490 Dr Avison on Single and Correct Vision, by means of 
The decussation at the pyramids exists in the mammalia and 
in birds; but the number of crossing fibres becomes less in 
the lower parts of the scale, as does the size of the cerebral lobes, 
in proportion to the spinal cord. In reptiles and fishes it does 
not exist.* In the two former classes the power of directing the 
axis of both eyes to the same object, exists, although in the case 
of many quadrupeds and most birds, that object must be very 
distant. In reptiles and fishes, the eyes appear to be dissociated 
from one another, and directed either laterally, or vertically up- 
wards, in such a manner that they evidently regard objects, in 
general, only with one eye at a time.} 
In the mammalia and birds, the commissure and partial decus- 
sation of the optic nerves are found; but in reptiles and fishes 
there is the complete decussation and no commissure.{ 
In the human body, it is well known that the usual effect of 
injury of the brain or cerebellum, when it produces palsy, is seen 
on the opposite side of the body ; but the effect of injury of one 
side of the spinal cord, even in the neck, is seen on the same side 
of the body ; and in the experiments of FLoureEns, the crossing 
influence of injuries was uniformly seen in the mammalia and 
birds, when they were inflicted any where above the decussation 
at the pyramids, but not when they were below this point. In 
reptiles and fishes, no crossing influence, and indeed hardly any 
influence, on sensation or voluntary motion, from injury of con- 
tents of the cranium, could be observed.{ 
I do not, however, offer this speculation as altogether satisfac- 
- tory or free from difficulties ; and two difficulties, in particular, 
present themselves so obviously as to demand notice. 
1. It may be said, that, in the case of reptiles and fishes, al- 
though the impressions made on each optic lobe may be in the 
* Cuvier, Rapport sur Anat. Comp. du Cerveau, &c par SerRes, in latter 
work, pref. p. 26. 
+ Cuvier, Lecon 12, Art. 2. { Serres, p. 326, et seq.: 
§ Recherches Experimentales, &c. p. 121. 
