444 



THE OLFACTORY LOBES. 



(septum lucidum) formed by the fusion of the lateral walls of the 

 hemispheres (figs. 255 and 259 cal), and at first only its curved 

 anterior portion the genu 

 or rostrum is developed. ^ 

 This portion is alone found 

 in Monotremes and Marsu- 

 pials. The posteriorportion, 

 which is present in all the 

 Monodelphia, is gradually 

 formed as the hemispheres 

 are prolonged further back- 

 wards. 



Primitively the Mam- 

 malian cerebrum, like that 

 of the lower Vertebrata, is 

 quite smooth. In many of 

 the Mammalia, Monotre- 

 mata, Insectivora, etc., this 

 condition is nearly retained 

 through life, while in the 

 majority of Mammalia a 

 more or less complicated system of fissures is developed on the 

 surface. The most important, and first formed, of these is 

 the Sylvian fissure. It arises at the time when the hemi- 

 spheres, owing to their growth in front of and behind the 

 corpora striata, have assumed a somewhat bean-shaped form. 

 At the root of the hemispheres the hilus of the bean there 

 is formed a shallow depression, which constitutes the first trace 

 of the Sylvian fissure. The part of the brain lying in this fissure 

 is known as the island of Reil. 



The olfactory lobes. The olfactory lobes, or rhinencephala, 

 are secondary outgrowths of the cerebral hemispheres, and con- 

 tain prolongations of the lateral ventricles, but may however be 

 solid in the adult state. According to Marshall they develop in 

 Birds and Elasmobranchs and presumably other forms later 

 than the olfactory nerves, so that the olfactory region of the 

 hemispheres is indicated before the appearance of the olfactory 

 lobes. 



In most Vertebrates the olfactory lobes arise at a fairly early 



FIG. 262. LATERAL VIEW or THE BRAIN 

 OF A CALF EMBRYO OF 5 CM. (After Mihal- 

 kovics.) 



The outer wall of the hemisphere is re- 

 moved, so as to give a view of the interior of 

 the left lateral ventricle. 



hs. cut wall of hemisphere ; st. corpus 

 striatum; am. hippocampus major (cornu am- 

 monis) ; d. choroid plexus of lateral ventricle ; 

 fm. foramen of Munro; op. optic tract; in. 

 infundibulum ; mb. mid-brain ; cb. cerebellum ; 

 IV. V. roof of fourth ventricle ; ps. pons Va- 

 rolii, close to which is the fifth nerve with 

 Gasserian ganglion. 



