SENSE OF SMELL. 



405 



numerous minute threads, which form a plexus upon the surface of the 

 Schneiderian or Pituitary membrane. Nothing satisfactory is known in re- 

 gard to their ultimate arrangement ; but it is probable that they form loops, 



Fig. 172. 



The Olfactory nerve, with its distribution on the septum nasi. The nares have been divided by a longi- 

 tudinal 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 sphe- 

 noidal sinus of the left side. 5. The sella turcica. G. The basilar process of the sphenoid and occipital 

 bones. 7. The posterior opening of the right nares. S. 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, b. 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-pala- 

 tine ganglion distributing twigs to the mucous membrane of the septum nasi in its course to (/) the ante- 

 rior palatine foramen, where it forms a smali gangliform 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 pala- 

 tine nerves, i, i. The septum nasi. 



similar to those of the cutaneous nerves. It would appear that every part of 

 the Schneiderian membrane is not equally endowed with the faculty of dis- 

 tinguishing odours, which is a very different power from that of becoming 

 sensible of irritation from them. The Olfactory nerves cannot be traced to 

 the membrane covering the middle and inferior spongy bones, or to that which 

 lines the different sinuses, these parts of the surface being supplied by the 

 Fifth pair only ; and it is a matter of common experience, that we cannot dis- 

 tinguish faint odours, unless, by a peculiar inspiratory effort, we draw the air 

 charged with them to the upper part of the nose. In animals living in the 

 air, it is a necessary condition of the exercise of the sense of Smell, that the 

 odorous matter should be transmitted by a respiratory current through the 

 nostrils; and that the membrane lining these should be in a moist state. 

 Hence, by breathing through the mouth, we may avoid being affected by 

 odours even of the strongest and most disagreeable kind; and in the first stage 

 of a catarrh, when the ordinary mucous secretion is suspended, the sense of 

 smell is blunted from this cause, as it afterwards is from the excess in the 

 quantity of the fluid, which prevents the odoriferous effluvia from coming 

 into immediate relation with the sensory extremities of the nerves. Hence 

 we may easily comprehend, that section of the Fifth pair, which exercises a 



