Starry-eyed
Question: Why do we see stars when we receive a blow to the head?
Answer: Our retina can only respond to stimuli in one way: by sending a
signal along the optic nerve to the brain,which interprets this signal as light.
Normally, the stimulus is light impinging on the retina, but other stimuli work
too.
One of the commonest is a mild trauma to the eyeball. If you close your eyes
in a dark room and press gently on the front of your eye you will see a faint
glow in your peripheral vision. This can also happen when a blow to the head
causes you to close your eyes hard for a moment. I sometimes see stars or
flashes when I sneeze, which is again the result of eyeball compression as the
eyelid is forced closed.
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The retina will also send a light signal response to the brain if it becomes
torn or detached. This is a serious condition requiring treatment by an
ophthalmologist. Another potential source of problems is the jelly-like vitreous
humour that fills the rear two-thirds of the eye. It is attached to the retina
at several points, and tugging on it can result in the sensation of light when,
for example, you move your eyeball around rapidly under your lid. These
attachments can fail as we age, which is normally not a problem unless the
vitreous pulls some retina with it. This, incidentally, is the cause of
floaters, the little bits of matter you can sometimes see floating about inside
your eye.
You should consult your ophthalmologist if you have any persistent
light-viewing episodes to make sure that it is not caused by a serious eye
condition. Light flashes can also be associated with migraine headaches and
central nervous system disorders, which can act on the vision centres of the
brain to produce the sensation of light.
David Slauenwhite
Dartmouth, Nova Scotia, Canada
Answer: This effect is the result of a marked increase in intra-ocular
pressure (IOP) caused by the blow. This pressure is exerted on the eye by the
vitreous humour鈥攖he fluid within the eye that gives it its shape. The
increase in IOP triggers the rods and cones that make up the retina, sending an
electrical signal via the optic nerve to the visual cortex at the back of the
head. The visual cortex is where we perceive what we see, and hence we think we
are seeing stars. The timescale of seeing stars for a few seconds nicely matches
the timescale of the temporary rise in IOP.
Toby Leslie
London
Feel the heat
Question: My daughter is studying to be a chef and wonders why lobsters are
boiled alive. Do they feel it?
Answer: Yes, they can feel it, and it鈥檚 a good sign that your daughter
cares鈥攊t鈥檚 one of the many things we do to animals that is a little
sick.
Gary Mantell
By e-mail, no address supplied
Answer: That is a difficult philosophical question. If you kick me on the
shin, I feel pain. I hop around and shout. If I kick you on the shin, I observe
you hop and shout. So I deduce that you feel pain. I can extrapolate this
deduction to other vertebrates but an arthropod has different reactions and an
utterly different nervous system.
You can remove a lobster鈥檚 legs and claws, cut a hole in its carapace and
insert electrodes into its ganglia. If you then hold food near its mouth, it
will bite off pieces and swallow them. You would not consider enjoying a
sandwich under similar circumstances, but the lobster seems to feel no
pain鈥攊t seems unaware anything is amiss.
On the other hand, lobsters can autotomise their legs: there is a weak point
near the base at which the legs can be shed if they are injured or trapped.
Perhaps autotomy is associated with pain. Lobsters also grind their mandibles
and will flip their tails to escape when they are being mistreated鈥攑erhaps
this too signals pain.
If you drop a lobster into boiling water, it flips its tail, thrashes its
legs, grinds its mandibles and sometimes autotomises its claws. However, if you
put a lobster into cold, well-salted water and slowly heat the water, the
lobster is apparently anaesthetised by the slowly increasing temperature and
shows no symptoms.
Peter Balch
Edinburgh
Answer: I have only once been in the unfortunate position to have a live
lobster to deal with, but it was a monster that weighed more than 2 kilograms. I
dealt with it as advised in Penguin Cordon Bleu Cookery by Rosemary
Hume and Muriel Downes. 鈥淗ow to kill a live lobster: cover the lobster tail with
a cloth, hold firmly with the left hand, and, using a sharp knife, pierce
through the head at the small cross mark or indentation that lies about half way
along the head. The lobster is killed immediately.鈥�
I found this very simple to follow. The cross is easy to find and provides an
entry point for the knife. The lobster slumped instantly, to all appearances
dead, and being limp was easier to place in the pot.
We cannot be sure whether creatures with the distributed nervous system of a
lobster feel pain, however they are killed. The method described by Hume and
Downes is quick, easy and certainly gives me confidence that I am doing my best
to minimise the animal鈥檚 suffering.
David Ridpath
Preston, Lancashire
Answer: Many people have discovered they can kill a lobster by putting it in
the freezer instead. In contrast to boiling it alive, freezing an invertebrate
to death is a humane thing to do. It happens all the time in nature, and
triggers no frantic escape attempts like boiling water does.
Ottar Sande
Sandane, Norway
This week鈥檚 questions
On the hoof: Why are domesticated horses given shoes? They seem to have no
difficulty managing without them in the wild.
Neil Sinclair
Kingston, Surrey
In or out?: Why do some people have 鈥渋nnie鈥� belly buttons while others have
鈥涣耻迟迟颈别蝉鈥�?
Kris
Montgomery, New York
Blood lines: Routine transfusion of blood is usually traced to Landsteiner鈥檚
discovery of the A, B and O blood group system early in the last century. But,
according to the Encyclopaedia Britannica 鈥淭he Incas apparently
practised blood transfusion successfully much earlier鈥攏early all South
American Indians are of blood type O rhesus positive, and incompatibility
reactions must have been few.鈥�
I have not been able to find any other references to this practice. How
extensive was it, was it ritual or remedial and how was the blood channelled
unclotted from donor to recipient?
脰rn贸lfur Thorlacius
Reykjavik, Iceland