Right & Left orbits are the bony depressions present on the anterior aspects of skull, in which eyeballs are lodged. Each eyeball is like a camera, with a lens which produces images of the objects we see. The images fall on inner lining of eyeball called retina, which converts light images into nervous impulses which pass through optic nerves to reach the visual areas of cerebral cortex, where the image of object is perceived.
The greater part of eyeball (posterior 5/6th) is spherical & anterior 1/6this convex. The posterior 5/6th part has outer white opaque wall called sclera & anterior 1/5this transparent called cornea. The center of cornea has pigmented part called iris with an aperture in center called pupil.
There are 3 pairs of muscles responsible for movements of eyeball. 2 pairs are straight hence called Superior Rectus, Inferior Rectus & Medial & lateral Rectus. One pair has oblique muscles, called as Superior Oblique & Inferior Oblique.
Movements of eyeball mediated by these muscles are up & down, medial (inward) & lateral (outward), combined convergence (downward & upward) & divergence(downward & upward). In addition, there is torsional movement called intorsion & extorsion.
Nerve supply to these muscles is by three cranial nerves
- Trochlear- the 4th cranial nerve supplies to Superior oblique muscle
- Abducent- the 6th cranial nerve supplies to Lateral rectus muscle
- Oculomotor- the 3rd cranial nerve which supplies to all remaining muscles
with one additional muscle called Levator palpabrae superioris which keeps upper eyelid lifted. This nerve also has parasympathetic fibers supplying sphincter pupillae & ciliaris muscle. Former causes narrowing of pupil and later causes accommodation reflex. This reflex makes the lens more convex for near vision.
Central control of eyeball fixation has two components – Voluntary & Involuntary.
Involuntary fixation of eyeballs is required during head, neck, body movements also in response to auditory stimulus. This is achieved by connecting fibres from visual cortex to brainstem nuclei of oculomotor nerve which controls 4 muscles of eyeball & parasympathetic fibers to sphincter pupillae & ciliary muscles.
Voluntary fixation of eyeballs is controlled by a specific area in cortex which is present in posterior region of Prefrontal cortex. Fibres arising from this area also project into brainstem nuclei of oculomotor nerve via a separate tract.
Tratak- is a shuddhikriya described in Hath Yoga. One has to gaze at a small object & keep eyes steady until tears come out. This is the description of Tratak given in Hatha Pradipika II-32. The II-33 says that process of Tratak cures many eyes diseases & removes laziness.
The tratak can be practiced with pinpoint small circle at eyesite level or using a flame of candle.
Voluntary fixation of eyeballs on a pin point or flame of a candle will bring signal to brainstem nuclei of oculomotor nerve. Voluntary fixation of eyeballs is done by downward convergence to gaze at tip of nose or upward convergence at center of eyebrows. Since the oculomotor nerves carries parasympathetic fibres, voluntary fixation of eyeballs in Tratak may help to increase parasympathetic tone.
Gazing at tip of nose as mentioned in BG-6-13 & gazing at center of eyebrows in Shambhavi mudra leads to downward convergence or upward convergence respectively. This may lead to activation of parasympathetic system via oculomotor nerve nuclei
Tip about Lacrimal Secretion– Lacrimal gland is the tear producing gland situated at upper part of eyeball. This gland produces tears to keep cornea moist & also protects it from external microinvasion.
This gland is innervated by branch of maxillary division of 5th cranial nerve called Trigeminal nerve. The Secretomotor fibres come from Pterygopalatine ganglion which is peripheral ganglion of parasympathetic outflow. (These fibres also lead to secretomotor effect for glands of nasal & palatine mucosa).
The net effect of tears flowing through eyes (during Tratak) may be intended to increase parasympathetic tone at this ganglion.
Conclusion – Yogic science offers many techniques to reach the efferent connections of Parasympathetic system. Tratak, Shambhavi mudra & nasal tip gazing are some of them.These techniques are based on voluntary fixation of eye balls. The concerned nerve is third cranial nerve called Oculomotor nerve which carries parasympathetic fibers for sphincter pupillae & ciliary muscles.Reaching & stimulating parasympathetic system helps to generate relaxation response of body.
This response helps the process of homeostasis of body.