126 



THE MECHANISM OF LIFE 



striata, the great nuclei of the cerebral hemispheres are situated 

 in the thin sheet of cortical grey matter, and it is the cells of 

 this that are connected with the underlying core of fibres. The 

 latter are either the axons of these cells (in which case they carry 

 impulses out from the cortex) or they end in forming synapses with 

 the cortical cells (in which case they carry impulses to the cortex). 

 Fig. 37 represents, on the same scale, a " pyramidal " cortical 

 cell from the frog's cortex and one from man; the actual cell 

 bodies are very much the same in the two cases, but the dendrites 

 and axons are very much more complex in the human than in 



Principal 

 axon 





FIG. 37. Two CORTICAL (PYRAMIDAL) CELLS FROM THE BRAIN OF MAN 

 AND THE FROG. HIGHLY MAGNIFIED. 



the amphibian cortex. Each cell is pyramidal in general form 

 (as it can be seen in section), and from the apex and lower parts 

 there start off a great number of fine fibres which branch in a 

 complicated way and have a structure peculiar to the cortex. 

 From the middle point of the base of the cell there issues a 

 prominent fibre, which is the axon of the cell, and branches pass 

 off laterally from the axon and again branch. These are the 

 " collateral " axons of the cell. Soon after leaving the latter 

 the axons acquire medullary sheaths (see p. 26), and become 

 the commissural or projection cortical fibres represented in 



