THE INTEKNAL COAT THE NERVOUS TUNIC 



649 



tn /. e. 



and stained preparations, the distinction being due to the fact that the 

 inner segment of each rod, while finely granular and easily stained, is 

 also singly refractive ; the outer homogeneous segment, on the other hand, 

 not only stains with difficulty but is 

 doubly refractive or anisotropic. The 

 outer, therefore, under all conditions 

 appears bright and lustrous as com- 

 pared with the isotropic inner seg- 

 ment. The outer segment is said to 

 be covered by a delicate sheath of 

 neurokeratin, and to consist of a sub- 

 stance chemically very similar to 

 that of the myeliri of medullated 

 nerve fibers. 



The outer segment contains the 

 visual purple or rJiodopsin which, 

 during life, is rapidly bleached by 

 exposure to light, and is as rapidly 

 renewed through the agency of the 

 pigment epithelium. 



Both segments, but especially the 

 inner, under favorable conditions, 

 present slight longitudinal striations. 

 These striations, when present, are 

 most distinct in the outer half of the 

 inner rod segment where they form 

 the so-called ellipsoid; they are gen- 

 erally interpreted as due to linear FIG. 549. DIAGRAM OF THE ROD 

 surface grooves in the outer segments, S-^^B^N^ 

 and to fibrils in the ellipsoid. The RONS. (Schwalbe.) 



outer filamentous segment of each rod 



m.l.e., external limiting membrane; 



sometimes exhibits transverse mark- n j. e . ; external nuclear layer; r.l.e., 

 ings, possibly indicating a minute external reticular (molecular) layer; 

 structure which is comparable to a 

 series of superposed disks. 



The rods have an average length of 60 microns, and an average 

 diameter of 2 microns; of the entire extent, the outer cylindric seg- 

 ment and the inner spheroidal segment contribute approximately equal 

 portions. 



The inner or nucleated portion of each rod, the rod fiber, is found 

 41 



