528 THE NERVOUS SYSTEM. 



a plane ; at its inner extremity it tapers to a point and becomes con- 

 tinuous with a fibre of the preceding layer. According to Schultze, 

 the inner half of each rod is slightly thicker, and exhibits rather less 

 refractive power than the outer half. 



The cones differ from the rods mainly in their tapering form and the 

 greater diameter of their inner portion, which is generally two or three 

 times as thick as that of the rods. Their extremities, in some regions, 

 stop short of the outer surface of the retina, as in Fig. 135, while in 

 that of most perfect vision they reach the same level with the rods. 

 Each cone is connected at its inner extremity with a nucleated fibre of 

 the preceding layer, the only peculiarity in this respect being that the 

 fibres and nuclei connected with the cones are larger than those con- 

 nected with the rods. 



Over the greater part of the retina the rods are more abundant than 

 the cones. When viewed from the outer surface (Fig. 136, A), their 

 closely packed extremities present the appearance of a fine mosaic, while 

 the cones are interspersed among them in smaller numbers. At the 

 borders of the macula lutea, the cones are more abundant, being only 

 separated from each other by single ranges of rods (B) ; and at its 

 central portion ( C) there are only cones, the rods being entirely absent. 

 But the cones at this point are longer and more slender than elsewhere. 

 In the following figure the smaller circles represent the rods, the larger 



circles the cones ; and in the interior of 

 FIG. 136. each cone is seen a section of its conical 



extremity. 



Eeception of Luminous Impressions 

 by the Retina. It appears, from the 

 above, that the retina is not simply an 

 expansion of the optic nerve. It is an 



OUTER SURFACE OF THE RETINAJ show- 



. From the edge of the macula lutea. for the reception of luminous rays, and 

 hoitz r ) m the maCUla 1Utea ' (Helm " connected by the optic fibres with the 



central parts of the brain. An exami- 



nation of the manner in which impressions of light are received brings 

 into view the following facts : 



The Optic Nerve and its Fibres are insensible to light. Notwith- 

 standing that this nerve is capable of transmitting impressions of sight 

 from the retina to the brain, yet in order to do this, it must first receive 

 its own stimulus from the retina. Its fibres cannot be called into 

 activity by the direct influence of luminous rays. This is shown by 

 the experiment of Bonders, in which a light of some intensity is con- 

 centrated upon the optic nerve, without being allowed to reach the 

 tissue of the retina. When the bottom of the eye is illuminated by 

 the ophthalmoscope, the observer sees the general surface of the retina 

 of a red or brownish color, while the papilla, at the entrance of the 

 optic nerve, presents itself as a circular white spot. This spot is occu- 

 pied entirely by optic nerve fibres, the elements of the retina com- 



