TUNNEL VISION
T
tu n n e l v is io n
L o s s o f t h e p e r i p h e r a l
v is u a l f ie ld
t o t h e
e x t e n t t h a t o n l y o b j e c t s s t r a i g h t a h e a d
c a n
b e
s e e n
c l e a r l y .
T u n n e l
v i s i o n
i s
m o s t o f t e n c a u s e d b y c h r o n i c
g la u c o m a
( r a i s e d
p r e s s u r e
w i t h i n
t h e
e y e b a l l ) .
R e t in itis p ig m e n t o s a
( d e g e n e r a t i o n o f t h e
r e t i n a ) i s a n o t h e r p o s s i b l e c a u s e .
tu rb in a te
O n e o f t h e t h r e e s h e l l - s h a p e d b o n y p r o -
j e c t i o n s i n t h e s i d e o f t h e n a s a l c a v i t y .
T u rn e r’s s y n d ro m e
A
d i s o r d e r
c a u s e d
b y
a
c h ro m o s o m a l
a b n o rm a lity
t h a t o n l y a f f e c t s f e m a l e s .
CAUSES
T h e
a b n o r m a l i t y m a y a r i s e
i n
o n e
o f
t h r e e w a y s :
a f f e c t e d f e m a l e s m a y p o s -
s e s s o n l y o n e X
c h ro m o s o m e
i n s t e a d o f
t h e n o r m a l t w o ; t h e y m a y p o s s e s s o n e
n o r m a l a n d
o n e
d e f e c t i v e
X
c h r o m
o -
s o m e ; o r t h e y m a y p o s s e s s a m i x t u r e o f
c e l l s ( s e e
m o s a ic is m ),
i n w h i c h s o m e o f
t h e
c e l l s
a r e
m i s s i n g
a n
X
c h r o m
o -
s o m e ,
s o m e h a v e
e x t r a
c h r o m o s o m e s ,
a n d
o t h e r s
h a v e
t h e
n o r m a l
c o m
p l e -
m e n t o f c h r o m o s o m e s .
SYMPTOMS
T h e c h a r a c t e r i s t i c s o f T u r n e r ’s s y n d r o m e
a r e : s h o r t s t a t u r e ; w e b b i n g o f t h e s k i n
o f t h e n e c k ; a b s e n c e o r r e t a r d e d d e v e l -
o p m e n t o f s e x u a l c h a r a c t e r i s t i c s ;
a m e n -
o rrh e a
( f a i l u r e t o m e n s t r u a t e ) ,
c o a rc ta -
tio n o f th e a o rta ,
a n d a b n o r m a l i t i e s o f t h e
e y e s a n d b o n e s .
TREATMENT
T r e a t m e n t
w i t h
g ro w th h o rm o n e
f r o m
i n f a n c y
h e l p s
g i r l s
w i t h T u r n e r ’s
s y n -
d r o m e t o a c h i e v e n e a r n o r m a l h e i g h t .
C o a r c t a t i o n o f t h e a o r t a i s t r e a t e d s u r g i -
c a l l y .
T r e a t m e n t
w i t h
o e s t ro g e n d ru g s
i n d u c e s m e n s t r u a t i o n , b u t i t d o e s n o t
m a k e a f f e c t e d g i r l s f e r t i l e .
TU R P
T h e a b b r e v i a t i o n f o r t r a n s u r e t h r a l r e s e c -
t i o n o f t h e p r o s t a t e . T U R P i s a s u r g i c a l
p r o c e d u r e i n w h i c h t h e c e n t r a l p a r t o f
a n
e n l a r g e d
p ro s ta te g la n d
i s
r e m o v e d
( s e e
p ro sta te , e n la rg e d ).
A v i e w i n g i n s t r u -
m e n t
c a l l e d
a
r e s e c t o s c o p e
i s
p a s s e d
a l o n g
t h e
u re th ra
u n t i l
i t
r e a c h e s
t h e
p r o s t a t e . A h e a t e d w i r e l o o p , o r s o m
e -
t i m e s
a
c u t t i n g
e d g e ,
i s
i n s e r t e d
t h r o u g h t h e r e s e c t o s c o p e
a n d u s e d t o
c u t a w a y e x c e s s p r o s t a t e t i s s u e .
tw in s
T w o o f f s p r i n g r e s u l t i n g f r o m o n e p r e g -
n a n c y . T w i n s m a y d e v e l o p f r o m a s i n g l e
o v u m ( e g g ) o r f r o m t w o o v a .
M o n o z y g o t i c , o r i d e n t i c a l , t w i n s d e v e l o p
w h e n a s i n g l e f e r t i l i z e d e g g d i v i d e s a t
a n e a r l y s t a g e o f d e v e l o p m e n t . I n c o m
-
p l e t e
d i v i s i o n
o f
t h e
e g g
r e s u l t s
i n
c o n j o i n e d t w i n s
( s e e
tw in s, c o n jo in e d ).
M
o n o z y g o t i c t w i n s s h a r e t h e s a m e
p la -
c e n ta .
A l t h o u g h
o n e
i s
o f t e n
m u c h
b i g g e r t h a n t h e o t h e r a t b i r t h , t h e y a r e
a l w a y s
o f
t h e
s a m e
s e x
a n d
l o o k
r e m a r k a b l y a l i k e .
D i z y g o t i c , o r f r a t e r n a l , t w i n s d e v e l o p
w h e n t w o e g g s a r e f e r t i l i z e d a t t h e s a m e
t i m e . T h e o v a t h e y d e v e l o p f r o m m a y b e
r e l e a s e d b y t h e s a m e o r d i f f e r e n t o v a r i e s ;
f e r t i l i z a t i o n o c c u r s s i m u l t a n e o u s l y . T h e y
e a c h h a v e a p l a c e n t a a n d m a y b e o f d i f -
f e r e n t s e x e s a n d l o o k q u i t e d i f f e r e n t .
T w i n s o c c u r i n a b o u t o n e i n 8 0 p r e g -
n a n c i e s . ( S e e a l s o
p re g n a n c y , m u lt ip le .)
tw in s , c o n jo in e d
I d e n t i c a l
tw in s
p h y s i c a l l y j o i n e d d u e t o a
f a i l u r e t o s e p a r a t e d u r i n g d e v e l o p m e n t
f r o m a s i n g l e f e r t i l i z e d e g g . C o n j o i n e d
t w i n s a r e a l s o c a l l e d S i a m e s e t w i n s .
tw itch
S e e
f a s c ic u la t io n
;
t ic
.
TWO TYPES OF TWINS
During each menstrual cycle, either one ovum or a small number ofova are
released. Ifone ovum is fertilized and the two cells formed from its first division
develop independently, the result is identical twins. Iftwo ova are fertilized and
mature normally, nonidentical twins result.
IDENTICALTWINS
Fertilization of a single ovum
Identical twins come from a single fertilized
ovum. When the ovum splits, the two cells
formed develop independently.
Identical twins in the uterus
The result is a pair of genetically identical twins
sharing the same placenta. Theyare also known
as monozygotic, or monovular, twins.
NONIDENTICALTWINS
Fertilization of two separate ova
Nonidentical twins are the result oftwo
separate ova that have been fertilized by
two separate sperm.
Nonidentical twins in the uterus
The resulting individuals are genetically distinct
and have separate placentas. They are also
known as dizygotic, or binovular, twins.
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