620 HANDBOOK OF PHYSIOLOGY. 



The cortical gray matter of the cerebral cortex has an average 

 thickness of about -J inch (3 mm.), being thin in the occipital lobe, -fa 

 inch (2 mm.), and thick in the pre-central, -J- inch (4 mm.). The cells 

 of which the substance is composed are of different kinds: (a) The 

 apical process is very long and reaches up often nearly to the surface. 

 It gives off lateral branches, and is studded along its course with little 

 projections called gemmules. This process is a protoplasmic process or 

 dendrite; the cell has other dendrites given off from the angles of the 

 body of the cell. It always has an axis-cylinder process or neuraxon 

 which passes off usually from about the middle of the base. There are, 

 besides these large pyramidal cells, others practically of the same shape 

 and structure but smaller. They are the small pyramidal cells. 



(li) In the superficial layer of the cortex there is a peculiar type of 

 cell, first described by Cajal. Most of these bodies are fusiform in shape, 

 with the long axis parallel to the surface of the convolution. They give 

 off usually two neuraxons which run along parallel to the surface and 

 send down numerous fine collaterals at right angles. Another form of 

 Cajal cell, triangular or quadrangular in shape, is also seen. Both 

 forms have, as a rule, more than one neuraxon. Their collaterals pass 

 in a horizontal direction, forming a fine band of fibres, known as tan- 

 gential fibres. 



(c) A third type of cell is the fusiform or polymorphous. Some of 

 these are strictly fusiform in shape and lie with their axis parallel to the 

 surface of the convolution. They give off protoplasmic processes which 

 pass down toward the white matter, some of them turning to run in a 

 horizontal direction. The fusiform and polymorphous cells are grouped 

 in the same layer, and are, therefore, described together. 



(d) Besides these cells we find scattered through the cortex a consid- 

 erable number of the neuroglia-cells. The character and position of 

 these are shown in fig. 373. 



The general arrangement of the layers of the cortex is described very 

 differently by different authors, and it differs in different parts of the 

 brain. The simplest and most representative type, however of the ar- 

 rangement is that in which the cortex is divided into four layers. The 

 outermost, or superficial, known as the molecular layer, contains rela- 

 tively few cells. It is composed of iieuroglia tissue, embedded in which 

 are a number of cells of the Cajal type, which have just been described. 

 There are also in this layer many neuroglia-cells. In the superficial part 

 of the layer of some areas of the cortex are many tangential fibres. The 

 second layer is composed of small pyramidal cells. In parts of the brain 

 there are here interposed what are known as the vertical fusiform cells. 

 The third layer is composed of large pyramidal cells, in which, however, 

 one sees many small pyramids also. The fourth layer is composed of the 

 fusiform and polymorphous cells, and beneath this is the white sub- 



