634 SPECIAL SENSES 



membrane covering the superior and middle turbinated bones and a por- 

 tion of the ethmoid. 



The mode of termination of the olfactory nerves differs from that of 

 the ordinary sensory nerves, and is peculiar and characteristic, as it is in 

 the other organs of special sense. The olfactory mucous membrane con- 

 tains terminal nerve-cells, called olfactory cells, which are situated be- 

 tween 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 hair-like process projecting from each cell beyond 

 the mucous membrane, which has not been observed in man or in the 

 mammalia. 



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

 that the olfactory nerves possess none of the general properties of 

 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 stimulated in living animals ; but experiments on the 

 nerves behind the olfactory 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 their situation is such that 

 these observations are of necessity indefinite and unsatisfactory. 



Among the experiments on the higher orders of animals, in which 

 the olfactory nerves have been divided, may be cited, as open to no ob- 

 jections, those of Vulpian and Philipaux, on dogs. It is well known 

 that the sense of smell usually is very acute in these animals. On 

 dividing or extirpating the olfactory bulbs, " after the animal had 

 completely recovered, it was deprived of food for thirty-six or forty- 

 eight hours ; then, in its absence, a piece of cooked meat was concealed 

 in a corner of the laboratory. Animals, successfully operated on, 

 then taken into the laboratory, never found the bait; and, neverthe- 

 less, care had been taken to select hunting-dogs." This experiment is 

 conclusive ; more so than those in which animals deprived of the olfac- 

 tory bulbs were shown to eat feces without disgust, for this sometimes 

 occurs in dogs that have not been mutilated. 



Comparative anatomy shows that the olfactory bulbs usually are 

 developed in proportion to the acuteness of the sense of smell. Patho- 

 logical facts show, in the human subject, that impairment or loss of 

 the olfactory sense is coincident with injury or destruction of these 

 ganglia. Cases have been reported in which the sense of smell was 

 lost or impaired from injury to the olfactory nerves. In nearly all the 

 cases on record, the general sensibility of the nostrils was not affected. 



