476 



PHYSIOLOGY 



main projicient sense-organs, namely, sight and hearing. On this 

 account it is not easy to make out the connections of the olfactory lobe 

 proper, the rhinencephalon. with the primitive part of the cortex, the 

 archipalUum, subserving the olfactory sense and probably the allied 

 sensations derived from the mouth cavity. The wide connections of 

 the olfactory sense-organs with the different parts of the brain in the 

 lower vertebrate are shown in the diagrammatic figure of the brain of 

 a reptile (Fig. 208, p. 471). 



In man it is interesting to note that the olfactory nerve fibres are 

 derived from cells situated actually on the surface of the body. These 

 are bilateral, spindle-shaped cells, lying in the olfactory mucous mem- 

 brane at the upper part of the nasal cavity. The peripheral process is 

 short and passes towards the surface, while the deep process passes 

 as a non-medullated nerve-fibre through the cribriform plate of the 



FKJ. 212. Schema of course of olfactory impulses. (RAMON Y CAJAL.) 

 A, olfactory mucous membrane; B, olfactory glomeruli ; r, mitral cells; 

 E, granule cells ; D, olfactory tract ; L, centrifugal fibres. 



ethmoid to sink into the olfactory bulb. The bulb, in man, is a greyish 

 enlargement at the anterior end of the olfactory tract. In sections 

 stained by Golgi's method of impregnation it may be seen that the 

 olfactory fibres terminate in an arborisation in close connection with 

 a thick end arborisation derived from a dendrite of a large nerve-cell, 

 known as a mitral cell. The synapses between these two sets of fibres 

 are prominent objects in a section through the olfactory bulb and form 

 the ' olfactory glomeruli ' (Fig. 212). The axons of the mitral cells 

 pass back in the olfactory tracts. Each olfactory tract divides 

 posteriorly into two roots, the mesial root which curves inwards behind 

 Broca's area and passes into the end of the callosal gyrus, and the 

 lateral root which runs backwards and over the outer part of the 

 anterior perforated spot. Its fibres pass into the uncinate extremity 

 of the hippocampal gyrus. The small triangular field of grey matter 

 between the diverging roots of the olfactory tract is known as the 



