78 LABORATORY DIRECTIONS 



small hole in the skull and pick off the bones piece by piece 

 from the roof and sides of the head so as to expose the brain 

 and beginning of the spinal cord. On the ventral side of the 

 skull cut the membranes around the nerves. The pituitary 

 body of the brain fits into a depression, the sella turcica, of 

 the sphenoid bone. If possible cut the sphenoid so as to leave 

 the pituitary attached to the brain. Keep the brain in a jar 

 of formalin when not studying it. 



Meninges. — These are membranes covering the brain 

 and spinal cord. There are three in mammals. The outermost 

 membrane is the dura mater, which is quite thick and tough. 

 The pia mater is the delicate inner membrane that follows closely 

 all the folds of the brain surface. Between these two membranes 

 is a third, the arachnoid, very delicate and hard to see. The 

 spaces between the membranes are filled with fluids during 

 life. Amphibians, reptiles, and birds do not have the arachnoid, 

 while fishes have only one membrane, the primitive meninx. 



Dorsal Surface of the Brain. — Remove the dura mater. 

 The olfactory bulbs appear as relatively small swellings at the 

 anterior end of the brain. The olfactory nerves going to the 

 nostrils pass forward from them. The bulbs may have been 

 broken in removing the brain from the skull. 



The large convoluted cerebral hemispheres lie posterior 

 to the olfactory bulbs and make up a large proportion of the 

 brain. The convolutions are known as gyri, and the grooves 

 between the gyri as sulci. The two hemispheres are separated 

 from each other by a deep longitudinal fissure. By gently 

 spreading the hemispheres a thick white mass may be seen 

 at the bottom of the fissure. This is the corpus callosum, 

 consisting of nerve tracts connecting the two hemispheres. 

 It will be mentioned again later. 



The cerebellum lies directly behind the cerebral hemispheres, 

 consisting of a median part, the vermis and two lateral hemi- 

 spheres. Between the cerebellum and the cerebral hemispheres 

 lies the mesencephalon, concealed by the hemispheres. 



The cerebellum overhangs the medulla oblongata posteriorly. 

 Beneath the vermis of the cerebellum Is the cavity of the fourth 

 ventricle of the medulla. The fourth ventricle is normally 

 covered by a vascular membrane, the choroid plexus. The 

 medulla narrows posteriorly to form the spinal cord. 



