Sensory organs (display 91)

For mammals, the capacity to maintain balance depends on receptors located in a region of the bony labyrinth called the vestibule, which contains two sacs of the membranous labyrinth: the saccule and the utricle. Each of these contains a sensory layer, known as the macula, which bears mechanoreceptor hair cells that extend into a gelatinous membrane covered in calcium carbonate crystals called otoliths. The membrane, influenced by gravity, follows the linear movements of the head causing the hair cells of the sensory receptors to bend.

Three semi-circular canals branch off from the utricle, one in a horizontal plane and two in a vertical plane. At the base of each canal, a larger region known as the ampulla contains mechanoreceptors. The hair cells of these receptors extend into the cupula, a gelatinous membrane contained within the ampulla. When the head makes a rotating movement the endolymphatic fluid in the canals follows, deforming the cupula and causing the hair cells to bend.

The vestibular information is thus translated into electrical impulses and transmitted by the vestibulocochlear nerve to various parts of the central nervous system. The integration of this data is essential for muscular coordination and for maintaining visual focus.

Sight allows the acquisition and appreciation of environmental information that is essential in the lives of many animals. This sensory capability varies from one species to another due to a number of factors such as the position of the eyes, visual acuity (the capacity to discern details) and colour perception.

The eye is characterised by three anatomical layers or tunics:

External tunic. This is a fibrous tissue that supports and protects the eye. Towards the front it is composed of the cornea, a transparent membrane that allows light to enter, and towards the rear it is made up of the sclera (the white of the eye) which lends rigidity. In some species, structures of cartilage or bone help to maintain the shape of the eyeball. 

Middle tunic. This layer is characterised by the choroid, a vascularised tissue that nourishes the iris and retina. Composed of pigmented muscles, the iris regulates the opening of the pupil and thus the quantity of light entering the eye. Towards the front part of the choroid is the ciliary body whose tissues secrete aqueous humour. This liquid nourishes the cornea and the lens and maintains pressure within the eye. The ciliary body also includes muscles that modify the shape of the lens to focus light on the retina during adjustment.

Internal tunic. Composed of nerve tissue located at the back of the eye, the retina contains photoreceptors (cones and rods) and nerve cells that convert light into electrical impulses. These impulses are transmitted to the brain by the optical nerve. While the rods are very sensitive to light and play an important role in night vision, the cones are involved in day vision and the perception of colours. Visual acuity, for its part, depends on the number of visual cells within the fovea, an area at the centre of the retina with the greatest density of cones. 

The ability to differentiate colours largely depends on the presence of at least two types of functional cones within the retina.

Type L cones capture long wavelengths (primarily sensitive to red), M cones capture medium wavelengths (primarily sensitive to green) and S cones capture the shortest wavelengths (primarily sensitive to blue). Hence, most land mammals have dichromatic vision characterised by two types of cones (L-S or M-S). Primates from Africa and Asia have three types of cones, which gives these trichromats the ability to discern a wider spectrum of light.

Certain nocturnal animals, as well as those living in low light conditions, possess a reflective layer behind the retina known as the tapetum lucidum. It reflects light so that the photoreceptors benefit from a double exposure. This is the case with cats and alligators.

The eyes of cetaceans feature many adaptations to the aquatic environment. To begin, the similar density of water and the liquid component of the eye cancels out the refractory power of the cornea, making the lens the only element involved in focusing. Secondly, a very thick sclera protects the eyes from strong pressure when swimming or diving. In order to compensate for the lack of light in the ocean’s depths, many species have large pupils and a retina that is rich in rods. 

Finally, whales appear to have only one type of functional cones, type L, the gene for S cones being inactive. These creatures with monochromatic vision may, however, be able to differentiate certain colours in poor light thanks to rods that are able to provide additional spectral sensitivity. 

Diurnal birds of prey have an excellent capacity for visual discernment. Their retina is rich in cones and features two separate foveae which produce two highly effective monocular fields of vision.

However, the binocular field of vision, vital for evaluating distance, is reduced. Their eyes also lack mobility within the eye sockets. These shortages are mitigated by rapid movements of the head which simulate a binocular field of vision. The resulting effect is particularly useful for hunting: good perception of distance is vital for targeting prey that is both quick and small in size.