NEUROLOGY 



matter of the cerebral hemispheres. The roof-plate of the telencephalon remains 

 thin, and is continuous in front with the lamina terminalis and behind with the 

 roof-plate of the diencephalon. In the basal laminae and floor-plate the pars 

 optica hypothalami is developed; this comprises the anterior part of the tuber 

 cinereum, the infundibulum and posterior lobe of the hypophysis, and the optic 

 chiasma. The anterior part of the tuber cinereum is derived from the posterior 

 part of the floor of the telencephalon; the infundibulum and posterior lobe of the 

 hypophysis arise as a downward diverticulum from the floor. The most depen- 

 dent part of the diverticulum becomes solid and forms the posterior lobe of the 

 hypophysis; the anterior lobe of the hypophysis is developed from a diverticulum 

 of the ectodermal lining of the stomodeum. The optic chiasma is formed 

 by the meeting and partial decussation of the optic nerves, which subsequently 

 grow backward as the optic tracts and end in the diencephalon. 



The cerebral hemispheres arise as diverticula of the alar laminae of the telen- 

 cephalon (Figs. 650 to 654); they increase rapidly in size and ultimately overlap 

 the structures developed from the mid- and hind-brains. This great expansion 



of the hemispheres is a char- 

 acteristic feature of the brains 

 of mammals, and attains its 

 maximum development in 

 the brain of man. Elliott- 

 Smith divides each cerebral 

 hemisphere into three funda- 

 mental parts, viz., the rhinen- 

 cephalon, the corpus striatum, 

 and the neopallium. 



The rhinencephalon (Fig. 

 655) represents the oldest 

 part of the telencephalon, 



I 



Gyr. df. med 

 Gyr. olf. lat. 



Gyr. ambiens 



Gyr. diagonalis 



Gyr, semilunaris 



Cerebcllu 



Fia. 655. Inferior surface of brain of embryo at beginning of fourth 

 month. (From Kollmann.) 



and forms almost the whole 

 of the hemisphere in fishes, 

 amphibians, and reptiles. In 

 man it is feebly developed 

 in comparison with the rest 

 of the hemisphere, and com- 

 prises the following parts, 

 viz., the olfactory lobe (cdn- 

 sisting of the olfactory tract and bulb and the trigonum olfactorium), the anterior 

 perforated substance, the septum pellucidum, the subcallosal, supracallosal, and 

 dentate gyri, the fornix, the hippocampus, and the uncus. The rhinencephalon 

 appears as a longitudinal elevation, with a corresponding internal furrow, on the 

 under surface of the hemisphere close to the lamina terminalis; it is separated 

 from the lateral surface of the hemisphere by a furrow, the external rhinal fissure, 

 and is continuous behind w T ith that part of the hemisphere, which will ultimately 

 form the anterior end of the temporal lobe. The elevation becomes divided by 

 a groove into an anterior and a posterior part. The anterior grows forward as 

 a hollow stalk the lumen of which is continuous with the anterior part of the ven- 

 tricular cavity. During the third month the stalk becomes solid and forms the 

 rudiment of the olfactory bulb and tract; a strand of gelatinous tissue in the interior 

 of the bulb indicates the. position of the original cavity. From the posterior part the 

 anterior perforated substance and the pyriform lobe are developed; at the begin- 

 ning of the fourth month the latter forms a curved elevation continuous behind 

 with the medial surface of the temporal lobe, and consisting, from before backward, 

 of the gyrus olfactorius lateralis, gyrus ambiens, and gyrus semilunaris, parts which 



