EYE. 



191 



layer only which extends so far, and others that 

 the vascular layer extends over the lens. No 

 one however at present, who describes from 

 observation, denies the termination of the ner- 

 vous layer at the posterior margin of the ciliary 

 body, although many insist upon the extension 

 of the vascular layer to the circumference of the 

 lens. The subject has received more attention 

 than it deserves, as it involves no consideration 

 of importance, either physiological or anato- 

 mical ; but I am convinced from a very care- 

 ful scrutiny that no such layer extends between 

 the ciliary processes of the choroid and those of 

 the hyaloid membrane ; these two parts being 

 mutually inserted into each other, as will pre- 

 sently be explained. In the paper above 

 quoted in the Medico-Chirurgical Transactions 

 I have explained what appears to me to be the 

 arrangement of this part in the following words : 

 " On removing the choroid, ciliary processes, 

 and iris, we see the retina terminating with a 

 denned dentated margin, about a quarter of an 

 inch from the circumference of the lens : be- 

 tween this line of termination and the lens, the 

 vitreous humour retains upon its surface part 

 of the black pigment which covered the .ciliary 

 processes. If the eye be examined shortly 

 after death, removing the black pigment from 

 this part of the vitreous humour with a camel- 

 hair pencil, there is an appearance of, at least, 

 the vascular layer being continued to the lens ; 

 this part not being so transparent as the rest of 

 the hyaloid membrane, or so opaque as the retina. 

 From such an examination I was led to con- 

 clude that the vascular layer was continued to 

 the margin of the lens, this part not being 

 so transparent as the rest of the hyaloid 

 membrane, or so opaque as the retina. From 

 such an examination I was led to conclude 

 that the vascular layer was continued to the 

 margin of the lens, but I adopted a con- 

 trary opinion after I had witnessed the change 

 which took place when the part had remained 

 twenty-four hours in water : the retina then 

 separating with a slight force, and frequently 

 detached by the disturbance given in making 

 the examination. If, after removing the choroid 

 without disturbing the retina, the part be al- 

 lowed to remain in water for some days, the 

 medullary part of the retina begins to give 

 way, and may be altogether detached by agita- 

 tion in water, leaving the vascular layer firmly 

 attached at the line of termination just de- 

 scribed. With all the care I could bestow, I 

 have, however, never succeeded in separating 

 this layer from the vitreous humour further. If 

 the maceration be continued for a few days 

 longer, the vascular layer of the retina gives 

 way, the larger vessels alone remaining attached 

 at the original line of termination of the retina, 

 and appearing to enter the hyaloid membrane 

 at this part ; the appearance which at first so 

 much resembled the vascular layer proceeding 

 towards the lens remaining unchanged, being 

 in fact part of the vitreous humour itself. The 

 circumstance which has most strengthened the 

 notion of the retina being continued forward to 

 the lens is, that often on raising the choroid and 

 ciliary processes from the vitreous humour, we 



find those processes covered in several places 

 by a fine semi-transparent membrane insinuated 

 between the folds ; this is supposed to be the 

 vascular layer of the retina, but is really the 

 corresponding part of the hyaloid membrane 

 which is torn up, being firmly united to this 

 part of the choroid." 



After this article had been prepared for 

 press, I received an admirable monograph upon 

 the retina by B. C. R. Langenbeck, son of the 

 celebrated professor of that name in the Uni- 

 versity of Gbttingen, in which the nature, 

 structure, and relations of this most important 

 and interesting part of the organ are subjected 

 to a critical and elaborate inquiry. He advo- 

 cates the membranous nature of the black pig- 

 ment on the inner surface of the choroid, and 

 gives an engraving of its organization as ascer- 

 tained by the microscope, resembling that given 

 from the essay of Mr. Jones in the preceding 

 pages. He devotes several pages to the de- 

 scription of the membrane which I found 

 covering the medullary layer of the retina, and 

 adds the testimony of a skilful anatomist in 

 support of my description, sufficient to coun- 

 terbalance the convenient scepticism of certain 

 writers better skilled in making plausible books 

 than difficult dissections. The fibrous struc- 

 ture of the medullary layer of the retina is 

 established, and a plate given of the peculiar 

 nodulated condition of these fibres. The work 

 concludes with an account of the morbid 

 changes of structure observed in the retina, a 

 subject which, notwithstanding its manfest 

 importance, has not hitherto attracted the atten- 

 tion which it deserves. I am indebted to Dr. 

 Graves for the following abstract of some 

 recent investigations of Treviranus on the same 

 subject. " From microscopical examinations 

 Treviranus demonstrates that the cerebral mass, 

 both medullary and cortical, consists of hollow 

 cylinders containing a soft matter. These 

 cylinders, extremely minute in the cortical 

 substance, are somewhat larger in the medul- 

 lary, and still larger in the nerves. In the 

 retina he finds, that after the optic nerve has 

 penetrated the sclerotic and choroid, its cylin- 

 ders or nervous tubes spread themselves out 

 on every side either singly or collected into 

 bundles, each cylinder or collection of tubes 

 bending inwards through the vascular layer, 

 and terminating in the form of a papilla on 

 the vitreous humour." 



Of the vitreous humour. It has already 

 been stated that the globe of the eye is 

 divided into two chambers by the iris, the 

 posterior of which is distended by a spherical 

 transparent mass called the vitreous humour, 

 which does not completely fill this chamber 

 between the back of the iris and the hollow 

 sphere of the retina, but is discontinued or 

 compressed at a short distance from the back 

 of the iris, having a narrow space between 

 it and that membrane, called the posterior 

 chamber of the aqueous humour. This trans- 

 parent mass is composed of water containing 

 certain saline and animal ingredients, deposited 

 in exquisitely delicate and perfectly transparent 

 cellular membrane ; hence it is capable of sus- 



