HISTOLOGY OF THE RETINA 



553 



nerve paths. Injury to the optic nerves causes blindness of the eye to which 

 the nerve belongs, and stimulation of the eye by light will not then elicit 

 the pupil reflex. Injury to the optic tract causes blindness of the halves 

 of both retinae on the same side as the lesion, that is to say blindness 

 to external objects on the opposite side to the injury. It is interesting to 

 note that, whereas, in most other nerve paths, crossing of the impulse occurs 

 from one side to the other as it travels to the brain, so that the left side 

 of the brain corresponds to the right side of the body, this is not the case 

 with the optic impulses. These are already crossed by the optical apparatus 

 of the eye and therefore crossing of the impulses is rendered unnecessary. 

 Injury to the optic radiation or the occipital cortex will cause blindness of 

 both retinae on the same side, but will not affect the pupil reflex, because 

 these fibres have already turned aside to go to the anterior corpora quad- 

 rigemina. Injury of the middle of the chiasma, such as may occur in 

 tumours of the pituitary body, affects the nasal halves of both retinae and 

 produces double temporal hemianopia. 



THE OPHTHALMOSCOPE.* If the retina of a patient be illuminated by causing 

 a beam of light to enter the pupil, the reflected light will cause the interior surface 

 of the retina to be visible. In order to see the image distinctly it is necessary either 

 that both the eye of the patient and also that of the observer should be focussed for 

 infinity (the direct method) or that both eyes should be focussed for one and the same 

 intermediate plane (the indirect method). The former has many disadvantages which 

 are not found in the case of the latter, and therefore will not be considered further. 

 The indirect method is carried out as follows : A bright source of light having been 

 placed behind and slightly to one side of the patient, the observer standing about half 

 a meter in front of him reflects by means of a convex mirror an image of the light into 

 his pupil. At the centre of the mirror is an aperture through which the doctor sees 

 the light which is reflected back from the patient's retina. The observer now holds a 

 biconvex lens of about 6 cm. focal length about 8 cm. in front of the patient's eye, while he 

 still directs the beam of light into the pupil as before. The image of the retina,which would 

 normally be focussed by the lens system of the patient's eye at infinity, is now brought 

 to a focus by the convex lens, forming what is called an aerial image (see Fig. 283). 

 It is this image that the observer sees, and in it are shown all the particular features 

 of the vessels and nerves of the patient's retina. Beside its very great utility to the oculist , 

 the ophthalmoscope is a very valuable instrument to the physician, for retinal vessels 

 and nerves frequently show the evidences of constitutional disease, which are of great 

 assistance to diagnosis. Ophthalmoscopes are usually fitted with a number of small 

 lenses of graduated power, which may be introduced as required behind the mirror. For 

 the indirect method they are seldom required. The magnification of the retina given 

 by the direct method is usually about 3, while that provided by the direct is 12. If 

 a higher magnification is an advantage it may be obtained by using a biconvex lens of 

 longer focal length (say 12 cm.). 



FIG. 283. Diagram to show paths of rays from eye of patient to doctor when 

 the indirect method of ophthalmoscopy is in use. (HARTRIDGE.) 



