634 



MANUAL OF HISTOLOGY. 



further outwards still as a system of delicate envelopes, the consideration 

 of which we must reserve for a future section., 



316. ' 



We must now devote our attention more particularly to the several 

 strata of the retina in succession. 



(1.) The columnar layer, stratum Ijaeillosum or membrana Jacobi, is 

 composed of two kinds of very remarkable elements the rods and cones, 

 which are arranged perpendicularly side by side. 



The rods, bacilli (fig. 589, b) are delicate cylinders extending through 

 the whole thickness of Jacob's membrane. They consist invariably of 

 two portions, as was shown by Braun and Krause, after Mutter, of an 

 external member, slender, homogeneous, and transparent, and possessed 

 of high refracting power, and of an internal member. The latter is some- 

 what greater in diameter than the first, and paler in outline; it not unfre- 

 quently presents a granular appearance. 



The internal joints of the rods become more deeply dyed by carmine 

 than the external, which, on the other hand, are 

 blackened by the action of osmic acid, while the 

 internal member remains for a long time quite 

 colourless in the same. The reagent just mention- 

 ed has come greatly into use since its application 

 by Schultze to the investigation of the structure 

 of the retina. According to this observer, the 

 external member is double refracting in the frog, 

 but not the internal. The length of the whole 

 rod in man, which is greatest in the posterior part 

 of the eye, is 0*0600 mm.; more anteriorly it is 

 O'OSOl mm., and at the ora serrata about 0-0399. 

 Its thickness may be stated at from 0'0016 to 

 0'0018 mm. (Milller). Eegarded from a chemical 

 point of view, these structures appear to con- 

 sist of an extremely perishable albuminoid sub- 

 stance. Owing to this, they present themselves 

 under the microscope in the greatest variety of 

 shapes. 



The external truncated end of the column is 

 in contact with the pigmentary epithelium of the 

 choroid. In the three first classes of the verte- 

 brates the cells of the latter form around the 

 external members of the rods (and also of the 

 cones) complete pigmentary sheaths. 



This may also be seen among the mammalia, 

 but slightly marked however. Prom the under 

 surface of these cells filiform prolongations of 

 their bodies extend downwards between the rods 

 (and cones), like pencils of very fine hairs. 



The inner members of the rods extend some- 

 what beyond the limitans externa in the form of very fine points, easily 

 broken off, and which are prolonged into filaments of the most extreme 

 delicacy. Under careful treatment with certain reagents, the latter are 

 found to present varicosities, similar to those so characteristic of nervous 

 filaments. Each of these bacillary threads traverses the external granular 



Fig. 589. Rods and cones from 

 the equatorial portion of the 

 retina, after Schultze. a, Mem- 

 brana limituns fxterna; 6, 

 rods; c, cones; V, rod gra- 

 nule; c', cone granule; d, in- 

 tergranular layer. 



