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Dr. G. L. Johnson. Contributions to the 



coloboma (Fuchs' Coloboma) has its analogy in a white or coloured 

 scleral ring, which is normally met with in a large number of animals. 

 7. Retinitis pigmentosa. In the Galagos and Lorides a spreading of 

 pigment occurs circumferentially in the retina, which greatly resembles 

 Retinitis pigmentosa. If these nocturnal animals are exposed for 

 prolonged periods to daylight the pigment advances concentrically, 

 similar to the manner in which it progresses in Man, so that the 

 animals gradually go blind. 8. Visible choroidal vessels and stippled 

 fundus. Visible choroidal vessels occur in most of the Simise below 

 Hylobates, and in a number of the other orders. They are most 

 marked in the Macropodidse, and some of the other Marsupials, which 

 present the appearance observable in the extreme cases of the analogous 

 congenital defect in Man. Stippled fundi are found in the feline 

 Douroucouli and in the Lemurs, an appearance occasionally met with 

 in Man. 9. Ectropion of the Uvea. In a number of the Ungulates, 

 which have large oval pupils, pigmented excrescences of the iris are 

 met with, and these evidently serve to screen the eye against glare, 

 since their pupils only contract moderately to light. In the Hyracoidse 

 we meet with a distinct specialised organ, which can be projected from 

 the iris towards the cornea, like a small screen, and this I propose to 

 call the " Umbraculum." 



The divergence of the optic axes follows the classification to a marked 

 degree. The higher the order the nearer the axes approach parallel 

 vision. Parallel vision with the power of convergence only occurs in 

 those animals which possess a true macula, viz., Man and all the 

 Simiae. In other words, convergence appears to be the necessary out- 

 come of a macula. This macula, which is bounded by a reflex ring, 

 exists in all the Simise without exception, and in no other Mammals, 

 so that ic ceases with the last of the Simise. 



If we eliminate the domestic animals in which the refraction varies 

 over considerable limits in all directions, we find throughout the 

 Mammalia, with a few notable exceptions, vision is hypermetropic. 

 The eyes of amphibious and marine Mammals are adapted for vision in 

 two ways. Those which live in fresh water have immensely developed 

 ciliary muscles and proportionally increased accommodative power, 

 enabling them to compensate for the loss of the refractive power of 

 the cornea when the eye is submerged. In the marine Mammals, 

 i.e., Pinnipedia and Cetacea, not only is this ciliary muscle greatly 

 developed, but there is always a large area of the cornea which is 

 flattened in the horizontal meridian, producing an extraordinary degree 

 of astigmatism. 



Binocular Vision. — It seems that if jNIammals below the Simiae have 

 binocular vision, they do not rely entirely on it. With the exception 

 of Man and the Simiae, Mammals very rarely move their eyes for the 

 purposes of vision, but move their heads instead. 



