SMELt. 221 



are not otherwise endowed with feeling, as, for example, the 

 stomach and intestines. Allied to it there are various other 

 sensations, not belonging to any of the five senses, but which 

 may be here alluded to. Among them are some which are 

 pathological, such as numbness, arising from derangement of 

 the conditions necessary for the normal activity of the nerves 

 of any part, and giddiness and nausea, which owe their origin 

 to deranged conditions within the brain. Hunger and thirst 

 are more healthy sensations, which are of an exceedingly 

 curious kind; for while the one is felt in the stomach, and 

 the other in the throat, both are greatly dependent on the 

 general state of nutrition of the body. Indigestible sub- 

 stances give only a very temporary relief from hunger, while, 

 on the other hand, the stomach may be quite empty without 

 the sensation existing; and thirst is only partially relieved 

 by the mere contact of fluid with the fauces. 



162. Smell, the sense by which we distinguish odours, is 

 located within the nasal cavities, and depends on a simpler 

 mechanism than any other of the special senses. 



The nasal cavities or fossse extend from the nostrils back 

 to the pharynx, into which they 

 open behind; the communication 

 being termed the posterior nares 

 (fig. 49). Superiorly, they are 

 separated from the cranial cavity 

 by a thin plate of bone, the 

 cribriform plate of the ethmoid, 

 which is perforated by the fila- 

 ments of the olfactory nerve ; and 

 inferiorly their floor is made by 

 the hard and soft palate; while 

 between them is placed a vertical 

 septum dividing one fossa or Fig. 109. NASAL FOSSJB, 

 cavity from the other. transverse vertical section. 



The outer wall, in the human subject, presents three ledges 

 of bone, one above another, projecting inwards with a down- 

 ward curve, and termed turbinated bones. The inferior is a 

 distinct bone; while the others are portions of the ethmoid, 

 and project little lower than the floor of the orbit. 



The ethmoid bone consists of a central plate, the cribriform 



