_Pedunc._) is usually called the _crura_ or
thighs of the brain. The right crus, running through the thalamus,
expands by successive additions into the right hemisphere, and the
left crus into the left hemisphere, of the cerebrum, and the two
hemispheres unite together on the median line by the corpus callosum.

There is very little space for the crura (plural of crus) between the
pons and the thalamus, but if we look at the posterior surface of the
ascending fibres or crura we see a larger surface, on which we find a
quadruple elevation called the _corpora quadrigemina_ (the four
twins). This is an important intermediate structure between the
cerebrum and the cerebellum, and in fishes is the largest part of the
brain, but in man is the smallest portion, as will be explained
hereafter, and is the origin of the optic nerve, as well as a
commanding head for the spinal system, from which convulsions may be
produced.

The quadrigemina are distinguished also as the location of the pineal
gland, which rests upon them, to which we may ascribe important
psychic functions. The engraving shows the fibres connecting the
quadrigemina with the cerebellum, and a channel under them (aqueduct
of Sylvius) connecting the ventricles of the cerebrum with those of
the spinal cord. What is called the fourth ventricle is the small
space between the medulla oblongata and the cerebellum. At this spot
the posterior surface of the medulla oblongata, as it gives origin to
the pneumogastric nerve, which conveys the sensations of the lungs,
becomes the immediate source of the respiratory impulse on which
breathing depends, and hence is of the greatest importance to life. A
very slight injury at this spot with a lancet or point of a knife
would be fatal. It is recognized by converging fibres which look like
a pen, and are therefore called the _calamus scriptorius_, or writer's
pen.

If the reader has not fully mastered the intricacy of the brain
structure, he will find his difficulties removed by studying two more
skilful dissections. The following engraving presents the appearances
when we cut through the middle of the brain horizontally and reveal
the bottom of the ventricles, in which we see the great ganglion, or
optic thalamus and corpus striatum, and the three localities at which
the hemispheres are connected by fibres on the median line, called
anterior, middle, and posterior commissures. These commissures are of
no importance in our stud