^ouml\ jufrifS^^^ -^^^^^^ jff^^maw Vitreous Humour. 35 
have the same effect. It is impossible to say whether, under ordi- 
nary circumstances, the secretion of the vitreous fluid is rapid or 
slow, but a large quantity is known to be thrown out when occa- 
sion requires, as when some of it has become evacuated by accident. 
In such cases, however, the structure of the humour will seldom be 
reproduced in the fully developed eye. 
The fluid escapes so readily from the humour that it has become 
a problem how nature effects its retention. As will have been 
observed from the description which has been given of the structure 
of the humour, it is such as to afford ample facility for a free and 
rapid circulation of the fluid through its substance ; and therefore 
any force inherent in the tissue itself, such as capillary attraction, 
can have very little effect in holding the fluid in its meshes. Its 
retention is readily accounted for by the limited permeability of 
its enveloping structure — the hyaloid membrane — during life. 
Even after death, when that membrane has lost much of its resist- 
ance to permeation, if the vitreous body be exposed entire, the fluid 
will take some days to drain away, while a few hours is sufficient 
if the hyaloid membrane has been lacerated. From the natural 
firmness of the vitreous body, the retention of the fluid has been 
accounted for by those who deny that the humour is possessed of 
any structure, by the fluid in its normal condition having a certain 
consistence resembling jelly, in which case its exhaustion could only 
arise from the substance of the humour passing from the semisolid 
to the fluid condition. This is disproved by the fact that the fluid 
of the humour can be replaced by any other fluid, such as pure 
water, spirit, or glycerine, merely by imbibition, and the vitreous 
body still retain the same physical properties. 
As the vitreous body does not possess any function of a vital 
nature, and as it has a temperature amongst the lowest of any 
organ in the body, it is evident that it must undergo little decay, 
and exercise little demand over the nutritive forces for its renova- 
tion. Nevertheless, though it be a non-vascular body, and though 
the greater part of it be remote from the sources of the circulation, 
SB it is known to undergo various rapid changes in disease, it must 
possess a proper system of nutrition within itself. In the absence 
of all other evidence of a nutritive system within the humour, the 
cellular elements described under the microscopic anatomy of the 
humour probably contain nutritive materials ; and the anastomosis 
which exists between cell and cell — to adopt the language of Yir- 
chow,* on an analogous nutritive system in Wharton's jelly of the 
umbilical cord, — "renders possible a uniform distribution of the nu- 
tritive juices throughout the whole of its substance." The small 
amount of albumen which exists in the vitreous fluid is incapable 
in its crude state of being applied to the nutrition of the fabric of 
* ' Cellular Pathology/ p. 100. 
D 2 
