THE OPTIC NERVES. 521 



between the two eyes and their nervous centres, namely, 1st, that be- 

 tween each nervous centre and the opposite eye ; 2d, that between each 

 nervous centre and the corresponding eye; 3d, that between the two 

 eyes by the anterior transverse commissure; and 4th, that between the 

 two nervous centres by the posterior transverse commissure. 



The physiological significance of this compound decussation is not 

 clearly understood. When compared as it presents itself in different 

 classes of animals, it appears to be connected with the decree of diver- 

 gence or parallelism between the visual axes of the two eyes. Thus in 

 fish, where the e3^es are so placed on opposite sides of the head that 

 their axes cannot be brought into parallelism with each other T the optic 

 nerves cross from side to side as distinct cords going to the opposite 

 eyes. In birds, where the eyes have nearly the same relative position as 

 in fish, the decussation is also complete, though less evident externally. 

 In quadrupeds as a class, the eyes are set more obliquely forward, while 

 in man they are situated completely at the front, so that their visual 

 axes are both directed forward, and parallel with each other, or may 

 even converge, and the two eyes can thus be brought to bear at the same 

 time upon near objects. The quadruple communication which exists in 

 man is, therefore, usually regarded as connected in some way with the 

 capacity for distinct and single vision with the simultaneous use of 

 both eyes. Cases, however, like that related by Yesalius, 1 in which the 

 decussation was wanting, each optic nerve going independently to the 

 eye of its own side, without any noticeable defect of vision, show that 

 such a communication is not directly or absolutely necessary, even in 

 man, to distinct vision of single objects. It more probably serves in an 

 indirect manner, by reflex action, to facilitate the harmonious muscular 

 control of the two eyeballs, by which the corresponding parts of the 

 retina on the two sides receive the visual rays coming from a single 

 object. 



Crossed Action of the Optic Nerves. The results of observation 

 show that the action of the optic nerves, as channels for the sense of 

 sight, is mainly a crossed action. The experiments of Flourens, Longet, 

 and Vulpian coincide in this respect ; and in regard to birds the fact is 

 easily established. If in the pigeon, as we have frequently observed, 

 the right optic tubercle alone be removed, when the bird has recovered 

 from the immediate effects of the wound, the sight is to all appearance 

 completely lost in the eye of the opposite side, but remains unimpaired 

 in the eye of the same side. After such an operation an instrument 

 may be carefully brought in close proximity to the left eye, without 

 producing any sign of its perception ; but the instant it is moved a little 

 in front, so as to come within the range of the right eye, the animal 

 starts backward to avoid it. Flourens obtained similar results in the 

 dog and rat, leading to the conclusion, which agrees with that of Longet, 

 that in quadrupeds also visual impressions are transmitted by the optic 



1 De Humani Corporis Fabrica. Liber iv. cap. iv. 

 34 



