604 



NERVOUS SYSTEM. 



FIQ. 220. Vertical section of the third cerebral 

 convolution in man (Meynert). 



1, superficial layer ; 2, layer of small pyramidal 

 cells ; 3. layer of large pyramidal cells ; 4. lay- 

 er of small irregular cells ; 5, layer of spindle- 

 shaped cells ; M, white substance. 



is the thickest of all and contains 

 large, pyramidal cells, which become 

 larger in its deeper portions. The 

 fourth layer contains a large number 

 of smaller cells, irregular in form and 

 with branching prolongations. The 

 fifth layer presents spindle - shaped 

 cells with branching poles and this 

 layer is just above the white sub- 

 stance. The pyramidal cells present 

 a long process above, which passes to- 

 ward the surface, lateral branches, 

 which form a plexus of fine fibrils, 

 and an unbranched prolongation be- 

 low, which passes to the Avhite sub- 

 stance, in the form of an axis-cylin- 

 der. The cells vary somewhat in their 

 appearances in different parts of the 

 brain. The largest pyramidal cells 

 are found, in the anterior central con- 

 volution, in the upper part of the pos- 

 terior central convolution and the par- 

 acentral lobule. Large cells with few 

 prolongations are found in the poste- 

 rior part of the occipital lobes. The 

 cells in this part are connected to- 

 gether by communicating poles. The 

 mode of connection of the cells with 

 each other and with the fibres of the 

 white substance has already been de- 

 scribed and does not demand farther 

 mention. 



Cerebral Convolutions. The cere- 

 brum presents a great longitudinal 

 median fissure by which it is partially 

 divided into two lateral halves. Fig. 

 221, which is based on the well known 

 diagram of the brain, by Ecker, shows 

 three great fissures, the fissure of Syl- 

 vius, the fissure of Rolando and the 

 parieto-occipital fissure. The lobes of 

 the cerebrum are (1) the frontal lobe, 

 lying in front of and above the fissure 

 of Sylvius and in front of the fissure 

 of Eolando, (2) the parietal lobe, be- 

 hind the frontal lobe and in front of 



