If we examine a rabbit's B., we find it to consist, apparently, of three parts—the o:Pctory bulb, the cerebral hemispheres, and the cerebellum. che cerebral hemispheres are connected by a transverse band of union, or what is technically termed a commisture. Continuing the dissection, we turn aside the hemispheres, and find they have concealed four ganglia, which represent the single pair of optic ganglia we found in birds. There are two other bodies in front of those just alluded to—viz., the optic thalamus, and in front of it another (inferior) longitudinal commissure. This forms a communication between the anterior and posterior portions of the hemisphere, on the same side. Two little white lines, running from the back of the thalami, join a little body called the pineal ghind, in connection with some fantastic physiological theories. It will be observed that the hemispheres lie over these structures like a cap; the space between the two, on each side, is termed the lateral ventricle.
We have now the most complicated 13. before us, the human encephalic mass of ganglia, and include with it the medulla oblongata, tlie link which unites the B. to the spinal cord. First viewing the B. from its upper surface, we sec that it is divided by the longitudinal fissure into two equal halves or hemispheres, which are broader behind than in front. They are irregularly marked by convolutions, and a smooth appearance is given to the whole surface by the glistening arachnoid membrane (q.v.). On slicing them transversely with a knife, the section appears white in the center, and gray at the margins, of the convolutions, which are now seen to various depths below the surface. The white substance is dotted with the blooevessels which supply the brain. On drawing the hemispheres asunder from each other with the fingers, the great commissure, or uniting band. is seen. the corpus callosum, which is streaked both longitudinally and transversely. The hemispheres should now be completely sliced off on a level with this commissure, and its transverse fibers will be seen to extend into their substance, constituting a large white surface, called by anatomists the white oval center.
If we Lake the handle of the knife, and scratch with it through this white substance, the instrument soon opens a cavity, which is the lateral rentrude. Let this be done on both sides. and the ventricles exposed to view. They are shaped somewhat like the italic 8. Their extremities are termed cornua, and the anterior look from each other, and are nearer than the posterior, which are turned the opposite way. We have now removed the hemispherical ganglion, and uncovered the others. The pia nutter, which supports the vessels bringing blood to the B. substance, is seen in a purple wreath lying in each ventricle. and passing down into a depression termed the middle horn of the ventricle.
This is the choroid plexus, and, if lifted, it will be found continuous with that on the opposite side, through an aperture called the foramen of Monro, after the great Scotch anatomist of that name. If the remains of the corpus callosum are now scraped away, the choroid plexus will be found continuous with a web of pia mater called the velum interposituni, which lies over the central cavity of the B.. or third ventricle. In front and behind will be seen portions of the inferior longitudinal commissure or fornix, the body of which has been removed to show the velum; but, placed vertically between its anterior part and the under surface of the corpus callosum, are two layers of gray mat ter, between which is a narrow space termed the fifth ventricle. Behind, them will be seen a small hole, through which a probe will pass into the fourth ventricle.
We now come to the upper surface of the cerebellum, consisting of two hemispheres split transversely into leaflets, and connected by a central portion to each other, and by two bunches of white fibers to the corpora quadrigemina. Between these is the fourth ventricle; and stretched across between them is a thin layer of gray matter, called the valve of Vieussens.
We now turn what remains of the B. upside down. and examine the base or under surface. It is very irregular in outline. The cerebral hemispheres are now found to be divided on each side by °fissure (8glcian). The part in front is called the anterior lobe; that behind the middle, as far as the cerebellum, when it is called the pos. terior lobe.
The olfactory lobes are now seen lying in a fissure in the :Interior lobes. The optic: tracts arc seen meeting at their commissure, interchanging fibers, and passing on as the optic nerves to the orbit. The larger bundles behind, and directed outwards, are the cram eerebri, passing towards the hemispheres, emerging from the transverse mass called the pons rarolii, which lies like a clamp between the two halves of the cerebellum, From the inner side of each crux arises the third nerve, destined to supply four of the muscles which move the eyeball. The fourth nerve comes front the valve of Vicussens, and is seen on its way to 'supply the superior oblique muscle which turns the eye upwards and outwards, hence called pathetkas. Fronk each side of the pons the tifth pair arises; the sixth, between the polls and the anterior pyramids of the medulla oblongata; the eighth, consisting of: 1. Thu gloSso-pharyn geal, or nerve of deglutitiOu. 2. The pneumogastric distributed to the respiratory appa ratus and stomach; and with it, 3. The accessory.