THE OPTIC NERVES. 391 



rods are therefore to be regarded as the termination of the optic fibres. 

 In the opinion of Miiller and Kolliker, the rods and cones composing 

 Jacob's membrane are the true percipients of light, communicating their 

 condition to the fibres of the optic nerve by means of the connection 

 which they thus maintain with it ; or, perhaps, the rods and cones are 

 conductors of the luminous impressions to the nerve-cells of the retina, 

 which constitute a ganglion capable of perceiving light, and the fibres of 

 the optic nerve merely communicate those impressions to the sensorium. 



Whichever of these descriptions we may follow, the physiological fact 

 which I desire to present with emphasis still remains the same. It is, 

 that the sentient or receiving part of the retina is the posterior, that 

 which is in contact with the black pigment. 



The second pair of nerves, from which the retina is thus derived, are, 

 from their function, designated the optic nerves. They do The optic 

 not enter the sclerotic in its optical axis, but at a little dis- nerves : their 



* . . . chiasm and 



tance on one side, and obliquely a provision doubtless in- passage to the 

 tended, in a measure, to avoid the occurrence of the blind bram< 

 spot on the centre of the field of vision, and to place it unsy in metrically 

 in the two eyes, so that each eye shall compensate the defect of the oth- 

 er. The nerves from each eye converge to their chiasm, which is a com- 

 missure consisting of three distinct systems of tubules an anterior set, 

 which are commissures between the two retinas, a posterior set, commis- 

 sures between the two optic thalami, and an interior set, the proper tu- 

 bules of the optic nerve, which cross, those from the right eye going to 

 the left side of the brain, and those from the left eye going to the right 

 side of the brain. The chiasm is therefore to be regarded as a complex 

 structure, its posterior region being independent of the other parts, and 

 existing in animals in which the optic nerve is not found, as, for exam- 

 ple, in the mole. 



Besides the optic nerve, which is exclusively the nerve of vision, the 

 collateral parts of the eye are supplied from various sources. 

 The third pair, or motores-oculorum, supply the superior, in- eye-ball and 

 ferior, and internal recti muscles, the inferior oblique, and the annexe P arts - 

 levator palpebrae. The fourth pair, or pathetici, supply the superior ob- 

 lique or trochlear muscles. Of the fifth pair, supplies are derived from 

 the frontal branch, lachrymal, the ciliary, and the infra-trochlear. The 

 sixth pair, or abducent, pass to the external recti : supplies are also de- 

 rived from the sympathetic. Of these nerves, the functions are very va- 

 rious ; some are for the movement of the ball, or for general sensibility 

 of the surface, or for the movements of the eyelids, or for those of the iris, 

 and some for the lachrymal apparatus. 



