Anatomy of the Eye: Lens and Accommodation

Structure of the Lens

• The lens fibers make up the bulk of the lens
• The elastic lens capsule is under tension, causing the lens to constantly try to assume a globular shape
• The equatorial region of the lens is attached to the ciliary processes of the ciliary body by the suspensory ligament

Accommodation of the Eye

• To accommodate the eye for close objects, the ciliary muscle contracts and pulls the ciliary body forward and inward, relaxing the suspensory ligament
• This allows the elastic lens to assume a more globular shape
• With advancing age, the lens becomes denser and less elastic, leading to decreased ability to accommodate (presbyopia)

Constriction of the Pupil During Accommodation

• The sphincter pupillae muscle contracts to reduce the size of the pupil, ensuring that light rays pass through the central part of the lens and minimizing spherical aberration

Convergence of the Eyes During Accommodation

• In humans, the retinae of both eyes focus on a single object, resulting in single binocular vision
• When an object moves from a distance toward an individual, the eyes converge to focus on the single object
• Convergence of the eyes results from the coordinated contraction of the medial rectus muscles

Clinical Notes

• Eye trauma can occur due to the lack of protection from small objects, such as golf balls
• The bony orbit provides some protection from large objects, but not from small objects

Sensory Nerves of the Face

• The skin of the face is supplied by branches of the three divisions of the trigeminal nerve
• The ophthalmic nerve supplies the region developed from the frontonasal process
• The maxillary nerve serves the region developed from the maxillary process of the first pharyngeal arch
• The mandibular nerve serves the region developed from the mandibular process of the first pharyngeal arch

Ophthalmic Nerve

• The ophthalmic nerve supplies the skin of the forehead, upper eyelid, conjunctiva, and side of the nose
• Five branches of the nerve pass to the skin:
  • Lacrimal nerve supplies the skin and conjunctiva of the lateral part of the upper eyelid
  • Supraorbital nerve supplies the skin and conjunctiva of the central part of the upper eyelid and the skin of the forehead
  • Supratrochlear nerve supplies the skin and conjunctiva of the medial part of the upper eyelid and the skin over the lower part of the forehead
  • Infratrochlear nerve supplies the skin and conjunctiva of the lower part of the upper eyelid

Carotid and Axillary Sheaths

• The carotid sheath is a local condensation of the prevertebral, pretracheal, and investing layers of the deep fascia that surrounds the common and internal carotid arteries, internal jugular vein, vagus nerve, and deep cervical lymph nodes
• The axillary sheath is a sheath of fascia that surrounds the subclavian artery and the brachial plexus as they emerge from the interval between the scalenus anterior and scalenus medius muscles

Cervical Ligaments

• Stylohyoid ligament: connects the styloid process to the lesser cornu of the hyoid bone
• Stylomandibular ligament: connects the styloid process to the angle of the mandible
• Sphenomandibular ligament: connects the spine of the sphenoid bone to the lingula of the mandible
• Pterygomandibular ligament: connects the hamular process of the medial pterygoid plate to the posterior end of the mylohyoid line of the mandible

Muscles of the Neck

• Digastric muscle: depresses the mandible or elevates the hyoid bone
• Stylohyoid muscle: elevates the hyoid bone
• Sternohyoid muscle: depresses the hyoid bone
• Sternothyroid muscle: depresses the larynx
• Thyrohyoid muscle: depresses the hyoid bone or elevates the larynx
• Omohyoid muscle: depresses the hyoid bone
• Scalenus anterior muscle: elevates the first rib and laterally flexes and rotates the cervical part of the vertebral column
• Scalenus medius muscle: elevates the first rib and laterally flexes and rotates the cervical part of the vertebral column
• Scalenus posterior muscle: elevates the second rib and laterally flexes and rotates the cervical part of the vertebral column

Cervical Plexus

• The cervical plexus is a network of nerve fibers that supplies innervation to some of the structures in the neck and trunk

Parathyroid Glands

• The two superior parathyroid glands are more constant in position and lie at the level of the middle of the posterior border of the thyroid gland
• The two inferior parathyroid glands usually lie close to the inferior poles of the thyroid gland
• Functions of the parathyroid glands:
  • Produce parathyroid hormone, which stimulates osteoclastic activity in bones, mobilizing bone calcium and increasing calcium levels in the blood
  • Stimulate the absorption of dietary calcium from the small intestine and the reabsorption of calcium in the proximal convoluted tubules of the kidney
  • Strongly diminish the reabsorption of phosphate in the proximal convoluted tubules of the kidney
• Control of parathyroid hormone secretion is regulated by calcium levels in the blood
• Blood supply to the parathyroid glands is from the superior and inferior thyroid arteries
• Venous drainage is into the superior, middle, and inferior thyroid veins
• Lymph drainage is into the deep cervical and paratracheal lymph nodes
• Nerve supply is from the superior or middle cervical sympathetic ganglia

This quiz covers the structure and function of the lens in the human eye, including its shape, attachments, and role in accommodation.