30 



THE FUNDUS OCULI OF BIRDS 



through the pupil is reflected by the struc- 

 tures of the background into the eye of the 

 observer is the principle of the ophthalmos- 

 cope. Moreover, this reflection, or rather 

 refraction, of the incident rays, modified by 



Fig. 20 

 Morton Ophthalmoscope 



the character of the media through which 

 they pass furnish the wonderfully varied and 

 colored pictures observed in the ocular back- 

 grounds of all the vertebrates. 



This brief account of the ophthalmoscope 

 and its method of employment is offered for 

 the benefit of those who have not used the 

 instrument, but no description of ophthal- 

 moscopy will much aid the zoologist in ac- 

 quiring a practical knowledge of its applica- 

 tion in the actual examination of animals' 

 eyes even in the favorable surrounding of the 

 darkened laboratory. Only a number of 

 months spent in the exploration of the eyes 

 of such domestic animals — including Man — 

 as are most readily accessible and easily ob- 

 served, followed by ophthalmoscopy of wild 

 species obtained in field work, will make an 

 expert of the student. While the writer will 



not say that a practical knowledge of the oph- 

 thalmoscope is essential to the training of the 

 scientific zoologist yet it is an instrument that 

 furnishes precise and valuable information 

 which the naturalist cannot afford to ignore. 



A. The Fundus Oculi or Background of the 

 Internal Eye in Alan. 



On account of the extensive literature on 

 this subject and because the human fundus, 

 normal and abnormal, has been carefully and 

 thoroughly explored by many observers for 

 over half a century no study of comparative 

 ophthalmology is complete without at least 

 a short account of the ophthalmoscopic find- 

 ings in Man. In the writer's Encyclopedia of 

 Ophthalmology, Vol. VII, p. 5315 et seq., 

 Moores Ball gives a good account of the 

 beautifully colored ophthalmoscopic picture 

 one sees in the depths of the human eye. 

 The parts of chief interest in the human fun- 

 dus are the optic disc, the blood-vessels, the 

 macula lutea, and the choroid, and they fur- 

 nish a satisfactory comparative study intro- 

 ductory to a consideration of avian fundi. 



Fig. 21 

 Electric Self-luminous Ophthalmoscope (de Zeng). 



The optic disc is situated about 3 mm. to 

 the nasal side of the posterior pole of the eye, 

 and is the point of entry of the optic nerve 

 into the retina. It measures from 1.4 to 1.7 

 mm. in diameter and is generally circular or 

 ellipsoidal in shape. Near its centre is a 



