1. The Origin of the Dog

 

2.The dog's evolution

 

3.The Origin of the Dog Revisited
1. The Odyssey of the Word "Molossus"

2. The Evolution of brachycephalic molossoid dogs

1. Man's relationship with animals

2. Dog and Human societe

3. The dog shows in Europe

1.The Vision in dogs

2. The hearing of the dog

3. The sense of smell in dog

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The Vision in dogs
The visual system of dogs operates differently than the corresponding one of other mammals. For example in human, the analysis of visual information are been processed in the brain with a unique way, in order to understand the information and the perception of the surroundings. The dog on the other hand, when searching the environment is based on a combination of senses, with dominant the smell, rather than the sight. However, this does not mean that its visual ability is less developed than in humans. In fact it is the different adaptation during evolution, in an environment that is constantly changing.
If we try to analyze it, we would say that the dog’s eye is equipped:
a) With a hole which allows entry of light and
b) With a diaphragm of alternately increasing and decreasing diameter (iris) which adjusts the brightness.
Inside there is a crystalloid lens of alternately increasing and decreasing curvature, which focuses the light and also controls the size of the image, as well as its sharpness. This focusing is done in the depth of the eye onto the retina, a film filled with light-sensitive cells that will catch the light and will transport it through the optic nerve to the brain, for further processing.
The basic vision in mammals
The eye of the dog, as constructed, resembles that of other mammals. But also has distinct differences with them, due to the different adaptation to survive. Generally the dog, which through the evolution became a predator, is specialized in the twilight (early evening) and at dawn (early morning). On the other hand human has been adjusted for daylight conditions, with a better understanding of color and depth of field.
Comparing the human eye to the dog’s eye, we see that the pupil (cornea) is larger. Internally also the crystalloid lens is adjusted to have better performance (larger aperture) in low light conditions, thereby sacrificing the depth of field and the absolute sharpness. (Coren  2004).
In order to understand this, we must use the principles of optics, as applied to cameras. Assuming that the pupil of the dog represents a focusing lens, the greater the diameter, the more light is allowed to pass through the eye. So in low light condition it sees better. However, this large lens gives blurred peripheral vision, due to the large angle with witch the light meets the lens periphery. So clear image reflected on the retina is going through centrally and around the center of this lens. So the dog's eye has good clarity when looking away and more blurred when trying to focus near.
Also in dogs we have an additional "reduced" capacity, compared always with the human eye. The horizontal movement of the dog’s eye is wide and it can focus with both eyes on the beak, but also to see the side quite well. On the contrary, vertical movement of the eyes is limited. Therefore, the range of motion is limited. For this reason, if the dog does not bend, it can’t turn its eyes perpendicular to the ground adequately. Also, if it does not lift throat or not "tilt" head to the side, it can’t see the sky. This lack of vertical scan of the field, reduces the perception of depth and thus the three-dimensional perception of the object. The horizontal scanning in conjunction to the vertical, gives the feeling of volume in the object, in combination of course with the binocular vision. So the dog sees the object approximately, since what is at the center of the visual field is clear, while anything at the periphery is unclear, i.e. blur.

 After the light passes through the cornea and the crystalloid lens, it will be gathered and reach the retina, which is full of photosensitive nerve cells, of two types, just as in humans. These, because of their shape, are called rods and cones (they work as the prism in the analysis of light). The rods are those that realize the night light (i.e. light - dark) and the cones, because of their conical shape, they analyze the light beam. There are three types of cones, each responsible for the analysis of a specific wavelength. The analysis by the brain of these different in intensity colors creates the feeling of the iris colors (rainbow). (Coren,  2004 & Lindsay, 2000).
In the retina of the dog’s eyes there are 97% rods and 3% cones and the cones are much less than they are in humans. So, the perception of low light and sense of movement in the dog is extremely high, while the color vision is reduced, but not nonexistent. "(Lindsay,  2000)".
Another difference in the dog's eye is the absence of the retina fovea. The fovea is the central part of the macula. The macula is the area of the retina that contains the highest density of photosensitive cells and most of these are cones. The central region of macula is the fovea, where rods are almost absent and is the point where the light ends up concentrated and "captures" the image. At this point we have the maximum resolution (image clarity) and color perception. This concerns the human and dog’s eye. But in the corresponding area of the dog’s eye, instead of fovea there is a visual streak (vertical streak) with more photosensitive cells, which are responsible for clear printing, the binocular vision and a horizontal scan. "(Lindsay,  2000)".
Recent studies on visual streak of dog’s eye proved that the shape and construction of the eye is directly related to the length and shape of the snout. According to these studies, the dogs with long snout have also a horizontal line of photosensitive cells in high density at the retina. This axis in a horizontal rib, allows to these dogs a horizontal clear vision over 200 degrees and as departing from this horizontal axis, the picture becomes fuzzy and blurry. But dogs with short snout or too short nozzle (brachycephalic) have around the visual streak a concentric region, rich in light-sensitive cells, with the central region having the highest density, so these dogs have a very clean central region  and a blurred regional, like the human eye. (Bark 2004)
Based on the above, dogs with long snout (dolichocephalic) have very good vision for long distances and good peripheral vision in the horizontal axis, but reduced and blur vision for nearby objects, while dogs with short snout have very good close and central vision, so as to distinguish even the facial expressions of their owner, but peripheral vision blurred and vision for distant objects reduced.

