ON THE BRAIN. 
528 
sorily pointed out to you as illustrations of the origins of the 
sensitive nerves, deep in the centre of the brain. We observe 
considerable difference in the development of these tubercles : in 
proportion as the lobes of the cerebellum are enlarged, so are 
the optic thalami—in proportion as other tubercles, which I shall 
have presently to describe, are enlarged, the optic tubercles are 
diminished. When we cut into the thalami we find a consider¬ 
able quantity of cineritious matter, as in all the other irregular 
prominences at the base of the brain. 
The Commissura Mollis. —These thalami are united together 
by a medullary mass so soft and pulpy as to assume the name of 
the soft commissure. It yields to the slightest attempt to sepa¬ 
rate the thalami; in fact, it is a continuation of the cineritious 
matter under the medullary covering, and of which the thalami 
are composed. 
The Tcenia Semicircular is. — Between the thalami and the cor¬ 
pora striata, and partly concealed by the plexus choroides, w’e 
observe a singular tract of medullary matter running from the 
hippocampi down to the anterior commissure, and connecting to¬ 
gether the posterior and anterior parts of the brain ; as the ante¬ 
rior commissure from one corpus striatum to the other, and the 
posterior commissure above the commissure mollis connect to¬ 
gether the sides of the brain. The use of all these links will be 
ascertained by some future physiologist. 
The Foramen Commune Inferius. —There is a curious con¬ 
nexion between the cavities as well as the substance of the brain, 
of w’hich we have here an evident proof. Observe this triangu¬ 
lar-shaped hole below the soft commissure, and between it and 
the inferior one ; it conducts to a sulcus betw^een the thalami, 
formed by their divergence. We call it a ventricle, but it is no¬ 
thing more than a little gutter, having the thalami, the fornix, 
and the velum, as its roof, and the crura cerebri as its floor. It is 
continued backward under the posterior commissure to a wider 
canal, the fourth ventricle, lying under the cerebellum, and hav¬ 
ing it as its roof, and the medulla oblongata as its floor. 
The Infundibulum.— At the bottom of the anterior part of the 
third ventricle, and under the foramen commune inferius, is an¬ 
other funnel-shaped opening. It is comparatively large in the 
ventricle, but rapidly narrows, and conducts towards the pituitary 
gland, seated immediately beneath. Of its office, or rather that 
of the pituitary gland, I shall immediately speak. 
The Pineal Gland. —Between the commissure mollis and 
superior, is another foramen, which leads down to the third ven¬ 
tricle, and would likewise form a communication between the lateral 
ones, and the third and fourth, if it were not closed by the velum 
