642 MANUAL OF HISTOLOGY. 



nective-tissue found between these cells is small, while anteriorly the 

 fibres of Muller, becoming more and more numerous as we advance in 

 that direction, form regular compartments for their accommodation. 



(8.) We turn now to the expansion of the optic, nerve, the stratum 

 ftbrillosum. The nerve tubes of the options, which, with Schultze, we 

 believe to be possessed of no primitive sheath, and which must conse- 

 quently be of the same nature as the nerve tubes of the centres, lie in the 

 trunk of the former in bundles, separated by interstitial connective-tissue. 

 Here they are to be seen as dark fibres 0'0045-0-0014 mm. in thickness, 

 and frequently varicose ; and in this form they pass through the lamina 

 cribrosa. In their passage through the funnel-shaped opening in the 

 sclerotic, and as far as the collicuius nervi optici (i.e., that slight eminence 

 projecting internally at the point of entrance of the nerve), they lose this 

 dark-edged medullated appearance. Here they commence to spread 

 out, forming a membranous layer of pale fibres, that is, of naked axis 

 cylinders covering the inner surface of the retina (Bowman, Remak, 

 Schultze), but still associated in groups. Diverging now more and more, 

 the bundles are seen to anastomose at very acute angles, forming one of 

 those characteristic plexuses so frequently to be met with immediately 

 before the termination of nerves. Tracing up this expansion of the fibres 

 towards the ora serrata, we find the fasciculi becoming thinner and 

 thinner, and the distance increasing between them. Finally, scattered 

 nerve tubes alone are to be seen. These are extremely delicate and 

 marked with slight varicosities, and decrease in number more and more 

 the further we advance forwards. Throughout the whole retina they 

 probably terminate by sinking into the multipolar ganglion cells of the 

 layer already described. From what we have just seen we should expect 

 to find great inequality in the thickness of various parts of the stratum 

 fibrillosum ; and so it is ; thus, in the neighbourhood of the entrance of 

 the optic nerve, the layer is 0'29 mm. in depth, sinking rapidly to 0-099 

 mm., and decreasing so much anteriorly that close to the ora serrata it is 

 only 0-0026 mm. 



The occurrence of dark-bordered medullated fibres in some retinae is 

 remarkable. Thus, in the eyes of certain of the rodentia they exist 

 normally, as in the rabbit and hare, where they present themselves in the 

 form of two bands of white fibres streaming into the retina. The same 

 medullated retinal fibres have not unfrequently also been observed in the 

 eye of the dog, and in a few cases in that of the ox (H. Muller) and man 

 ( Virclww). 



The expansion of the optic fibres takes place between the extremities 

 of the fibres of Muller, as they are about to be inserted into the mem- 

 brana limitans internet. These fibres, as we have already remarked in 

 speaking of the stratum cellulosum, are at the fundus of the eye narrow 

 and fine, as they lie between the massive bundles of fibres situated here, 

 but become more and more bulky anteriorly, thus affording extra support 

 with their broad expanded bases to the nerve fibres, where the latter de- 

 crease in number, and seem specially to require it. 



(9.) The membrana limitans interna has been already described above. 



318. 



There now remains for our consideration the special structure of two 

 spots in particular in this complicated structure, the retina ; these are the 

 macula lutea and ciliary or anterior border. 



