THE CENTRAL NERVOUS SYSTEM 717 



The Cranial Nerves. 



Unlike the spinal nerves, which arise at not very unequal intervals 

 from the cord, the nuclei of the cranial nerves, with the exception of 

 the olfactory and optic, are crowded together in the inch or two of 

 grey matter of the primitive neural axis in the immediate neigh- 

 bourhood of the fourth ventricle and the Sylvian aqueduct. Of 

 these nuclei some are the end nuclei or * nuclei of reception ' of 

 sensory fibres, that is to say, collections of nerve-cells around which 

 the sensory fibres break up into terminal arborizations. Such are 

 the sensory nucleus of the fifth, both nuclei of the eighth, and 

 probably the common nucleus of the ninth, tenth, and eleventh. 

 The nuclei of origin of the motor fibres lie, upon the whole, in two 

 longitudinal rows a median row, which consists of the nuclei of 

 the third and fourth nerves in the floor of the aqueduct, and those 

 of the sixth and twelfth nerves in the floor of the fourth ventricle ; 

 and a lateral row comprising the motor nuclei of the fifth, tenth, and 

 eleventh nerves, and the nucleus of the seventh. The clumps of 

 grey matter which make up these nuclei may be considered as homo- 

 logous with the grey matter of the ventral or anterior (including the 

 lateral) horn of the spinal cord ; and the motor fibres of the nerves 

 themselves as homologous with the anterior spinal roots. Without 

 going further into the thorny subject of the homologies of the cranial 

 and spinal nerves, we may point out that while all the spinal nerves 

 contain both motor and sensory fibres, some of the cranial nerves 

 are purely motor, some purely sensory, and others mixed So that 

 if we are to look upon the motor nerves as the homologues of the 

 ventral roots, the dorsal (posterior) root-fibres corresponding to them 

 must be represented in the other cranial nerves. Thus the sensory 

 portion of the mixed fifth nerve, and the purely sensory auditory nerve, 

 must be supposed to contain fibres corresponding to several dorsal roots. 



The first or olfactory nerve consists of fine fibres, each of which 

 is a process of an olfactory cell (Fig. 257). The olfactory cells, which 

 are really peripheral nerve-cells, lie among the epithelial cells in the 

 olfactory region of the Schneiderian membrane, the common lining of 

 the nostrils. Each olfactory cell gives off two processes, a short one, 

 representing a dendrite, which runs out to the surface of the mucous 

 membrane, and a longer but more slender process, representing an 

 axon, which as a fibre of the olfactory nerve pierces the cribriform 

 plate of the ethmoid bone, and plunges into the olfactory bulb. 



In the olfactory bulb at least four layers can be distinguished (i) 

 on the surface, beneath the pia mater, the layer of entering olfactory 

 nerve-fibres ; (2) the layer of olfactory glomeruli, peculiar structures, 

 each of which is made up of an intricate basket-like arborization formed 

 by an olfactory nerve-fibre, or, it may be, more than one, and a brush- 

 like arborization belonging to a dendrite of one of the mitral cells of 

 the next layer ; (3) the molecular layer, which contains a number of 

 large nerve-cells called mitral cells, along with smaller nerve-cells and 

 neuroglia ; (4) the nuclear layer, containing numerous small nerve- 

 cells or ' granules ' intermingled with white fibres. The mitral cells 



