326 ANATOMY AND PHYSIOLOGY 



SPECIAL SENSATION 



The special senses are: smell, touch, taste, hearing and sight. 

 The organs concerned are the nose, the skin, the tongue, the ear and 

 the eye. 



It is understood that all consciousness of sensation is based 

 upon the final reception of sensory impressions by the brain. So 

 far as a " sense " may be said to reside anywhere, it resides in the 

 brain, for without it there are no senses as we know them. 



THE SENSE OF SMELL 



The nose is the organ of the sense of smell. In the nasal 

 chambers is a layer of special cells olfactory cells supported 

 by a basement membrane, forming the Schneiderian membrane 

 (or pituitary membrane). The upper part only of the nose is 

 the olfactory region. Here the sensory nerves arise which proceed 

 through the foramina in the cribriform plate or the roof of the 

 nose, to the brain. 



In quiet respiration most of the air passes in and out through 

 the lower parts of the nasal chambers, diffusing gradually into the 

 upper parts. Although most odors are readily perceived as soon 

 as one comes into the atmosphere containing them, a slight odor 

 is better appreciated by means of an effort to draw the air through 

 the olfactory region, in other words, a sniff. More of the odorous 

 particles are thus brought into contact with the olfactory cells, and 

 the impressions made upon them are transmitted by the delicate 

 olfactory nerves through the cribriform plate to the olfactory bulbs 

 and thence by the olfactory tracts to the olfactory center in the 

 temporal lobe of the brain. 



The sense of smell is valued for the pleasurable sensations 

 which it affords, as an adjunct to the sense of taste, and as a 

 sentinel to warn us of danger when in the vicinity of irritating or 

 poisonous gases, etc. 



Clinical notes. It may become greatly impaired in catarrhal affections 

 of the nasal mucous membrane, and is sometimes lost temporarily or per- 

 manently after an attack of influenza, congestion and consequent dryness of 

 the olfactory membrane (as in coryza or "cold in the head") always diminish 

 the acuteness of smell. The degree of development of this sense in lower 

 animals is remarkable; they readily "scent danger." 



