognize your scent (well developed sense of smell), but they don’t see you distinctly until you come up close and pat and speak to them.
Of course, the dog compensates for this ‘deficiency’ by having a larger visual field (compared to humans, for example). The dog’s eye has a larger pupil which allows more light to enter. As a result, the dog can follow moving objects (catching a thrown ball) pretty well. Also, in the retina, there are more light receptors, so the dog has an advantage of seeing well in the dark.
Tear glands
Actually, the correct term should be “lacrimal apparatus” because it is not only the tear glands themselves that are of importance, but also the canals (ducts) that lead the secretions eventually to the nose. That’s why when we cry (increased secretion of the tear glands), we also begin to sniffle due to the excess fluid in our nostrils. The tear glands have two obvious functions:
(i) the lubrication of the outer surface of the eyeball,
and,
(ii) the removal of particles of dust and debris which might accumulate on the eyeball. This is a cleansing function. By the way, the constant blink reflex ensures that the secreted fluid is distributed evenly over the outer surface of the visible part of the eyeball. The constant (normal) production of lacrimal fluid does not build up, mostly because the fluid evaporates.
As an aside, please allow me to involve you in the GSPCA’s fight to have squibs and other explosive devices kept at a minimum during the imminent Easter celebrations. As is documented above, dogs are well endowed with a superlative sense of hearing. The noise made by squibs is an agony to dogs. They react to the noise in ways that can be destructive to their well-being. Vets encounter dogs with broken legs after they have, in a deranged state, leapt from the verandah in an effort to escape the amplified sound; they try to hide in places that give them severe lacerations; they run away from home (trying to avoid the explosions); their whole behavioural patterns and attitudes could undergo serious changes, some of which can be irreparable (in extreme cases, dogs can cower and tremble uncontrollably at the slightest sound).
If you see persons in your neighbourhood throwing squibs at dogs (or tying lighted fire crackers on to their tails/bodies), please report such practices to the Police (911; 225-6411; 225-2317; 227-4064-5) or the GSPCA (226-4237).
Next week, we’ll talk about the methods we can use to examine the eye and how best medicine can be applied to the eye.
Please implement disease preventative measures (vaccinations, routine dewormings, monthly anti-Heartworm medication, etc) and adopt-a-pet from the GSPCA’s Animal Clinic and Shelter at Robb Street and Orange Walk, if you have the wherewithal to care well for the animals. Do not stray your unwanted pets, take them to the GSPCA’s Clinic and Shelter instead. If you do not wish your pet to have puppies or kittens, you may exploit the GSPCA’s free spay and neutering programme. If you see anyone being cruel to an animal, or if you need any technical information, please get in touch with the Clinic and Shelter by calling 226-4237.
