Introduction

The visual system is the specific receptor for light stimuli, which not only act on animals: let us recall photosynthesis in plants, mediated by chlorophyll, which transforms carbon dioxide and water into carbohydrates and oxygen, with accumulation of energy.
Light energy can act on living organisms in four different ways: influencing metabolic processes, affecting movements, being related to pigmentation or giving rise to the mechanisms of vision itself (physical, chemical, neural or psychological).

Effects of light on movement

They are of three different types:

    - Phototropism

Phototropism is more typical of plants and consists of the movement towards the light or away from it of the sessile organs (positive, negative or transverse heliotropism).

    - Photokinesis

Alteration of motility without directional orientation. A distinction is made between orthokinesis (acceleration or slowing down of movements) and clinokinesis (deviation of movements towards or away from light).

    - Phototaxis

Alteration of motility with directional orientation. It requires receivers that can distinguish the direction of the light source. Phototaxis is divided into:

a. Clinotaxis
It requires the existence of a receptor organ that compares the intensity of successive stimuli to direct the displacement in relation to the light in an irregular way (for example the orientation of the flagellates).

b. Tropotaxis
It requires two symmetrical receptors and the more exact movement is made by comparing the intensity of the stimulation of each of them (for example, the displacement of the worms).

c. Telotaxis
Orientation is carried out directly towards or from the light source. Although a single receptor is sufficient, it must be complex and composed of several elements. In addition, it is necessary to have a central nervous organization that analyzes stimuli and inhibits unnecessary or less intense ones (compound eye of insects).

d. Menotaxis
It includes the ability to move not only to or from the light, but also at an angle (light compass reaction). The birds' navigation system and the possible polarized second light orientation (dance of the bees) are based on it. It also includes orientation in front of an image and the dorsal or ventral light reaction that helps air or aquatic navigation.

c. Mnemotaxis
Which includes the existence of a visual memory with which current stimuli are compared.

 

Effects of light on pigmentation

The pigment that exists to absorb light or to filter it and has a less frequent protective (cryptic) or demonstrative (faneric) function and of social or reproductive value.

It requires the existence of organized cells or cell groups: chromatophores. The pigment can be melanin (melanophores), it will be composed of fat-soluble carotenoids (lipophors) or guanine (guanophores), the last two being of a great chromatic and iridescent variety.

The action of light on the pigment can be of three types:
- Direct on the melanophores of the skin or even on retinal or iridian ectoderm: direct pupillary contraction in fish and amphibian cephalopods.
- Indirect through the eye.
- Indirect through the midbrain (some fish) or the pineal gland (lamprey, teleost fish).

Vision

The essential and primitive function of vision is the control of movements to achieve as easily as possible an optimal environment. This is used to avoid obstacles, to chase prey or flee from predators and in man requires a close relationship between the eyes and the vestibular apparatus.


Higher levels of visual function are, by their very nature, conscious, although in lower animals (eg, the ant's gait) they are not, and neither naturally in machines regulated by cybernetic feedback mechanisms. The process of “awareness” of vision has coincided, or has been due, to the migration of the terminal station from the basal nuclei to the cortex (which is a recent fact in the history of evolution).

Evolution of the Visual Apparatus

Although in lower animals there is diffuse light sensitivity throughout the ectoderm, soon part of the ectoderm specializes in light reception that is limited to photoreceptors. These are characterized by the presence of pigments: melanin, which is chemically inert and acts only as an absorbent, and the visual pigments themselves. All these are based on carotenoids associated or not with proteins, which are also found, in addition to chlorophyll, in the plant kingdom (xanthophyll). When the level of the eye with optical imaging ability is reached, all pigments are specifically related to vitamin A (A1 or A2).

In unicellular organisms, diffuse light sensitivity can be concentrated at a point that is associated with cilia or flagella (motor function). In flagellates this "eye point" or "stigma" has pigment and occasionally a retractable structure that concentrates light and acts, like a lens (crystalline).

In multicellular beings, some ectodermal cells soon specialize in light reception as bipolar cells (a distal receiving end and a proximal transmitting end) or apolar cells of ellipsoidal shape. From here begins the evolution of the eye in invertebrates.

