DR. HUBERT AIRY ON A DISTINCT FORM OF TRANSIENT HEMIOPSIA. 263 
of Dr. Wollaston’s conclusion, that it is in the brain. The point that most distinctly 
bears upon this question is the exact agreement of the two eyes in the nature, extent, 
and degree of their affection. 
Closing either eye, the scope of the other eye is blurred by the selfsame bastioned 
cloud*; and the effort of the will is powerless to disregard it and see through it. 
The sight of both eyes being thus equally affected, we must conclude, assuming the 
semidecussation of the optic nerves at the chiasma, that the seat of the affection must 
lie at some point behind the chiasma of these nerves. 
The main division (that seems to offer itself) into dextral and sinistral teichopsia will 
correspond then to the distinction between right and left permanent hemiopsia, and will 
depend upon temporary affection of the left or right optic tract, or its origin in the cor- 
responding optic thalamus. 
The circumstances which determine the attack, whether those of bodily exhaustion, 
or mental fatigue and distress, or gastric derangement, all seem to me more likely to 
affect the inner integral parts of the brain than such outlying dependencies as the optic 
tracts, and lead me to suppose that the affection is in the former rather than in the latter. 
The partial paralysis, the loss of speech and of memory, and the derangement of hearing, 
that sometimes follow an attack of teichopsia, all point to the same conclusion. But, in 
truth, the radical connexions of the optic tracts seem to be so wide, that it is impossible 
at present to do more than guess at the locus morbi. Such cases as Sir John Herschel’s, 
where the cloud passed over the whole field from left to right, can only be explained by 
supposing the disturbance to lie in some region of the brain where the opposite halves 
are in contact. 
Finally, as to the nature of the local mischief -Is it a temporary suspension of 
function, propagated by contiguity, among the nerve-cells of the visual sensorium 
(wherever that may be), due to vascular congestion, and relieved by the relief of that 
congestion 1 Does the headache, following close upon the departure of the morbid 
vision, tell of the further propagation of the nervous disturbance into parts of the brain 
where disturbance is ache, as in the visual tract disturbance is abnormal sensation of 
light 1 And the detriment to speech and hearing that has occasionally been noticed, 
does it mean extension of the same disturbance still further into the regions of brain- 
substance appropriate to those functions 1 
The phenomena are so definite and so localized, and their course is so regular, that 
we can hardly avoid the conviction that their cause is equally definite and equally local- 
ized ; and it is difficult to admit so vague an agent as nervous sympathy with gastric 
derangement, except as acting through the medium of some secondary local manifesta- 
tion in the brain. 
* This is invariably the ease with myself and with most of those who have noted these phenomena ; but 
while preparing these pages for the press (May 12th, 1870) I have received a note from Sir John Herschel, 
with the following postscript:' — “ On the 16th ult., at waking, I found the ‘ Fortification pattern ’ certainly in 
my left eye only, a,nd much more vivid with the eye open and looking at paper than when closed.” 
