COMPARATIVE ANATOMY OF THE NERVOUS SYSTEM 353 



which the blood vessels run. The pia mater follows closely all of the folds of 

 the brain surface. Between the pia mater and the dura mater is another mem- 

 brane, the arachnoid, very delicate and difficult to see. It is best found covering 

 the depressions on the surface of the brain, for the pia mater dips down into these 

 depressions while the arachnoid passes over them. Between the arachnoid and 

 the pia mater is the subarachnoid space crossed by a delicate web of tissue. All 

 of the spaces between the meninges of the brain are filled in life with the cerebro- 

 spinal fluid. 



b) The dorsal aspect of the brain: Remove the dura mater. Fit the two halves 

 of the brain and study the dorsal surface. At the anterior end of the brain are 

 the two olfactory bulbs, relatively small, rounded masses into whose anterior 

 surfaces the fibers of the olfactory nerve enter. Posterior to them are the 

 enlarged pear-shaped cerebral hemispheres. Their surfaces are much convoluted 

 in the cat, consisting of folds, the gyri, with grooves, the sulci, between the gyri. 

 The two hemispheres are separated from each other by a deep median sagittal 

 fissure, the longitudinal cerebral fissure (which is on the larger piece of the brain). 

 Gently spread open the fissure and note at its bottom a thick, white mass con- 

 necting the two hemispheres. This is the corpus callosum, a structure character- 

 istic of the mammalian brain. It is composed of nerve fibers passing between 

 the hemispheres. At the posterior end of the longitudinal fissure is a small red- 

 dish mass of folded tissue, which is part of the chorioid plexus of the roof of the 

 diencephalon. The diencephalon or region of the brain posterior to the cerebral 

 hemispheres is in mammals completely concealed from dorsal view by the poste- 

 rior extension of the hemispheres above it. The posterior ends of the cerebral 

 hemispheres are in contact with the cerebellum, a large mass with a much con- 

 voluted surface. Between the cerebellum and the cerebral hemispheres is the 

 midbrain, also concealed from dorsal view by the hemispheres. It is readily 

 revealed by bending the hemispheres and the cerebellum apart. It consists of 

 four rounded lobes or hillocks known as the corpora quadrugemina or colliculi. 

 The two anterior ones are named the superior colliculi, the two posterior ones 

 the inferior colliculi. The cerebellum consists of a median lobe, the vermis or 

 worm, and a pair of lateral lobes, the hemispheres. The vermis corresponds to 

 the entire cerebellum of lower vertebrates. From each hemisphere in the rabbit 

 arises by a narrow stalk another lobe, the flocculus, which as already seen is left 

 behind in the floccular fossa of the petrous bone when the brain is removed from 

 the skull. Identify on the hemispheres the cut surfaces where the flocculi were 

 attached. Posterior to the cerebellum and partly overlapped by the vermis is 

 the medulla oblongata. Lift the vermis and note the cavity of the fourth ventricle 

 in the medulla. It is normally roofed over by a membrane, the medullary velum, 

 which contains a chorioid plexus; this has probably been removed in sectioning 

 the brain. In the cat the chorioid plexus projects on each side between cere- 

 bellum and medulla as a little tuft of vascular tissue. At each side of the 



