232 
Histology of the Eye. 
rirtonthly Microscopical 
L Journal, Nov. 1, 1869. 
Vitreous Humour. — This, in a perfectly healthy state, is a clear, 
colourless mass of gelatinous consistence, enclosed in a hyaloid 
membranous capsule. 
In the adult, the traces of structure perceptible in it are scanty 
and indistinct, conveying a very imperfect idea of its anatomical 
composition ; but in the foetus its formed elementary parts are 
recognizable without difficulty, and their combinations are easily 
made out ; so that we naturally turn to embryology for aid ; and 
this, as in so many other instances, explains points in the anatomy 
of the adult organ which would otherwise remain unintelligible. 
Genetically, the corpus vitreum is an extension of the deeper 
stratum of the cutis, intruded into the secondary eye-vesicle between 
the lens and the nervous lamina which becomes the retina. 
In order to make this quite clear, I must ask your attention to 
some matters in the development of the eye. 
The first trace of the eye in the chick, which makes its appear- 
ance very early, is a hollow protrusion from the front and lateral 
part of the foremost cerebral vesicle. Gradually, as this cerebral 
vesicle enlarges forwards, and divides into the two segments which 
Yon Baer called the Vordernhirn and the Zivischenhirn, the 
primary eye-vesicle shifts its place backwards and downwards until 
at length it lies beneath the Zwischenhirn ; there it becomes pedun- 
culated. The stalk— the future optic nerve — at first is hollow, and 
through it the cavity of the eye-vesicle communicates freely with 
the cerebral ventricle. 
The upper side of the eye-vesicle, where the stalk is placed, is 
towards the Zivischenhirn ; whilst its opposite side is towards the 
external tegument, which here consists of the epidermal stratum 
only, as Kemak thought, or which includes, as KoUiker believes, a 
part of the cutis. At this spot the epidermis thickens ; and an 
mhud of it, pressing on the summit of the primary eye-vesicle, 
pushes this inwards, so changing the globular shape of the vesicle 
into a cup) consisting of an inner and an outer plate, separated by 
an interspace, the remnant of the original cavity of the first vesicle, 
which continues for some time longer to communicate with the 
brain-ventricle through the still hollow eye-stalk. 
The cup thus formed, distinguished as the secondary eye-vesicle, 
is incomplete below ; and through this gap — the foetal cleft — the 
deeper stratum of the cutis intrudes between the epidermal inbud, 
which is the matrix of the lens, and the anterior plate of the 
secondary eye-vesicle, which is the foundation of the retina. 
It will be perceived that this intruded portion of cutis fills the 
space in the secondary eye-vesicle v/hich corresponds to that in 
the completed eye occupied by the vitreous humour. So long as the 
foetal cleft remains open, the intruded portion of cutis (which we 
may now call the vitreous humour) is directly continuous through 
