554 COMPARATIVE PHYSIOLOGY. 



in the microscope and telescope, ■which none of Nature's forms 

 can approach. 



Pathological. — There may be ulceration of the cornea, in- 

 flammation of this part, or various other disorders which lead 

 to opacity. The low vitality of this region, probably owing to 

 absence of blood-vessels, is eyidenced by the slowness with 

 which small ulcers heal. Opacity of the lens (cataract) when 

 complete causes blindness, which can be only partially reme- 

 died by removal of the former. Inflammations of any part of 

 the eye are serious, from possible adhesions, opacities, etc., fol- 

 lowing. Should such be accompanied by great excess of intra- 

 ocular tension, serious damage to the retina' may result. Of 

 course, atrophy of the optic nerve (due to lesions in the brain, 

 etc.) is irremediable, and involves blindness. Inspection of the 

 internal parts of the eye (fundus oculi) often reveals the first 

 evidence of disease in remote parts as the kidneys. 



From what has been said of the movements of the two eyes 

 in harmony, etc., the student might be led to infer that disease 

 of one organ, in consequence of an evident close connection of 

 the nervous mechanism of the eyes, would be likely to set up 

 a corresponding condition in the other unless speedily checked. 

 Such is the case, and is at once instructive and of great practi- 

 cal moment. 



Paralysis of the various ocular muscles leads to squinting, 

 as already noticed. 



Brief Synopsis of the Physiology of Vision.— AU the other 

 parts of the eye may be said to exist for the retina, since all are 

 related to the formation of a distinct image on this nervous ex- 

 pansion. The principal refractive body is the crystalline lens. 

 The iris serves to regulate the quantity of light admitted to the 

 eye, and to cut off too divergent rays. In order that objects at 

 different distances may be seen distinctly, the lens alters in 

 shape in response to the actions of the ciliary muscle on the 

 suspensory ligament, the anterior surface becoming more con- 

 vex. Accommodation is associated with convergence of the 

 visual axes and contraction of the pupil. The latter has circular 

 and radiating plain muscular fibers (striped in birds, that seem 

 to be able t» alter the size of the pupil at will), governed by the 

 third, fifth, and sympathetic nerves. Contraction of the pupil 

 is a reflex act, the nervous center Ijring in the front part of the 

 floor of the aqueduct of Sylvius, while the action of the other 

 center (near this one) through the sympathetic nerve is tonic. 



