306 VERTEBRATE ZOOLOGY 



the tongue and the lower jaw, if this has not already been done. 

 With bone forceps remove the roof of the skull. Do not, if avoid- 

 able, tear the dura mater— the outermost of the coatings of the 

 brain, lying next to the inner surface of the skull— but press it 

 away'from the skull as the removal of the bone continues. Expose 

 thus the dorsal and lateral surfaces of the brain, taking care to 

 preserve intact the olfactory lobes, which form the anterior end of 

 the brain. Remove now the dura mater from the dorsal and lateral 

 sides of the brain. 



The brain and spinal cord are enveloped in three membranes : 

 the dura mater, the thick, tough, outer membrane ; the pia mater, 

 the thin, inner membrane ; and the delicate arachnoid, between the 

 other two. 



Study the dorsal surface of the brain. It will be seen to be made 

 up of two main divisions, the cerebrum and the cerebellum. The 

 cerebrum is the anterior and larger portion, and is divided by a 

 deep median sagittal fissure into two lateral halves, the cerebral 

 hemispheres ; at the anterior end of the hemispheres are the two 

 olfactory lobes, from which the fibers of the olfactory nerves pass 

 to the nasal capsules. Note that the surface of the hemispheres is 

 smooth and without convolutions such as characterize the brain of 

 the higher mammals. Spread the hemispheres apart, but without 

 tearing the brain, and note at the bottom of the median fissure the 

 corpus callosum, a mass of white transverse fibers which join the 

 two hemispheres. The cerebrum forms the telencephalon, the first 

 (anterior) of the five subdivisions of which the vertebrate brain 

 is made up. At the hinder end of the median fissure will be seen 

 a small, dark-colored mass of tissue, the anterior choroid plexus, 

 which is the vascular, non-nervous roof of the third brain ventri- 

 cle. This ventricle is located in the diencephalon, which is the 

 second of the five subdivisions of the vertebrate brain ; rising on 

 this plexus may be seen the minute stalked pineal body. 



Immediately behind the cerebral hemispheres is the cerebellum, 

 the surface of which is much convoluted. Three regions are to be 

 distinguished, the median vermis, or worm, and the lateral hemi- 

 spheres ; from each hemisphere a convoluted lobe called the floc- 

 culus projects laterally. The cerebellum forms the dorsal portion 



