440 ORGANS OF THE SENSES. 



oculorum (derivations from the lenticular ganglion and nasal 

 branch of the ophthalmic division of the fifth nerve, ciliary 

 nerves to the number of fifteen), and the dilatator or radiat- 

 ing fibres by the great sympathetic nerve. The pupil dilates 

 when the light is not strong, or when the object presented is 

 at a distance, and contracts when the contrary is the case. 

 These movements are slow, the fibres being smooth muscular 

 fibres, like those of the ciliary muscle ; the movements of the 

 iris, like those of this muscle, have a reflex character, and 

 belong, no doubt, to the same centre of reflexion (see p. 59). 

 The iris appears, however, to be directly sensitive to the 

 action of light. The will has no power to produce any move- 

 ment of the iris, but this may be done indirectly by looking 

 into space, as if at an object placed at an infinite distance, 

 and the pupil will then dilate ; this simple method has been 

 frequently employed, especially in past times, for the purpose 

 of giving an expression of ecstasy to the eyes, this feeling 

 invariably being accompanied with great dilatation of the 

 pupil. Some valuable medicinal agents also possess the pro- 

 perty of producing dilatation or contraction : the Calabar bean 

 serves to contract, and belladonna (atropine) to dilate the 

 pupil for a longer or shorter space of time. 



The pupil is also dilated in certain diseases of the brain 

 and cord. Its normal movements are more active and fre- 

 quent in some persons than in others. We have already seen 

 that these contractions have only a subordinate position in 

 the process of accommodation, and we may, therefore, close 

 this part of our subject by saying that the iris is simply a 

 diaphragm which by a reflex action decides the diameter of 

 its own aperture. 



III. Sensitive membrane or retina. 



The retina is an extremely complicated membrane which 

 closely covers most of the inner aspect of the choroid tunic. 

 It is formed principally by the bifurcations or subdivisions of 

 filaments of the optic nerve, to the extremity of which special 

 terminal organs are attached. The optic nerve passes through 

 all the tunics or envelopes of the eye, at a point situated a 

 little within the posterior extremity of the antero-posterior 

 axis of the globe of the eye, and, as it reaches the inner aspect 

 of the choroid tunic (Fig. 119, P.), it expands (optic papilla 

 or optic disk}, and thus forms the internal layer 01 the retina; 

 subsequently the fibres of this layer bend and turn outwards 

 (Fig. 120), forming thus, by their juxtaposition, the substance 



