THE CRANIAL NERVES 



93 



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 most common shape, mitral cells, 

 along with smaller nerve-cells (' granules ') and neuroglia; (4) the nuclear 

 layer, containing numerous small nerve-cells or ' granules ' intermingled 

 with white fibres. The mitral cells give off axons, which pass through 

 the fourth layer, and then as fibres of the olfactory tract to the grey 

 matter of the hippocampal region of the brain. The course of the 

 impulses from the olfactory mucous membrane to the brain is shown in 

 Fi S- 359- The olfactory tract, as it runs back, divides into portions 



Fig. 359- Scheme of the Olfactory Nervous 

 Apparatus (Cajal). A, olfactory cells; B, glomeruli; 

 C, mitral cells; D, olfactory granule cell; E, lateral 

 root of olfactory tract; F, cortex of brain in the 

 region of the uncinate gyrus; a, small cell of mitral 

 layer; 6, brush of dendrite of a mitral cell ending in a 

 glomerulus; c, thorns or spines on the processes of an olfactory granule; e, collateral 

 coming off from the axon of a mitral cell; /, collaterals ending in the molecular layer 

 of the uncinate gyrus; g, pyramidal cells of the cortex; h, supporting epithelial cells 

 of the olfactory mucous membrane. 



called its ' roots.' Of these the lateral is the most important, and it 

 terminates in the hippocampal and uncinate gyri of the same side. 

 Fibres of the olfactory tract are also connected either directly or through 

 the relay of another neuron with the opposite side of the brain, especially 

 the opposite uncinate gyrus. The anterior commissure contains 

 numerous fibres, which connect the hippocampal regions of the two 

 sides. Other central connections of the olfactory tract exist, but some 

 are imperfectly known. The name ' rhinencephalon ' is given to the 

 portions of the brain concerned with the sense of smell. Disturbances 

 of smell sensation may be caused by lesions in any part of the rhinen- 

 cephalon, and also by changes in the olfactory mucous membrane and 

 olfactory fibres ; but the symptoms do not obtrude themselves, and are 

 doubtless often overlooked. Excessive stimulation of the olfactory 

 nerve by exposure to a strong odour has been said to cause complete 

 and permanent loss of smell. 



The second or optic nerve contains mainly afferent fibres, which 

 arise from the ganglion cells of the retina, and terminate by forming 

 synapses with nerve-cells in the lateral or external geniculate body, the 