The invertebrate eye

It derives from the surface ectoderm and secondarily connects with the nervous system.
It can be simple, either unicellular or multicellular, or compound.

- Simple eye an ocelo.

It consists of one or more light-sensitive cells that act without a functional association.
When it is multicellular it can be still subepithelial (just a collection of photosensitive cells) or in a more advanced stage invaginate epithelial. This last type passes through the flat, cupular (formation of a dark chamber) and finally vesicular phases, which is still perfected in the eye of the cephalopods in which a thickening of the epithelium already forms a “shadow” of the lens, and there is also a contractile pupil.

Although the existence of an inverted retina (in which the photoreceptor elements are in the deepest layer and the transmitters are on the surface, which allows better nutrition of the former) is characteristic of vertebrates, some mollusks and arachnids already have this new retinal organization, instead of the inverted retina; or direct. This change in the orientation of the retina is one of the most difficult processes to explain in light of the theory of continuous evolution. In addition, in animals that have to live in poor lighting conditions, a reflective crystalline layer or tapetum appears behind the retina.

- Compound eye.

In the compound eye the sensory elements are structurally and sensory grouped. Each one of them, called ommatidium, consists of a corneal facet, a crystalline cone, an iridian pigmented cell and a retinae composed of photoreceptors that end in a prolongation common to several of them, called a rhabdom, in which transmission begins. of impulses to the nerve centers.

The compound eye, peculiar to arthropods, adopts different varieties in arachnids, crustaceans and insects. Although it only makes it possible to achieve a reduced visual acuity, which varies depending on the greater or lesser concentration of the retina on the rhabdomes, the compound eye has the ability to analyze the plane of polarization of light, an essential function for orientation in space.

The eye of vertebrates

It supposes an evolutionary revolution since, with the differences that we will see, it has a common pattern in all species and the continuous process of improvement, along different pathways, that is appreciated in invertebrates is not attended here. Unlike invertebrates, it derives from the neural ectoderm.

The vertebrate eye does not show the increasing differentiation seen in the brain, ear or heart, and the eye of a fish is essentially as complex and fully developed as that of a bird or mammal. This is only one, although for us the most interesting, of the mysteries that the understanding of evolutionary mechanisms continues to throw from animals without a skeleton to others that have it so complete.

Schematically, the vertebrate eye is differentiated into three types depending on the habitat:

- That of ichthyosids, fish and amphibians, adapted to an aquatic environment, in the latter only in the larval stage.
- Sauropsids, reptiles and birds, adapted to an air environment.
- That of mammals, ambivalent.

The fundamental element, the retina, has the same structure in all vertebrates, although the relationship between rods and cones varies depending on the lighting. Some think that the cones are the primitive visual cell and that the rods are a later element due to a developmentally related transmutation of rhodopsin, which occurred first in deep-sea fish. In the rest of the ocular structures, there are considerable differences in vertebrates that are the consequence of adaptation to very diverse environments: initially the eye adapted to shallow waters but later it has evolved, depending on the case, to existence in abysmal waters. in rivers, in the mud of swamps, on the ground and in the air as well as at night, twilight or semi-darkness of caves or the most intense daylight. It has also achieved a panoramic vision when the perception of movement is essential or a stereoscopic vision when it is an extract calculation of the distances, oscillating between a simple vague apperception and the most exact resolution capacity or the most complex color vision.

Disregarding the few cyclostomes, we can thus summarize the main characteristics of the eye of fish, amphibians, reptiles, birds and mammals.

a. Fishes.

The cornea has practically no diopter value, so the eyeball tends to be very large and refractive crystalline. The cornea is flattened to better resist water pressure. The size of the eyeball is greater the greater the depth of the waters, in such a way that in the abyssal spaces, devoid of light, the eye degenerates and almost disappears. Even in species like the eel the eye grows as the animal travels from the days of Europe or America to the sea of ​​sargassum in the Atlantic where they breed and die. The very rod-rich retina is usually avascular. Eye movements are very restricted and fixation is achieved preferably with head and whole body movements. We already know the curious difference in visual pigments, that freshwater fish are made of vitamin A2 and that, like salmon, migrate from the seas to the rivers and consequently change the visual pigment.

b. Amphibians

The passage from the aquatic to the air environment allows the cornea to adopt dioptric functions, becoming strongly convex. The lens is smaller than the fish, allows accommodation through anteroposterior displacements. Protective structures appear such as the eyelids, glands and tear ducts.

c. Reptiles

The accommodation is carried out through a deformation of the lens, which requires the existence of a ciliary muscle that is attached to the periphery of the cornea. The retina is avascular and is nourished at the expense of the choroid or an extraretinal avascular membrane. They have a third eyelid.

d. Birds.

