CHAP, ii.] THE BRAIN. 1085 



play in determining the coordination of movements. We must 

 remember, however, that all three masses are connected with the 

 cortex, and probably all three play a part in vision even of the 

 highest psychical kind. 



Sensations of Smell 



674. The olfactory nerve, which is undoubtedly the nerve 

 of smell, stands like the optic nerve apart from the rest of the 

 cranial nerves ; and a few words as to its structure and relations 

 will be necessary. 



Lying on the ventral surface of the anterior region of each 

 hemisphere, on each side of the anterior fissure, is seen the 

 olfactory bulb, which is prolonged directly backwards as the 

 olfactory tract, coming apparently to an end where the hind 

 margin of the frontal lobe abuts on the anterior perforated space 

 in the floor of the front part of the third ventricle. The bundles 

 of fibres forming the olfactory nerve proper spring from the bulb, 

 which is their immediate cerebral origin, both bulb and tract 

 being really parts of the cerebrum. Just as the fore-brain buds 

 off on each side the optic vesicle to form the optic nerve, so each 

 cerebral vesicle buds off an olfactory vesicle, the front part of 

 which becomes the rounded bulb and the remainder the rounded 

 trigonal tract or peduncle connecting the bulb with the hemisphere. 

 In man the original cavity of the vesicle is obliterated, being filled 

 up with neuroglial gelatinous substance, but in the lower animals 

 remains as a linear space, the ventricle of the olfactory tract. 



The bulb is a specialized mass of grey matter, forming a sort 

 of cap to the end of the tract, and presents some analogies with 

 the cortex of the hemisphere. Along the middle line lies the core 

 of neuroglial gelatinous substance ; but the side of the bulb dorsal 

 to this core, in contact with the hemisphere, is much less developed 

 than the side lying ventral to the core, next to the cribriform 

 plate ; and we may confine ourselves to the ventral portion. Next 

 to the neuroglial core lies a layer of longitudinal medullated fibres, 

 with which are mingled some nerve cells. This layer, which forms 

 the beginning of the tract inside the bulb, is thinnest at the 

 rounded front extremity of the bulb and gradually thickens 

 backward. Next to it lies a 'nuclear' layer, composed of small 

 nuclear cells, arranged to a large extent in longitudinally disposed 

 rows. Fibres from the preceding layer pass between the groups, 

 which are moreover separated by interlacing bundles of fibres. 

 Next to this layer comes a somewhat thick one, which perhaps 

 may be compared to the molecular layer of the cerebellum or to 

 the pyramidal layers of the cerebrum. It is composed of a 

 molecular ground substance, partly neuroglial in nature, traversed 

 by numerous fibrils and fibres, many of the latter being of the 



