Dr. Wollaston on semi-decussation of the optic nerves. 223 
its own muscular structure, or by alteration of its place , from 
the agency of other muscles. Nor do I mean to consider 
either the involuntary motions of the iris dependent on the 
quantity of light present, or that voluntary contraction of it 
by which we adapt the aperture of the pupil for distinct vision 
at different distances, limiting thereby, what in optics is 
termed the spherical aberration of the lens. 
The subject of my inquiry relates solely to the course by 
which impressions from images perfectly formed are con- 
veyed to the sensorium, and to that structure and distribution 
of the optic nerves on which the communication of these 
impressions depends. 
Without pretending to detect by manual dexterity as an 
anatomist, the very delicate conformation of the nerves of 
vision, I have been led, by the casual observation of a few 
instances of diseased vision, to draw some inferences re- 
specting the texture of that part which has been called the 
decussation of the optic nerves, upon which I feel myself 
warranted to speak with some confidence. 
It is well known that in the human brain these nerves, after 
passing forwards to a short distance from their origin in the 
thalami nervorum opticorum, unite together, and are, to ap- 
pearance, completely incorporated ; and that from this point 
of union proceed two nerves, one to the right, the other to 
the left eye. 
The term decussation was applied to this united portion, 
under the supposition that, though the fibres do intermix, 
they still continue onward in their original direction, and 
that those from the right side cross over wholly to supply 
