new  Cafe  in  Squinting . g ^ 
the  whole,  though  rather  darker  while  the  diaftole  of 
the  artery  comprefies  the  retina,  is  yet  diftiniStly  vifible. 
The  following  circumftance  fee  ms  to  give  rife  to  the 
infenfibility  of  the  central  part  of  the  optic  nerve  at  its 
ingrefs  into  the  eye,  which  I have  obferved  in  feveral 
calves’  eyes.  The  point  of  a pair  of  fciffars  was  intro- 
duced behind  the  ciliary  circle,  and  the  whole  of  the 
cornea,  aqueous  humour,  iris,  and  cryftalline,  being  re- 
moved, the  retina  was  beautifully  feen  through  the  vi- 
treous humour  fomewhat  magnified.  On  expofing  this 
to  the  fun-fhine,  and  infpecting  it  with  nicety,  a white 
filament,  about  the  tenth  of  an  inch  in  length,  arifing 
from  the  center  of  the  optic  nerve,  was  feen  afcending; 
ftraight  upwards  into  the  vitreous  humour,  like  a thin 
white  worm.  The  ufe  of  this  may  be  to  fupply  the  vi- 
treous humour  or  cryftalline  with  nourifhment,  whe- 
ther it  be  a nerve  or  an  empty  blood-veflel ; but  this  is 
certain,  that  its  riling  fo  high  above  the  furface  of  the 
retina  muft  render  it  incapable  of  vilion : whence  there 
is  juft  reafon  to  conclude,  that  this  conformation  muft 
be  the  true  caufe  of  the  infenfibility  of  this  part  of  the 
eye. 
I do  not  affirm,  that  the  human  eye,  either  during  in- 
fancy or  in  our  riper  years,  is  fimilar  in  conformation  to 
Shat  of  a calf,  nor  have  we  fufficient  opportunities  to  ob~ 
a ferve 
