^o'SSLljuriTff^ Adult Human Vitreous Humour. 33 
or those which take their origin from the extreme border of the 
zone of Zinn, have a coarser texture and web than those nearer the 
axis of the eye ; and the former are also more bent in their course 
backwards, following the curve of the hyaloid membrane, in adapta- 
tion to the globular form of the vitreous body. In the vertex, or 
what may be called the medullary portion of the humour, I have 
not been able to detect any distinct membranous layers. Faint 
lines may usually be recognized running backwards in this situa- 
tion ; but they partake of none of the characters of a membrane. 
Moreover, the central parts are devoid of the strong fibrous tissue 
which forms the basis of the membranous strata of the cortical por- 
tion of the humour. 
(c) As to the anastomosing cellular tissue, it occupies the medul- 
lary portion of the humour behind the crystalline lens and the 
spaces between the membranes, in the manner of intercellular 
tissue. It is woven into cellular spaces as receptacles for the 
vitreous fluid. The filaments of this tissue cross the short dia- 
meter of the intermembranous spaces, and therefore they have a 
radiating direction from the vertex to the sides of the vitreous body 
(Fig. 2) ; but, inasmuch as they are finer than the fibres of the 
membranous strata running from before backwards, they are not 
so apparent to the naked eye in the prepared specimen. In fact, 
the intermembranous spaces look to the naked eye as if they merely 
contain a transparent fluid — a fallacy which is only dispelled by 
observing them minutely as the direct rays of the sun are thrown 
upon them, or by touching them with a blunt instrument. 
Thus it will be found that while the membranes of the vitreous 
body are stretched between the zone of Zinn and the fundus of the 
eye, the anastomosing cellular tissue radiates from the vertex to the 
sides of the humour — an arrangement which offers the best provi- 
sion for the prevention of undue distension of the hyaloid mem- 
brane from the accumulation of fluid within. It is also not 
unworthy of note that the membranes, being the strongest fabric 
of the two, offer the greatest resistance to distension in the direc- 
tion in which it would be most injurious to vision — namely, in the 
direction of the axis of the eye. The crystalline lens and retina 
are by such an arrangement maintained at a fixed distance from 
each other ; and the delicate retina is also protected from undue 
pressure, between the hyaloid membrane, on the one hand, and the 
unyielding tunics of the eyeball, on the other. 
Such is the structure of the vitreous humour in the adult 
human subject. It is so fragile, and the proportion which it bears 
to the vitreous fluid is so small, that it is scarcely to be wondered 
at that it has escaped notice so long. Its weight makes no appre- 
ciable difference in the specific gravity of the fluid of the humour, 
which is 1053 ; for if the fluid of the humour be allowed to drain 
VOL. II. D 
