1890 - 91 .] Alexander Bruce on a Human Cyclops. 
421 
this condition is almost invariably due to pressure of the head-fold 
of the amnion. It is obvious that a dilatation of the third ventricle, 
occurring at an early stage, may drive forward its floor so that the 
optic vesicles form a single fused cavity, and so cause cyclopia. 
It is probable that this is a very frequent cause, hut it is not the 
one operative in my case, where there was neither pineal cyst nor 
dilated ventricle. 
The two optic thalami, on the other hand, are all hut completely 
fused together, as are also the two crura cerebri. This fusion is 
associated with an excessive development of the lepto-meninges 
round them, by a fusion of these with thickened pachy-meninges at 
the junction of falx and tentorium, and also by an almost complete 
disappearance of the optic tracts. The causation of this chronic 
meningitis is not evident, but its occurrence is apparently so un- 
common in the described cases of cyclopia that it cannot be looked 
upon as a change secondary to the malformation. However produced, 
it is certainly capable of causing the fusion of the two thalami, and of 
altering the optic vesicles from an antero-lateral to a directly-forward 
course. I would submit, then, as a possible separate, or at least 
contributing, cause of cyclopia, a limited pachy- and lepto-menin- 
gitis. The occurrence of this meningitis may serve to explain the 
restrained ingrowth of pia mater to form the velum interpositum, 
and therefore the imperfect atrophy of the brain substance within 
the cerebral vesicle. 
The remaining changes in the crura, pons, medulla, and cord 
were mainly due to the absence from them of the descending tracts 
from the cerebrum. 
