892 THE CENTRAL NERVOUS SYSTEM 



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

 contain both efferent and afferent fibres, some of the cranial nerves 

 are purely efferent, sdme purely afferent, and others mixed. So 

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

 the ventral roots, the dorsal, (posted or) root-fibres corresponding to 

 them must be represented in the other cranial nerves. Thus, the 



Fig. 358. Nuclei and Roots of Cranial Nerves (Toldt). Lateral view. Motor red, 



sensory blue. 



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

 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. 359). The olfactory cells, which are 

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

 tory 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. 



