THE NERVOUS SYSTEM. 641 



gyrus also with vision as the centre, while others look upon it merely 

 as an accessory centre. 



Olfactory centre. The olfactory nerve differs from the other cranial 

 nerves. In reality it is a representative of the olfactory lobes of other 

 .animals, which are part of the cerebrum. It originates as an off-shoot 

 from the cerebral vesicle, the front part of which is developed into the 

 bulb of the olfactory nerve, while the back forms its peduncle. The 

 nerve, the cavity of which is filled up in the fully developed condition 

 with neurogliar substance, lies upon the cribriform plate of the ethmoid 

 bone, and is contained in a groove of the frontal lobe on its under sur- 

 face. On examination of the bulb it is found to be thus made up. 

 Beneath the neurogliar layer is a layer of longitudinal fibres and a few 

 nerve-cells, next to this is a layer of small cells (nuclear layer) , fibres 

 from the layer of nerve-fibres passing through it. 



The nuclear layer is also separated into groups of cells by an inter- 

 lacing of the fibres. The next layer is thick and is composed of neuroglia 

 and some fibres, some of which are medullated, as well as of cells more 

 or less pyramidal in shape. Below this layer is the layer of olfactory 

 glomeruli. These glomeruli are small coils of olfactory fibres inclosing 

 small cells and granular matter. A full description of the anatomy of 

 these parts is given later (see Olfactory nerve). 



Fibres of the olfactory nerve proper are found below this layer and 

 pass to be distributed to the olfactory mucous membrane. They are 

 thought to have origin in the glomeruli. The peduncle of the nerve 

 or the olfactory tract as it is sometimes called, is made up of longitudinal 

 fibres originating in the bulb, with neuroglia and some nerve-cells. 



The fibres of the olfactory tract have been traced into the nucleus 

 amygdalae and its junction with the hippocampal gyrus in the temporal 

 lobe (fig. 386). The hippocampus must be in some way connected with 

 smell, since a lesion of it, leaving the olfactory tract uninjured, seriously 

 interferes with that sense. 



Taste centre. It is very uncertain where the taste centre is situated, 

 if such exist. It has been placed in the temporal lobe, not far from that 

 of smell (fig. 386). 



Auditory Centre. This centre has been localized in the superior 

 temporal convolution (fig. 382). Experiments have been made which 

 connect auditory impulses on either side with the posterior corpus quad- 

 rigeminum and the median corpus geniculatum, for when the internal 

 ear is destroyed there results atrophy of these bodies as well as of the 

 lateral fillet of the opposite side; and on the other hand, destruction of 

 the part of the temporal lobe above indicated is similarly followed by 

 atrophy of the nuclei of the same side. If these results be confirmed by 

 additional experiments, it would make it plain that these nuclei bear 

 much the same relation to the sense of hearing as do the anterior corpus 



