Anatomy
Head and Neck
Regarding the eyeball, which of the following statements is CORRECT:
Answer:
The choroid is a highly vascular pigmented layer. The optic disc is where the optic nerve leaves the retina. The dilator pupillae muscle is innervated by sympathetic fibres from the superior cervical ganglion. The ciliary muscle is innervated by parasympathetic fibres from the oculomotor nerve. The sclera is the opaque outer layer of the eye.Eyeball
Anatomy / Head and Neck / Orbit and Eye
Last Updated: 6th November 2020
The walls of the eyeball consist of three layers:
- the outer fibrous layer of the sclera posteriorly and the cornea anteriorly
- the middle vascular layer of the choroid posteriorly which is continuous with the ciliary body and iris anteriorly
- the inner retina.
The anterior and posterior chambers are the areas posterior to the cornea, and anterior to the lens. The posterior four-fifths of the eyeball, from the lens to the retina, is the vitreous chamber filled with the vitreous body, a transparent gelatinous substance.
![By OpenStax College [CC BY 3.0], via Wikimedia Commons](https://primary-cdn.frcemsuccess.com/wp-content/uploads/2016/07/eyeball-1024x585.jpg)
Eyeball. (Image by OpenStax College [CC BY 3.0], via Wikimedia Commons)
Sclera and Cornea
The sclera is an opaque white layer of dense connective tissue that provides attachment for the muscles involved in eyeball movement.
The transparent cornea is the anterior protrusion of the eyeball. It is continuous with the sclera anteriorly and allows light to enter the eyeball.
This outer layer receives its sensory supply from branches of the trigeminal nerve, which acts as the afferent pathway of the corneal blink reflex (the facial nerve is the efferent pathway of the corneal blink reflex, mediated by the orbicularis oculi muscle).
Choroid, Lens and Ciliary Body
The choroid is posterior and represents two-thirds of the vascular layer; it is a thin, highly vascular, pigmented layer firmly attached to the retina internally and loosely attached to the sclera externally.
The lens is a transparent, biconvex elastic disc attached to muscles associated with the outer wall of the eyeball, which can change its refractive ability to maintain visual acuity.
The ciliary body extends from the anterior border of the choroid and is composed of the ciliary muscle and the ciliary processes, from which extends the lens attached to the suspensory ligament of the lens. Contraction of the ciliary muscle changes the shape of the lens to accommodate the lens for near vision. The ciliary muscle is innervated by parasympathetic fibres of the oculomotor nerve.
Iris and Pupil
The iris, the coloured part of the eye with the central opening forming the pupil, projects outwards from the ciliary body. Smooth muscle fibres within the iris control the size of the pupil:
- The sphincter pupillae muscle is innervated by parasympathetic fibres of the oculomotor nerve which acts as the efferent pathway of the pupillary light reflex (the optic nerve is the afferent pathway of the pupillary light reflex). Loss of parasympathetic innervation results in a fixed dilated pupil.
- The dilator pupillae muscle is innervated by postganglionic sympathetic fibres arising from the superior cervical ganglion. Loss of sympathetic innervation results in a constricted pupil (miosis).
Horner's syndrome results from damage to the sympathetic chain and is characterised by the triad of miosis, partial ptosis (due to loss of innervation to the superior tarsal muscle) and anhidrosis.
Chambers
The anterior chamber is the area of the eyeball directly anterior to the iris. Posterior to the iris and anterior to the lens is the smaller posterior chamber. The chambers are continuous with each other through the pupillary opening. They are filled with aqueous humour, which is secreted into the posterior chamber, flows into the anterior chamber and is absorbed into the canal of Schlemm. The aqueous humour supplies nutrients to the avascular cornea and lens and maintains intraocular pressure. If the normal cycle of production and absorption is disturbed, this can cause glaucoma due to raised intraocular pressure.
Retina and Optic Disc
The inner layer of the eyeball is the retina, divided into the posterior optic part and the non-visual part, which covers the internal surface of the ciliary body and the iris.
The optic part of the retina consists of an outer pigmented layer firmly attached to the choroid and the neural layer attached to the pigmented layer around the optic nerve.
The optic disc is where the optic nerve leaves the retina. Lateral to the optic disc, a small area is the macula lutea with its central depression, the fovea centralis. This is the thinnest area of the retina and visual sensitivity is higher here than elsewhere in the retina because it has fewer rods and more cones.
The retina is supplied by the central retinal artery from the ophthalmic artery.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
