(1)
Faculty of Medicine of Montpellier, Montpellier, France
Abstract
Vision is a complex function that allows the identification of shape, colour and movement of objects. With two eyes, only one image is perceived in 3D: it is stereoscopy. This requires a perfect fusion of the two retinal images made by the optic chiasma and the retinal macula—a high-resolution point placed along the optical axis. Eye movements have to be executed around the geometric centre of the eyeball explaining why a protraction force given by the superior oblique muscle has to be used as a counterpart of the retrotraction force produced by the four recti muscles. The images are guiding the muscular dynamic coordination and are managed at the level of the superior colliculus representing the mandatory functional link between visual images and eye movements. The tectospinal neural pathway is the activating system of the eye/head servomechanism, allowing the movement of both eyes and head in case of large visual explorations. The oculomotor nerves (III, IV, VI) are purely motor nerves requiring a proprioceptive anastomosis with the sensitive nerve of the territory: the trigeminal nerve (V), which has for this reason the longest brainstem nucleus.
5.1 Introduction
Vision is a sensory programme and we have already largely spoken about it in Chap. 2. Still, we have to recall that the vision programme allows the identification of shapes and colours of objects in visual space, as well as of their movements.
In addition, we have two eyes and see only one single image, which is the principle of stereoscopy. A stereoscopic vision involves a lateral shift of the two retinal images and especially their perfect superposition, pixel by pixel.
This technical problem is solved by two specific complex structures. The first is the optic chiasm that crosses retinal fibres at 50 % in humans and primates. They then go to the integrator of the two half-retinal images, the lateral geniculate nucleus with six layers, and then project through the optic radiations to the primary visual cortex of the calcarine fissure (area 17) of the occipital lobe (Fig. 5.1). This configuration is such that each cerebral hemisphere sees a visual hemifield.
Fig. 5.1
Visual cortex and lateral geniculate nucleus (LGN). (a) Medial aspect of the interhemispheric section: 1. Cuneus of occipital lobe; 2. Calcarine fissure; 3. Cingular gyrus; 4. Corpus callosum; 5. Third ventricle. (b) Vascular microinjection (H. Duvernoy): the six layers of the NGL (three for each retina). (c) Coronal section of temporal lobe seen from behind: 1. Hippocampus; 2. LGN; 3. Gyrus parahippocampi; 4. Tentorium cerebelli; 5. Cerebellum. (d) Microinjection of vessels of primary visual cortex (H. Duvernoy): 1. White matter; 2. Gray matter with Gennari stria
But this is still not sufficient to guarantee a perfect fusion of images. There is a high-resolution point in the retina (135,000 cones per mm2) placed in the optical axis and that is responsible for 80 % of vision: the macula with the fovea, a small crater enhancing the precision of point setting, which gives a perfect centring of images and therefore stereoscopy.
One technical problem is the difficult implementation of the coupling of the two eyes, which may not be perfect at birth. If, subsequently, there is a shift of the two optical axes that cannot be retrieved, it is anticipated in the construction plan to provide a correction to avoid the very debilitating diplopia thanks to corticoretinal fibres described by Mawas which are capable of inhibiting one macula.
5.2 Three Specific Features Are Important to Understand
5.2.1 A Broad Visual Space Exploration
It can be obtained by moving the two eyeballs along two main axes, vertical and horizontal. In order to achieve this, it seems logical to introduce two pairs of rectus muscles: superior/inferior for the vertical axis and medial/lateral for the horizontal axis of visual exploration. The precise functional coupling of the two globes for stereoscopic vision imposes that eyeball rotations be perfectly centred on the centre of rotation of the globes [1–3]. But, during their contraction, the recti muscles exert a backward traction (Fig. 5.2).
Fig. 5.2
Anatomical sections of orbital cavities. (a) Axial section: 1. Lens (back post mortem position); 2. Temporal muscle; 3. Lateral rectus muscle; 4. Medial rectus muscle; 5. Ethmoidal cells. (b) Coronal section: 1. Orbital part of the frontal lobe; 2. Olfactory tract within the olfactory groove; 3. Superior oblique muscle; 4. Superior rectus and levator palpebrae superioris; 5. Medial rectus; 6. Optic tract; 7. Inferior rectus; 8. Lateral rectus; 9. Maxillary sinus
To counter this retrotraction, a first idea is to limit the degrees of freedom of the globes by placing them in a fibrous sac, the Tenon’s capsule, and sheath of the bulb that acts as a gliding space. This continues backward through the sclera and the sheath of the optic fascicle and forward in the episcleral space. On the sides, the capsule is fixed by fibrous fins to the sidewalls of the orbit.
However, this passive limitation must be coupled with an active compensation by an opposing force to the protraction. This justifies the necessary presence of the superior oblique muscle, which reflects on a fibrous pulley placed in the superomedial angle of the orbit (Figs. 5.3 and 5.4). From there, the tendon fibres are spread in a wide fan on the superior aspect of the globe passing under the superior rectus. The contraction of this superior oblique muscle obviously pushes forward the eyeballs, compensating the protraction of the recti. This ingenious anatomical solution could probably explain why the superior oblique muscle has a particular nerve, the trochlear nerve (IV). It is the only cranial nerve to emerge back near the superior colliculus, which is precisely the centre of the mandatory junction between visual images and eye movements [4, 5].
