796 



THE CENTRAL AXIS OF THE NERVOUS SYSTEM. 



The second system comprises the fibres of the corpus callosum. It was 

 formerly believed that these fibres arose from the cerebral peduncles, but now it 

 is thought they pass merely from one hemisphere to another. The anterior com- 

 missure belongs also to this system ; but great uncertainty exists as to the 

 connectiojis of its fibres. 



The radiation apparatus is formed by the fibres of the corona radiata. Among 

 them a great number should proceed, according to Meynert, to the caudate 

 nucleus, the lenticular nucleus, and the optic thalamus {cortico-striated, cortico- 

 lenticular, and cortico-optic fibres). The others form two direct peduncular 

 fasciculi — one, occupying the anterior two-thirds of the corona radiata, going 

 from the motor convolutions of the cortex to the infero-lateral fasciculi of the 



Fig. 440. 



Fitr. 4U. 



ARRANGEMENT OF THE LAYERS AND CEUfl 

 OF A FRONTAL CONVOLUTION. 



1, Hyaline layer; 2, layer of the small 

 pyramid cells ; 3, thick layer of the great 

 pyramid cells ; 4, granular layer ; 5, 6, 

 layer of so-called volition cells (beneath 

 this layer is the white fibrillated 

 substance). 



CORTICAL SUBSTANCK 

 OF THE CEnKBRAL 

 HEMISPHERES. 



a, Medullary sub 

 stance ; b, reddish 

 grey layer ; c. clear 

 white streak, com- 

 posed of horizontal 

 fibres; d, grey layer; 

 e, external white 

 layer. 



spinal cord ; the other occupies the posterior part of the corona radiata, and 

 passes from the lateral and posterior convolutions of the hemisphere to disappear 

 in the cerebral peduncle. This fasciculus is sensitive. 



The ivhite suhstmice of the brain is formed of very fine nerve-fibres. The 

 grey substance of the cortex is made up of five or six superposed layers. In the 

 frontal region there are six layers, as follows : 1 . A granular layer, with very 

 small stellate cells. 2. A stratum containing numerous small pyramidal cells. 

 3. A very thick layer with large cells — great pyramidal, or giant cells. 4. A 

 granular layer formed of numerous small and uniform cellular elements. 5 and 

 6. Two layers having for their principal elements stellate or fusiform cells 

 (volition cells). 



In the occipital region, the third layer is absent ; but in the cornu Ammonis 

 it is very developed. Lastly, the giant pyramid cells are more particularly found 

 in the cortical regions in the vicinity of the crucial fissure. 



