52 1NNERVAT10N. [CHAP. XVII. 



can naturally affect it ; and the other parts endowed with the same 

 kind of excitability, can, in the natural state, be stimulated only in 

 a secondary manner, as though by induction through the retina. 

 It is certain that the integrity of these other parts is essential to 

 vision, and it may therefore be concluded that during vision they 

 are all, immediately or mediately, in a state of excitation. 



The retina is not affected sufficiently for the purposes of vision 

 by a very faint light ; and on the other hand, a very strong light, 

 especially if long applied, will produce effects analogous to those 

 resulting from an inordinate stimulus to other organs : the blood- 

 vessels of the retina will become unduly injected, its nutrition dis- 

 ordered, and even its texture destroyed. But the retina exhibits 

 a considerable power of accommodation to different amounts of 

 light, and thereby the utility of the organ is much enhanced, as 

 well as its safety provided for. After a short stay in the dark, 

 objects disclose themselves, which at first were imperceptible; and, 

 on the other hand, a light which was at first too bright, becomes 

 agreeable by use. This adaptability is quite independent of the 

 iris, and has its analogue in the case of every nerve of sense. 



The iris, however, by its contractile power, is a most important 

 agent in protecting the retina from the effects of sudden transitions 

 from dark to light ; and in thus co-operating for the maintenance 

 of its most essential quality, excitability, the iris is seconded by the 

 eyelids and brows. The iris contracts under a strong light, by 

 virtue of the stimulus imparted to the retina ; for if this or the 

 optic nerve be destroyed or paralysed, as in amaurosis, the iris no 

 longer contracts. It is interesting, however, to notice that the iris of 

 the unsound eye will often contract in company with its fellow, when 

 the opposite sound retina is stimulated. The motion is therefore 

 evidently caused by a reflexion of the stimulus from the optic nerve, 

 through the nervous centre along the inferior branch of the third 

 pair to the iris, and the consensual action of the two sides is ef- 

 fected in the nervous centre. Mechanical irritation of the ciliary 

 nerves, or third pair, occasions contraction of the pupil on the 

 same side. The orbicularis palpebrarum and corrugator supercilii 

 likewise contract under a powerful glare, in obedience to a stimulus 

 reflected through the optic nerve, and quite independently of the 

 will. This is well exemplified in children affected with strumous 

 ophthalmia, in whom the excitability of the retina is highly 

 exalted. 



The pigmentum nigrum is a permanent shield to the retina, 

 absorbing the light which falls upon it, and remaining the same 



