482 Dr Axtson on Single and Correct Vision, by means of 
of the mammalia in the adult state; but in the foetal state, in 
man, in the horse, ox, sheep, rabbit, and guinea-pig, he distinct- 
ly saw the internal fibres of the tractus opticus separate them- 
selves from the external, and traced them across from the right 
tractus to the left nerve, and vice versa. In crossing, they form 
a plexus, and, as the animal grows, this plexus is soon loaded 
with fresh deposits of medullary matter, so that the course of the 
fibres, distinct in the foetal state, is hardly to be traced in the 
adult. 
The idea that the partial decussation of the optic nerves is 
designed to give single vision by two eyes directed to the same 
object, is farther strongly confirmed by the fact, that in those 
animals in which this structure is generally, if not universally, 
found, 7. e. in mammalia and birds, the power of directing the 
axis of both eyes to the same object, generally, if not universally, 
exists, although in many quadrupeds and most birds the object 
which can be thus contemplated must be at a considerable dis- 
tance ; whereas in those animals in which there is (generally, 
if not universally,) no intermixture of the filaments of the optic 
nerves, 7. é. in reptiles and fishes, the eyes are so situated, as re- 
marked by Cuvier, that any object must in general, andprobably 
in most instances can, only be contemplated by one eye at a 
time.* 
But notwithstanding this strong evidence of the existence of 
the partial decussation in the higher animals, and of its connexion 
with the single vision by two eyes, the foundations of the theory 
must be allowed to be deficient. The partial decussation of the 
nervous fibres will explain the single vision by two eyes only on 
the supposition, that the fibres which terminate in corresponding 
* Anat. Comp. Lec. xii. Art. 2. It is true that the axes of the two eyes, at 
least in some fishes and reptiles, may be brought to bear on the same objects, if 
very distant, but as the vision of very distant objects is seldom requisite for these 
animals, it is probable that they are not habitually guided by simultaneous impres- 
sions on the two eyes. 
