THE CRANIAL NERVES 515 



which tlie fibrils of nerves pass to and from these 

 cells from the central white matter. The latter is 

 composed, as in the cerebellum, of medullated nerve 

 fibres. This neuroglia is most marked in the outermost 

 parts of the cortex, immediately below the pia mater, and 

 since in a section its wavy fibres are mostly seen as 

 sectional dots, this layer of the cortex is known as the 

 molecular layer (Fig. 165, I). Internally to this layer 

 the cortex is characterised by the presence of nerve cells 

 whose shape is pyramidal with the apex of each coll pointed 

 towards the surface of the brain. This layer may there- 

 fore be spoken of as the layer of pyramidal cells 

 (Fig. 165, 11). These cells vary in .size in the several parts 

 of this layer, the largest being found in the inner portion, 

 the smallest next to the molecular layer. That part of 

 the cortex which lies immediately external to the central 

 white matter is characterised by the presence of nerve 

 cells of a somewhat irregular form, hence this layer is 

 known as the layer of polymorphous cells (Fig. 165, 

 III). 



In addition to the nerve cells and their processes which 

 characterise the several layers of the cortex, nerve fibrils 

 pass up into and through the cortex from the central 

 white matter. Of these some are arranged in bundles 

 at right angles to the surface of the cortex, medullary 

 rays (Fig. 165, m.r.) while others lie parallel to the surface 

 as tangential rays (Fig. 165, x.y.z.). 



12. The Cranial Nerves.— Nerves are given oflF from 

 the brain in pairs, which succeed one another from before 

 backwards, to the number of twelve (Figs. 160 and 166). 

 These are often called "cramai" nerves, to distinguish 

 them from the spinal nerves. 



The first pair, counting from before backwards, are the. 

 olfactory nerves, and the second are the optic nerves. 

 The functions of these have already been described. 

 But these two nerves require special notice. That which 

 is commonly called the olfactory "nerve " is really a lobe 



L L 2 



