520 MR. youatt’s veterinary lectures. 
being left only under the arch of the fornix. An incision being 
made along the central sulcus, these laminae are easily separated 
from each other; and between them is found a little fissure or 
cavity, called by some the fifth cavity, and by others the fossa 
si/lvii. It usually contains a moisture or dew within it; but I 
have never seen the abundant serous fluid of which some have 
spoken. 
The Fornix. —The fornix is now brought into sight; the French 
call it the medullary or cerebral triangle: there is an indistinct 
resemblance to a triangle, with one of the angles presenting an¬ 
teriorly, and the other two posteriorly ; its lower surface is to¬ 
ward the third ventricle, and is spread over the optic thalami; 
a beautiful vascular membrane is interposed between them, and 
the impression of the vessels of this membrane seem to be formed 
upon it;—this is the psalterium or harp, and on its upper sur¬ 
face rests the septum lucidum. Of its four irregular extremities 
or crura, the anterior or inferior ones may be traced from the 
corpus albicans—winding round the thalami—bending upwards 
behind, and becoming connected with the anterior commissure; 
then approximating towards each other, and, erroneously sup¬ 
posed to unite, forming the arch or body of the fornix within the 
ventricles;—the posterior or superior crura are prolonged into 
the hippocampi, and running along the crura of the hippocampi 
form the corpora fimhriata, by their thin fringed floating edges. 
The Lateral Ventricles. —They are first observed when we 
are dividing the septum lucidum to demonstrate the fifth 
ventricle; and on reflecting back the roof we have their irre¬ 
gular shape and strangely-uneven floor fully exposed. The 
floor consists mainly of two convex, pyriform substances, and a 
central channel. That which appears outwardly and anteriorly 
is the corpus striatum. It may be said, indeed, to occupy almost 
the whole of the outer portion of the floor, being narrow poste¬ 
riorly, and assuming a broad pear-like form anteriorly. It is 
' covered by a lamina of medullary matter, so thin that the colour 
of the cortical substance beneath is seen through it. If this 
cortical substance is cut into, it presents an appearance more or 
less striated, differing in different species of animals, and often 
varying in animals of the same species; more evident usually in 
the herbivorous than the carnivorous animals, with the excep¬ 
tion of those that hunt by scent. The striaj take a direction 
from above downwards, and from without inwards ; and the 
broad extremities of the corpus striatum in each ventricle lying 
in approximation with each other, the strige from each seem to 
unite under the anterior commissure. I have never seen the 
changing direction of the strim which is spoken of in the human 
