of Animal Bodies. 301 



point, quits its former course, and bends inwards towards the 

 opposite and inner side of the retina ; immediately on making 

 this deflection, it passes through the meshes of the vascular net- 

 work, derived from the central vein of the optic nerve. Before 

 arriving at the inner surface of the retina, it passes likewise 

 through a second vascular network, derived from the ultimate 

 ramifications of the central artery of the retina. Each nervous 

 cylinder, or tube, obtains at this place a sheath-like covering 

 from the vascular coat of the retina, and thus enveloped, termi- 

 nates in the form of a papilla behind the vitreous humour. 



To ascertain the reality of this structure many precautions 

 must be used, all of which are minutely detailed by Treviranusi 

 As very fresh ^es only must be employed, these investigations 

 will not always lead to very satisfactory results ; the cylinders 

 or tubes alluded to, lose their configuration very soon after 

 death, and are resolved into minute globules. The papillary 

 structure of the inner surface of the retina is consequently dif- 

 ficult to be verified in the human eye, unless the observer be 

 very muc-h accustomed to such investigations. Treviranus as- 

 serted positively, that the medullary or nervous layer of the 

 retina is not prolonged, as some have believed, to the zonula. 

 Near the border of the medullary lamina of the retina, he could 

 distinctly perceive that it is covered by two membranes, the 

 external homogeneous (membrana Jacobi) and the internal vas- 

 cular. Both these coverings extend beyond the nervous la- 

 mina, and coming into contact with each other, acquire longi- 

 tudinal plaits, and proceed between the vitreous humour arid 

 the choroid in] the form of corpora ciliaria to the zonula. Be- 

 tween these there is no trace of proper medullary or nervous 

 substance, and those who believe they have seen such, have been 

 deceived by the white appearance of the vascular lamina. Tre- 

 viranus remarks, that it would be in fact quite useless for the 

 medullary or nervous portion of the retina to be prolonged quite 

 beyond the limits of the space which lies within the reach of thfe 

 rays of light. '■ 



The papillary termination of the extreme nervous cylinders 

 which compose the retina, is confirmed by a similar distriblition 

 of the other nerves devoted to the senses, e. g. the acoustic and 

 olfactory. In order to examine this structure of the retina it is 



VOL. XXI. NO. XLII.^-OCTOBEB 18S6. X 



