336 



PHYSIOLOGY 



CHAP. 



the relations between the cones and the optic fibres in the central 

 fovea have a peculiarity which distinguishes them from those of 

 the other parts of the retina. In the fovea only one cone or two 

 at most (Cajal's external neurone) articulate with one element of 

 the nuclear layer (middle neurone), and only one or at most two 

 elements of the nuclear layer articulate with one ganglion cell 

 (internal neurone). So that in the fovea the paths which conduct 

 the light impulses to the brain are least reduced and least 

 concentrated. From the fovea towards the periphery of the 



FIG. 161. Diagram of a section through the fovea centralis. Outlines' traced from a photograph. 

 Magnified 350 diameters. (From a preparation by G. N. Golding-Bird.) 2, ganglionic layer ; 

 4, inner nuclear ; 6, outer nuclear layer ; the cone-fibres forming the so-called external fibrous 

 layer ; 7, cones ; m.l.e., membrana limitans externa ; m.l.i., membrana limitans interna. 



macula, on the contrary, and thence towards the outer parts of 

 the retina, the conducting paths are greatly reduced, and the 

 excitations are carried to the brain from an increasing number of 

 rods and cones by a single fibre of the optic nerve. 



II. One of the fundamental principles of physiological optics 

 is the fact that when we desire to see an object distinctly we 

 direct our eye towards it in such a way that its image falls on the 

 fovea centralis of the retina. The fovea, which, according to Fritsch, 

 measures 1-1 '5 mm. across, therefore corresponds to the area of 

 most distinct vision; it subtends an angle of 3'5-7 with the 

 geometrical axis of the eye-ball or optic axis, which is known as 



