Ross & Wilson Anatomy and Physiology in Health and Illness (94 page)

Retinitis pigmentosa

This is a group of hereditary diseases in which there is degeneration of the retina, mainly affecting the rods. Visual impairment, especially in dim light, usually becomes apparent in early childhood, leading to tunnel vision and, eventually, loss of sight.

Tumours

Choroidal malignant melanoma

This is the most common ocular malignancy in adults, occurring between 40 and 70 years of age. Vision is not normally affected until the tumour causes retinal detachment or secondary glaucoma, usually when well advanced. The tumour spreads locally in the choroid, and blood-borne metastases develop mainly in the liver.

Retinoblastoma

This is the most common malignant tumour in children. A small number of cases are familial. It is usually evident before the age of 4 years and usually affects one side. The condition presents with a squint and enlargement of the eye. As the tumour grows visual impairment develops and the pupil looks pale. It spreads locally to the vitreous body and may grow along the optic nerve, invading the brain.

Refractive errors of the eye

Learning outcome

After studying this section you should be able to:

explain how corrective lenses overcome refractive errors of the eye.

In the
emetropic
or normal eye, light from near and distant objects is focused on the retina (
Fig. 8.26A
).

In
hypermetropia
, or farsightedness, a near image is focused behind the retina because the eyeball is too short (
Fig. 8.26B
). Distant objects are focussed normally, but close vision is blurred. A biconvex lens corrects this (
Fig. 8.26C
).

In
myopia
, or nearsightedness, the eyeball is too long and distant objects are focused in front of the retina (
Fig. 8.26D
). Close objects are focussed normally, but distant vision is blurred. Correction is achieved using a biconcave lens (
Fig. 8.26E
).

Astigmatism
is the abnormal curvature of part of the cornea or lens. This interferes with the light path though the eye and prevents focussing of light on the retina, causing blurred vision. Correction requires cylindrical lenses. It may coexist with hypermetropia, myopia or presbyopia.

For a range of self-assessment exercises on the topics in this chapter, visit
www.rossandwilson.com
.

CHAPTER 9

The endocrine system

Pituitary gland and hypothalamus
208

Anterior pituitary
209

Posterior pituitary
212

Thyroid gland
213

Parathyroid glands
214

Adrenal glands
215

Adrenal cortex
216

Adrenal medulla
217

Response to stress
217

Pancreatic islets
217

Pineal gland
219

Thymus gland
219

Local hormones
219

Disorders of the pituitary gland
221

Hypersecretion of anterior pituitary hormones
221

Hyposecretion of anterior pituitary hormones
222

Disorders of the posterior pituitary
222

Disorders of the thyroid gland
222

Abnormal secretion of thyroid hormones
222

Simple goitre
223

Tumours of the thyroid gland
224

Disorders of the parathyroid glands
224

Hyperparathyroidism
224

Hypoparathyroidism
224

Disorders of the adrenal cortex
224

Hypersecretion of glucocorticoids (Cushing’s syndrome)
225

Hyposecretion of glucocorticoids
226

Hypersecretion of mineralocorticoids
226

Hyposecretion of mineralocorticoids
226

Chronic adrenocortical insufficiency (Addison’s disease)
226

Disorders of the adrenal medulla
226

Tumours
226

Disorders of the pancreatic islets
227

Diabetes mellitus
227

Type I diabetes mellitus
227

Type II diabetes mellitus
227

Secondary diabetes
227

Gestational diabetes
227

Effects of diabetes mellitus
227

Acute complication of diabetes mellitus
228

Long-term complications of diabetes mellitus
228

ANIMATIONS

9.1
Steroid hormones
208

9.2
Control of thyroid secretion
214

9.3
Adrenal function
217

9.4
Pathophysiology of primary hypothyroidism
223

9.5
Pathophysiology of type II diabetes mellitus
227

The endocrine system consists of glands widely separated from each other with no physical connections (
Fig. 9.1
). Endocrine glands are groups of secretory cells surrounded by an extensive network of capillaries that facilitates diffusion of
hormones
(chemical messengers) from the secretory cells into the bloodstream. They are commonly referred to as
ductless glands
because hormones diffuse directly into the bloodstream. Hormones are then carried in the bloodstream to
target tissues
and
organs
that may be quite distant, where they influence cellular growth and metabolism.