Chap. XX] INTERNAL AND EXTERNAL SENSES 413 



branch of the fifth or trifacial, (2) filaments of the chorda tympani, 

 a branch of the seventh or facial, and (3) filaments of the ninth 

 or glossopharyngeal nerve. ^ The twelfth or hypoglossal nerve is 

 distributed to the tongue, but is a motor nerve and is not concerned 

 in the sense of taste or touch. 



Other sensations in the tongue. — The sense of touch is very 

 highly developed here, and with it the sense of temperature, pain, 

 etc. ; upon these tactile and muscular senses to a great extent 

 depend the accuracy of the tongue in many of its important uses — 

 speech, mastication, deglutition, sucking. 



We often confound taste with smell. Substances which have 

 a strong odor, such as onions, are smelled as we hold them in our 

 mouths ; and if our sense of smell is temporarily suspended, as it 

 sometimes is by a bad cold in the head, we may eat garlic and 

 onions and not taste them. Hence the philosophy of holding 

 the nose when we wish to s\^allow a nauseous dose. 



SMELL 



Necessary conditions. — The first essentials are a special nerve 

 and nerve-centre, the changes in whose condition are perceived 

 as sensations of odor. No other nerve structure is capable of 

 such sensations, even when acted on by the same cause. The 

 special organs for this sense must be in their normal condition, 

 and a stimulus (odor) must be present to excite them. 



Odors are caused either by minute particles of solid matter or 

 by gases which are in the atmosphere, and they must be capable 

 of solution in the mucus of the pituitary membrane. Odorous 

 particles in the air, passing through the lower, wider air passages, 

 pass by diffusion into the higher, narrower, nasal chambers, 

 and falling on the membrane which is provided with olfactory 

 nerve-endings, produce sensory impulses, which, ascending to 

 the brain, give rise to the sensation of smell. 



If we wish to smell anything particularly well, we sniff the air 

 up into the higher nasal chambers, and thus bring the odorous 

 particles more closely into contact with the olfactory nerves. 



Each substance we smell causes its own particular sensation, 

 and we are not only able to recognize a multitude of distinct 



1 This is the generally accepted view, but other statements may be found in the 

 various text-books. 



