660 



SPECIAL SENSES. 



tions being about two-thirds of the former to one-third of the latter. The 

 gray substance, derived from the gray root, is situated at the upper portion of 

 the nerve, the white substance occupying the inferior and the lateral portions. 

 By the side of the crista galli of the ethmoid bone, the nerve-trunk 

 expands into an oblong ganglion called the olfactory bulb. This is grayish 

 in color, excessively soft, and contains the ordinary ganglionic elements. 

 From the olfactory bulb, fifteen to eighteen nervous filaments are given on% 

 which pass through the foramina in the cribriform plate of the ethmoid bone. 

 These filaments are composed entirely of nerve-fibres, and are quite resisting, 

 owing to fibrous elements prolonged from the dura mater. It is strictly 



proper, perhaps, to regard these as the true olfac- 

 tory nerves, the cord leading from the olfactory 

 bulb to the cerebrum being properly a commis- 

 sure. Having passed through the cribriform 

 plate, the olfactory nerves are distributed to the 

 olfactory membrane, in three groups : an inner 

 group, distributed to the mucous membrane of 

 the upper third of the septum ; a middle group, 

 to the upper portion of the nasal fossae ; and an 

 outer group, to the mucous membrane covering 

 the superior and middle turbiuated bones and a 

 portion of the ethmoid. 



The mode of termination of the olfactory 

 FIG. 236. Terminal filaments of the nerves differs from that of the ordinary sensory 



olfactory nerves ; magnified 30 ,. ,. ,, .. ... 



diameters (Koiiiker). nerves, and is peculiar and characteristic, as it is 



in the other organs of special sense. The olf ac- 

 tory mucous membrane contains terminal nerve- 

 cells > called the olfactory cells, which are situated 



ceil of the sheep. between the cells of epithelium. These are long> 



delicate, spindle-shaped, varicose structures, each one containing a clear, 

 round nucleus. In the frog there is a fine, hak-like process projecting from 

 each cell, beyond the mucous membrane, which has not been observed in 

 man or the mammalia. The delicacy of the structures entering into the 

 composition of the olfactory membrane renders the investigation of the ter- 

 mination of its nervous filaments exceedingly difficult. 



Properties and Uses of the Olfactory Nerves. It is almost certain that 

 the olfactory nerves possess none of the general properties of the ordinary 

 nerves belonging to the cerebro-spinal system, and are endowed with the 

 special sense of smell alone. The filaments coming from the olfactory bulbs 

 and distributed to the pituitary membrane have not been exposed and stimu- 

 lated in living animals ; but experiments upon the nerves behind the olfac- 

 tory bulbs show that they are insensible to ordinary impressions. Attempts 

 have been made to demonstrate, in the human subject, the special properties 

 of these nerves, by passing an electric current through the nostrils ; but the 

 situation of the nerves is such that these observations are of necessity indefi- 

 nite and unsatisfactory. 



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