482 Dr Alison 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 fcetal 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, i. 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, i. e. 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. Lee,, 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. 



