Bma Illustrated Medical Dictionary

it. The colostomy is closed when the

rejoined colon has healed. A permanent

colostomy is needed if the rectum or

anus has been removed.

colostrumA thick, yellowish fluid pro-

duced by the breasts during the first few

days after childbirth. Colostrum is then

replaced by breast milk. Colostrum con-

tains less fat and sugar but more

minerals and protein than breast milk.

It also has a high content of lympho-

cytesand immunoglobulins, which help

to protect the baby from infection.

colour blindnessSee colour vision

deficiency.

colour visionThe ability to see differ-

ent parts of the colour spectrum. Light

perceived by the human eye consists of

electromagnetic radiation (energy waves)

with a spectrum of different wavelengths

between about 400 and 700 nanometres.

Different wavelengths produce sensations

of violet, indigo, blue, green, yellow,

orange, and red when they fall on the

retina and stimulate nerve signals,

which are processed in the brain.

As light falls on the retina, it strikes

light-sensitive cells called rods and cones.

The rods can detect all visible light, but

only the cones can distinguish colour.

There are 3 types of cones: red-sensitive,

blue-sensitive, and green-sensitive. Each

responds more strongly to a particular

part of the light spectrum. Because the

cones are most concentrated in a central

area of the retina called the fovea,

colour vision is most accurate for

objects viewed directly and is poor at

the edges of vision. When light hits a

cone, it causes the cone to emit an elec-

trical signal, which passes to the brain

via the optic nerve. Colour perception

requires a minimum level of light,

below which everything is seen as

shades of grey. (See also colour vision

deficiency; eye; perception; vision.)

colour vision deficiencyAny abnor-

mality in colour vision that causes

difficulty distinguishing between certain

colours. Total absence of colour vision

(monochromatism) is rare. The most

common types of colour vision deficiency

are reduced discrimination of red and

green. Most cases of red and green

colour vision deficiency are caused by

defects in the light-sensitive cells in the

retina. These defects are usually inherit-

ed, although occasionally defects are

caused by retinal or optic nerve dis-

eases or injury. The inherited defects

tend to be sex-linked (see genetic disor-

ders), which means that the majority of

sufferers are male. A person with a

severe green deficiency has difficulty dis-

tinguishing oranges, greens, browns,

and pale reds. In severe red deficiency,

all reds appear dull. A much rarer defi-

ciency in which blue cannot be

distinguished may be inherited or may

be due to degeneration of the retina or

optic nerve.

colposcopyVisual inspection of the cervix

and vaginausing a magnifying instru-

ment called a colposcope. Colposcopy

is carried out to look for the presence of

COLOSTRUM COLPOSCOPY

C

Colon

Colon

Rectum

LOCATION

Small

intestine

TEMPORARY COLOSTOMY

PERMANENT COLOSTOMY

Colon

Muscle

Muscle

Opening on

skin surface

Healing

colectomy

site

Edge stitched

to surface of

skin

COLOSTOMY

Opening on

skin surface

Edge stitched to

surface of skin

Skin