HISTOLOGY OF THE RETINA 



551 



UTILITY OF PERIPHERAL VISION. The high visual acuity of the fovea and 

 the great facility with which the,eyes can be directed, so that images form onthisregion , 

 might raise the question as to the utility of peripheral vision. This question may be 

 investigated experimentally by placing restricting screens in front of the eyes, or by 

 ascertaining the experience of persons who are suffering from blindness in the periphery 

 as the result of disease, e.g. retinitis pigmentosa. Both methods show that the periphery 

 is of great value in directing attention to outlying obstacles. Our attention being excited 

 we direct the gaze in the direction indicated, in order to bring into action the greater 

 power of analysis of the fovea. 



' NO BLIND 

 \ ZONE 



Fio. 281. Diagram Z shows size of blind zone in man. (HARTRIDGE.) Diagram 

 X shows how extremely peripheral rays enter the eye and reach the retina. 

 Diagram Y shows absence of blind zone in certain birds and animals. 



CENTRAL CONNECTIONS OF THE RETINA. The connecting paths 

 between the retina and the brain are formed by the optic nerves. At first 

 these are hollow tubes to which the optic cups are attached. After the retinal 

 artery has found its way through the cleft in the optic cups, the nerves fold 

 round it and become solid, and through their substance the nerve fibres from 

 the ganglion cells of the retina grow toward the brain. The primitive ground- 

 work of the nerve is also invaded close to the optic cup by mesoblast forming 

 the cribriform plate of the sclera. Through the meshes of this plate the 

 nerve fibres have to pass. Traced backwards the optic nerves leave the orbit 

 through the optic foramen accompanied by the ophthalmic artery. The 

 optic nerve has sufficient slack in order to permit free motion to the eyes. 

 Having entered the cranial cavity the nerve pierces the dura mater, and 

 meets its fellow from the other eye ; with this it connects, forming the chiasma, 

 and the fibres partially decussate. The fibres thus form the optic tracts 



