452 THE NERVOUS SYSTEM. 



ing from one side of the brain go to the eye of the opposite side, and 

 vice versa. 



II. In birds the optic nerves appear superficially to be united at the 

 chiasma, but dissection shows that they are only broken up into fascic- 

 uli of fibres which, though interwoven with each other, remain anatom- 

 ically distinct (Fig. 120). The fasciculi of each nerve, generally eight 

 in number, cross the median line at the point of decussation, so that 

 the retina of each eye is exclusively supplied with nerve fibres from the 

 opposite side of the brain. Experiment furthermore shows that in the 

 pigeon, removal of the optic tubercle on one side produces complete 

 blindness in the opposite eye. 



III. In the mammalia, and in man, the two optic nerves are so inti- 

 mately consolidated at the chiasma, that the course of their respective 

 fibres cannot be determined by simple inspection, nor by the ordinary 

 means of dissection, but requires the aid of hardening fluids and micro- 

 scopic sections. These methods demonstrate that in all cases there is 

 a decussation at the median line. According to Henle, the decussating 

 fibres, in man, are arranged in laminae, about -^ of a millimetre in 

 thickness, which cross above and below each other from side to side ; 

 while in front and behind the chiasma sections of the optic nerves and 

 tracts present only the appearance of longitudinal fibres. All anatomists 

 are agreed that the greater part of the optic fibres decussate in this way 

 at the chiasma. But the majority also admit that this decussation, in 

 man and most quadrupeds, is incomplete ; a portion of the fibres of each 

 tract, situated upon its outer border, passing to the eye of the same side, 

 while the remainder cross at the chiasma, to the eye of the opposite 

 side. Each eye is supplied, according to this view, with nerve fibres 

 from the opposite optic tract and opposite side of the brain, and also 

 with fibres from the optic tract and the brain on its own side. There 

 is, furthermore, a transverse band of fibres, admitted by all modern 

 writers, passing across, at the posterior border of the chiasma, from 

 one optic tract to the other. This band is the only part of the chiasma 

 which remains intact after destruction of both eyeballs and consequent 

 atrophy of the optic nerves and tracts. It is considered as a transverse 

 commissure between corresponding parts of the brain, and as having 

 no direct connection with the sense of vision. The partial decussation 

 of the optic nerve, in man and the higher quadrupeds, is regarded by 

 many, and especially by Henle,* as demonstrated on anatomical grounds. 

 The possibility of this is denied by others; and the existence of direct 

 fibres, in addition to those which decussate, is no doubt largely inferred 

 from the partial disturbance of vision in pathological cases, and from 

 the results of physiological experiment. In birds, as above stated, 

 the complete decussation of the optic nerves at the chiasma ig 

 demonstrable by dissection ; and removal of one optic tubercle causes 

 absolute blindness on the opposite side without perceptible loss of sight 



* Ilandbuch der Nervenlekre des Menschen. Braunschweig, 1879, p. 389. 