The eye is usually larger than the one that appears to have an accommodative mechanism through deformation of the lens. The retina is thick, with a great abundance of photoreceptors, cones and or rods more abundant according to life habits. The retina is nourished by a Pecten, an elongated vascular membrane that corresponds to the optic disc. In some types there are two foveas, one for distant vision and one for near vision.

c. Mammals

The eye of monotremes, platypus, is very similar to that of reptiles, that of marsupials has an intermediate structure between that of reptiles and that of placentates. This is the most abundant and varied subclass and includes fifteen orders: insectivores, chiropterans, dermatoptera, primates (with various suborders and families), rodents, carnivores, artiodactyls, perissodactyls, proboscids, sirenids, cetaceans.

The eye of the placentates differs from the rest of the vertebrates in three fundamental aspects:

- The development and subsequent involution of the hyaloid artery, which in many cases leads to intraretinal vascularization.
- In the appearance of the mesodermal layer of the iris.
- In the accommodative mechanism due to a deformation of the lens capsule.

Naturally, habitat and life habits impose minor variations in the rest of the ocular structures: almost spherical crystalline in those that live in an aquatic environment, presence or absence of a tapetum located between the choroid and the retina, variations in the thickness of the sclera (which in the whale is 3/4 of the ocular diameter), in the shape of the pupil and sphincter, in the vascularization of the retina (total, partial or absent: holoangiotic, mesoangiotic and anangiotic retinas) and in the proportion of retinal rods and cones.

One of the most interesting aspects (since the detailed description of each ocular structure in each order, suborder and family would take us too far) is that of the decussation of the optical fibers.
In the other vertebrates, decussation is practically total, which allows a binocular visual field that can span 360 ° but limits stereoscopic vision to less exact mechanisms than mammals. In these, especially in the placentates, he begins to see direct fibers in relation to the frontalization of the eyes. The proportion is 1/6 of the total in the horse, 1/4 to 1/3 in the dog and cat, 1/3 in the higher primates and almost 1/2 in the man. This process implies a loss of the extension of the binocular visual field but the acquisition of an exact stereoscopic vision. It has been speculated that this began to be necessary in animals that lived in an arboreal environment, in which the movements of the branch require an adequate calculation of the distances and was associated with the development of the hand as a prehensile organ.
We have already pointed out that only in mammals does an area of ​​the cerebral cortex appear where visual impulses arrive and are analyzed. Even so, except in man, this location is still imprecise and in most mammals ablation of the visual cortex does not cause complete blindness. In men, on the other hand, the only visual activity that remains subcortical is the pupillary.

Visual function

The environment and life habits, preferably daytime or nighttime activity, impose a wide range of variations in all aspects of visual function: light perception, color sense, sense of form, stereoscopic vision.

An example is a summary of the different mechanisms by which accommodation is achieved in vertebrates:

- Static mechanisms:

A pinhole pupil.
A duplicated optical system.
The interposition of a nictitating membrane.
A duplicated retina.
A ramping retina.
A rough retina.
A great length of the receiving elements.

- Dynamic mechanisms:

Displacement of the lens forwards or backwards, induced by corneal pressure or by own musculature.
Lens deformation, which in mammals is ultimately due to the elasticity of the capsule and its different thickness.

Visual acuity in birds reaches a resolution of 10o of arc, three times higher than that of man.

The monocular field of view ranges from 30 ° for the chameleon to 215 ° for the horse. In submerged fish, aquatic and aerial vision are combined, but subject to the restriction imposed by the total refraction of the water surface. The binocular, as we mentioned before, covers 360 ° in animals that can be prey and is reduced in predators. In fish with laterally positioned eyes, the blind area, considering a spherical surface, can be very small.