716 OF THE FUNCTIONS OF THE NERVOUS SYSTEM. 



of the eyeball itself, occasioned by an action of some of its muscles. It is 

 principally noticed under the two following conditions : 1. When an object 

 is brought very near the eye, and we steadily fix our attention upon it, the axes 

 of the two eyes are made to converge ; and if this convergence be carried to a 

 considerable extent, so that the pupils of both eyes are sensibly directed towards 

 the inner canthus, a contraction of the pupil takes place. The final cause or 

 purpose of this contraction is very evident. "When an object is brought near 

 the eye, the rays proceeding from it would enter the pupil (if it remained of 

 its usual size) at an angle of divergence so much greater than that which would 

 allow them to be properly refracted to a focus, that indistinct vision would 

 necessarily result. By the contraction of the pupil, however, the extreme or 

 most divergent rays are cut off, and the pencil is reduced within the proper 

 angle. The principle is precisely the same as that on which the optician applies 

 a stop behind his lenses, which reduces their aperture in proportion to the short- 

 ness of their focal distance. 2. Contraction of the pupil is also noticed, when 

 the eyeball is performing that rotation upwards and inwards, which, when it 

 occurs along with violent respiratory actions, or during sleep, must be regarded 

 as involuntary. (This rotation also takes place, to a slight degree, when the 

 eyelid is depressed, as in ordinary winking; and it is obvious that, in this 

 manner, the surface of the eye is more effectually swept free from impurities 

 which may have gathered upon it, than it would be by the downward motion of 

 the lid alone.) But the pupil is not contracted, when the eyeball is voluntarily 

 rotated upwards and inwards. Besides the contractions of the pupil, another 

 action of a " reflex" character is produced through the Optic nerve the con- 

 traction of the Orbicularis muscle under the influence of strong light, or when 

 a foreign body is suddenly brought near the eye. But this cannot be produced 

 without a consciousness of the object; in fact, it is a movement of a " consen- 

 sual" kind, produced by the painful effect of light, which gives rise to the 

 condition well characterized by the term photophobia. The involuntary charac- 

 ter of it must be evident to every one who has been engaged in the treatment 

 of diseases of the eyes; and the effect of it is aided by a similarly involuntary 

 movement of the eyeball itself, which is rotated upwards and inwards, to a 

 greater extent than the Will appears able to effect. Another reflex movement 

 excited through the visual sense is that of Sneezing, which is induced in many 

 individuals by the sudden exposure of the eyes to a strong light : of the purely 

 automatic character of this movement there can be no question, since it cannot 

 be imitated voluntarily ; and that it is not excito-motor is proved by the fact 

 that it is not excited unless the light is seen. 1 



742. There is a further peculiarity, of a very marked kind, attending the 

 course of the Optic nerves ; this is the crossing or decussation which they under- 

 go, more or less completely, whilst proceeding from their ganglia to the eyes. 

 n some of the lower animals, in which the two eyes (from their lateral position) 

 have entirely different spheres of vision, the decussation is complete ; the whole 

 of the fibres from the right optic ganglion passing into the left eye, and vice 

 versa. This is the case, for example, with most of the Osseous Fishes (as the 

 cod, halibut, &c.) ; and also, in great part at least, with Birds. 2 In the Human 

 subject, however, and in animals which, like him, have the two eyes looking in 

 the same direction, the decussation seems less complete ; but there is a very 

 remarkable arrangement of the fibres, which seems destined to bring the two 

 eyes into peculiarly consentaneous action. The posterior border of the Optic 

 Chiasma is formed exclusively of commissural fibres, which pass from one optic 



1 A patient was for some time in the London Hospital, in whom there was such an undue 

 impressibility of the retina that she could not remain in even a moderate light without a 

 continual repetition of the act of Sneezing. 



2 See Solly on " The Human Brain," Am. Ed. 



