SENSE OF SMELL. 871 



4. Sense of Smell. 



872. Of the nature of Odorous emanations, the Natural Philosopher is so 

 completely ignorant, that the Physiologist cannot be expected to give a definite 

 account of the mode in which they produce sensory impressions. Although it 

 may be surmised that they consist of particles of extreme minuteness, dissolved 

 as it were in the air, and although this idea seems to derive confirmation from 

 the fact that most odorous substances are volatile, and vice versd yet the most 

 delicate experiments have failed to discover any diminution in weight, in certain 



Fig. 206. 



Distribution of the Olfactory Nerve on the Septum Nasi. The nares have been divided by a longitudinal 

 section made immediately to the left of the septum, the right nares being preserved entire. 1. The frontal 

 sinus. 2. The nasal bone. 3. The crista galli process of the ethmoid bone. 4. The sphenoidal sinus of the 

 leftside. 5. The sella turcica. 6. The basilar process of the sphenoid and occipital bones. 7. The posterior 

 opening of the right nares. 8. The opening of the Eustachian tube in the upper part of the pharynx. 9. The 

 soft palate, divided through its middle. 10. Cut surface of the hard palate, a. The olfactory peduncle, 

 fe. Its three roots of origin, c. Olfactory ganglion, from which the filaments proceed that spread out in the 

 substance of the pituitary membrane, d. The nasal nerve, a branch of the ophthalmic nerve, descending into 

 the left nares from the anterior foramen of the cribriform plate, and dividing into its external and internal 

 branch, e. The naso-palatine nerve, a branch of the spheno-palatine ganglion distributing twigs to the mucous ' 

 membrane of the septum nasi in its course to (/) the anterior palatine foramen, where it forms a small gangli- 

 form swelling (Cloquet's ganglion) by its union with its fellow of the opposite side. g. Branches of the naso- 

 palatine nerve to the palate, h. Posterior palatine nerves, t, i. The septum nasL 



I 



substances (as musk) that have been impregnating with their effluvia a large 

 quantity of air for several years ; and there are some volatile fluids, such as 

 water, which are entirely inodorous. The true Olfactory nerves pass down from 

 the Olfactory Ganglion ( 729) in the form of very numerous minute threads, 

 which form a plexus upon the surface of the Schneiderian or pituitary membrane 

 (Fig. 206). The filaments composing this plexus are described by Messrs. Todd 

 and Bowman 1 as differing widely from those of the ordinary cephalic nerves in 

 structure; they contain no white substance of Schwann, are nucleated and finely 

 granular in texture, and altogether bear a close resemblance to the gelatinous 

 form of nerve-fibres (Fig. 207). It has been hitherto found impossible to trace 

 the ultimate distribution of these fibres in the olfactory membrane, owing to 



1 "Physiological Anatomy," p. 397, Am. Ed. 



