248 VERTEBRATE ZOOLOGY 



brum and the cerebellum. 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 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 subdivisions, the cerebrum and the cere- 

 bellum. 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 nostrils. Spread the hemispheres apart, but 

 without tearing the brain, and note at the bottom of the median 

 fissure the corpus callosum, a mass of 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 

 ventricle,— that in the diencephalon, the second of the five sub- 

 divisions of the vertebrate brain; at the posterior end of this 

 plexus may be seen the minute pineal body. 



The area of the surface of the cerebrum will be seen to be very 

 much increased by the presence of folds, the gyri, which are sepa- 

 rated from one another by deep fissures called sulci. These gyri 

 are characteristic of mammals, all of which, except the lowest, 

 possess them. The surface of each hemisphere is divided into four 

 principal gyri whose general position is longitudinal, and whose 

 margins are not in all places distinct. 



Lateral to the sagittal fissure is the marginal gyrus, which is 

 sometimes double posteriorly. Lateral to this gyrus and separated 

 from it by the lateral sulcus is the suprasylvian gyrus. Lateral 

 to this gyrus and separated from it by the suprasylvian sulcus is 

 the ectosylvian gyrus, the form of which is that of an inverted U. 

 Between the limbs of this gyrus is the Sylvian gyrus, which forms 

 an inverted V, the two limbs of which are separated by the short 