Another structural difference compared to the human eye, is that the dog’s eye is equipped with a reflective mat (film behind the retina).
 It is a shiny reflective surface, which reflecting the light back increases its feeling, in low light conditions. It is the glow seen in the eyes of the dog at night, when light falls on them. And just below this reflective mat, there is another dark surface, which absorbs the excessive brightness, reducing the impact of the dispersion of light into the eye. Eventually, both these mats are intended to regulate the extra light diffusion and its concentration, in order to have a more clear impression by light-sensitive cells. "(Lindsay,  2000)".

There is an impression that dogs can’t distinguish colors. This is not quite correct, since dogs have the ability to distinguish colors, but not so strongly in gradation, as with humans. Since dogs lack in number of cones than the human eye and since the cones are responsible for visual acuity and color distinction, it makes sense that dogs have limited this ability. Furthermore, from the three types of cones in the dog’s eye there are only two, so the perception of colors is defisit. (Coren,  2004).
So the dog can perceive colors like dark blue, light blue, gray, light yellow, dark yellow (like coffee). Instead, it is difficult to perceive the dark gray and black. With this fact we understand why dogs, during the hunting search, they use more the nose rather than the eyes, for example a prey hidden and motionless.
So the visual acuity in dogs is lower than that of humans and that is because dogs, during their evolution, have developed more their sense of smell.
We would say that the dog sees the world through a very thin veil of fog (Coren, 2004).
But this reduced clarity of vision, gives the dog an excellent perception of motion. This of course is important for a predator. Many studies have shown that the dog is able to perceive the slightest movement in a prey from a long distance, even longer than 500 meters, while if the same object is closer and stable it can’t perceive it visually, so it needs help by the smell.
Furthermore and according to the skull (brachycephalic, dolichocephalic or foleodytis), the eyes are located on the front, but the longer the snout greater is the field perceived by the dog. This field is wider than in humans (man 170 degrees and dog over 200 degrees).
It is important to understand that dogs evolved more on smell rather than sight and this because this way they achieved better adaptation for survival. This is a vital difference that we must understand in order to realize the diversity of the dogs and improve our communication with them. (Coren  2004).

So in conclusion we can understand why the Cretan Tracer has this form and its eyes have this position on the skull. But also why its snout has this shape as well as why it uses primarily the smell to find prey. Also we can understand why the games have gray colors and why the dogs or other predators have color variation to confuse the sight of prey in the environment. Also, the ability of sight of the dog must be taken into account during training, in order to get the best results.

THE EVOLUTION OF COLOR VISION.
Humans and animals see the world differently; some birds, rodents and certain types of fish can see ultraviolet light, some snakes can see infrared light waves. On the other hand, humans can’t see either of these lights. To give an example, dogs can see fewer colors than humans; however, they compensate for their weakness of color vision with a sense of smell which is said to be 10,000 times more powerful than humans’. The world of dogs is ruled by scent, while the world of humans is ruled by the ability to see. The ability of every living organism to see color has evolved, according to a view, based on random mutations while, according to another view, due to environmental adaptations.
As stated above, two types of cells (or  receptor cells) are responsible for the sense of vision: One type identifies the color (cones). The other one responds to varying degrees of light and to darkness (bars). Lately, there has been great progress in the study of genes constituting these cells and researchers have discovered many new facts about genes, particularly those related to the recognition of different colors.
Looking back at the process of biological evolution down to humans; the common ancestors of birds, reptiles and fish were tetrachromat (they could see four color spectrums: UV, blue, green, and red types). All mammals had the same ancestors. During Mesozoic era, when mammals had to live with dinosaurs, in order to survive; mammals became nocturnal and the ability to see four different colors became less crucial for survival. As a result, they lost the genes responsible for green and blue color recognition, which resulted in dichromacy, i.e. the two-color vision system (UV and red types).
So these mammals, which evolved this adaptation, became dichroic and gave descendants adapted to the environment where they were forced to survive. This confirms the known from Aristotle and Darwin theory of natural selection.
So a question arises, how living beings acquire new functionalities? There are two ways to change the genes, Redo and Mutation, changes that are inextricably linked with the evolutionary process.
The new cells are constantly being reproduced and they replace the old ones. Genes of the old cells are copied in the new cells during reproduction. The copying process is not always perfect. Reproductive errors occur, 1 every 1.000 million These occurrences are known as mutations.
Another kind of error called "covering", occurs when the copy of the same gene unexpectedly is made more than once. These genes can be replicated and evolve further adding new functionalities. But only when the new functionalities are adapted in the environment, then organizations will grow and spread. Otherwise they will fail to produce descendants and disappear.
When the dinosaurs went extinct about 65 million years ago, the nocturnal ancestors of mammals returned to daylight activity. Some 30 million years later, the genes that these animals possessed for seeing red and blue duplicated and then mutated, which gave rise to the green-type gene capable of perceiving green light.
Eventually, the three-color vision system (trichromat) developed. Mammals gained dominance in the jungle since they had the ability to make fine distinctions in color. It was then that our ancestors gained the ability to see color as we enjoy it today.
With the ability to distinguish more colors, our ancestors became less sensitive to smells and pheromones. Unlike dogs which maintained a relatively less color perception, while they increased olfactory ability both to odors and pheromones.
In general we can say that as visual acuity and color perception increases, so the olfactory sense decreases.