SENSE OF SIGHT. 



865 



admit of as close analysis as the physical action of the different refract- 

 ing media of the eye. It is known that it is only the layer of rods and 

 cones of the retina that is concerned in the formation of the imao-e since 

 rays of light pass through the anterior layers of the retina without giv- 

 ing rise to any sensations. The retina is a highly complicated nervous 

 apparatus, which may, by the use of the microscope, he divided into 

 eight and probably ten distinct laj'ers. The 

 innermost layer — i.e., the layer in contact 

 with the vitreous humor — consists of nerve- 

 fibres in which the optic nerve terminates, 

 radiating from the entrance of the optic 



Fig. 386.— Vertical Section op Human 

 Retina. {Landois.) 



a, rods and cones j b, external, j, internal limiting 

 memwanes; c, external, and/, internal nuclear layers: 

 e, external, and g. internal granular layers ; k, blood- 

 vessel and nerve-cells ; i, nerve-fibres. 



Fig. 387.— Layers of the Retina. 

 (Landois.) 

 Pi, hexagonal pigment cells; St, rods and cones; 

 La, external limiting membrane; uuK, external nuclear 

 layer; iingr, external granular layer; inK, internal 

 nuclear; tn^r, internal granular; Gi/l, ganglionic 

 nerve-cells; 0. fibresof optic nerve; Xf.intemal limiting 

 membrane ; Rk, fibres of M u I ler ; K, nuclei ; Sg, spaces 

 for the nervous elements. 



nerve (Fig. 385). At this point, therefore, the retina will consist 

 only of nerve-tubules. At one spot in the centre of the retina no nerve- 

 fibres are to be distinguished, and on account of its color this point is 

 termed the macula lutea, or yellow spot, and is the point of most acute 

 vision. 



The layers of the retina have been described as follows : First, and 



55 



