8i6 



A MANUAL OF PHYSIOLOGY 



(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 efferent 

 and afferent fibres, some of the cranial nerves are purely 

 efferent, some 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 (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 afferent audi- 

 tory 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. 

 341). The olfactory 

 cells, which are really 

 peripheral nerve-cells, 

 lie among the epithe- 

 lial cells in the olfac- 

 tory region of the 

 Schneiderian mem- 

 brane, the common 

 lining of the nostrils. 



FIG. 339. NUCLEI OF CRANIAL NERVES (TOLDT). 



Motor red, sensory blue, 

 to the cranial nerves. 



The numbers correspond Each olfactory cell 

 gives off two pro- 

 cesses, a short one, 

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

 mucous membrane, and a longer but more slender process, repre- 

 senting 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 or mitral 

 layer, which contains a number of large nerve-cells called, from their 



