33° 



NERVOUS SYSTEM OF VERTEBRATES. 



of the lamina terminalis is evident upon comparing a sagittal 

 section of the bat or rabbit brain with that of the cat or man. It 

 is shown diagrammatically in Figure i68. A further effect is to 

 stretch the mesial walls of the hemisphere at the same time with 

 the lamina terminaUs. As the anterior, hippocampal and callosal 

 commissures form in the lamina terminalis the lamina becomes 

 thickened by the invasion of gray substance from the adjacent 

 precommissural body. This gray substance forms a bed for the 



Fig. 169. — Scheme of cerebral commissures and margin of the cortex in the 

 human brain. From G. EUiot Smith, a'.a". extra ventricular alveus; c,c', corpus 

 callosum; d, fascia dentata; /,/',/", fimbria; h',h",h"', reduced hippocampus; 

 1,1', I", lamina terminalis; 0,0', o", olfactory bulb and peduncle; p, precommissural 

 body; p', septum pellucidum; r, pyriform lobe; s, splenium of corpus callosum; 

 V, anterior commissure; x, optic chiasma. 



commissural fibers. As the callosum expands and bulges and 

 folds upward and backward it stretches the commissure-bed and 

 also the closely related precommissural body, until the upper 

 part of that body is drawn out into a thin membrane. As this 

 occurs in the mesial walls of both hemispheres two thin membranes 

 are formed facing each other and filHng in the somewhat triangular 

 space between the two limbs of the callosum. These membranes 



