314 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



the upper surface of the tongue, salt and sweet in bands near the tip, sour 

 near the sides, and bitter in the center near the root. The receptors of bitter 

 taste are in a few conspicuous papillae ranged in a V-shaped line far back on 

 the tongue. They have been called vallate papillae because each one is shaped 

 like a turrcted castle surrounded by a flooded moat, in this case, a tasty 

 flood (Fig. 17.3). 



Smell. At least among mammals, smell is the most democratic of the senses. 

 Whatever minute particles there may be in the air, and of whatsoever kind, 

 they are hospitably drawn into the nostrils. The nose of mammals is not only 

 a heater, humidifier, and cleaner of air but through the sense of smell it is 

 a testing place of the chemical nature of the surroundings. In spite of all they 

 smell and think they smell, man and other primates are only feeble smellers 

 as compared with cats and dogs and other mammals. A man looks as he 

 walks; a dog smells as he runs. 



In the human nose, the mucous membrane on each side is raised upon 

 three ridges of the turbinate bones that spring from the outer nasal wall. 

 Each cavity of the nose is thus incompletely divided into compartments. The 

 lower ones are passages that are open behind, and air slips through them 

 into the pharynx; the uppermost one is a narrow cleft directly under the 

 floor of the skull. The olfactory organs are pale yellow patches of cells on the 

 walls of this cleft (Fig. 17.4). Their location, as it were, in the attic, sets 

 them off the main roadway of incoming and outgoing air. Each breath of 

 cool air pushes the warmed air up into the olfactory attic where it is poised 

 over the smell receptors till more air comes in. The exposed ends of the 

 receptors bear slender processes that are always wet with mucus. Fibers 

 arising from the other ends of the receptors are grouped together in the 

 olfactory nerves that pass through the skull to the olfactory centers in the 

 brain (Fig. 16.13). 



Although man's sense of smell is relatively weak, even so, it will respond 

 to remarkably small amounts of substance. A synthetic substitute for the 

 odor of violets (ionone) can be detected when it is present as one in over 

 30 billion parts of air. The sense of smell of a particular substance is 

 fatigued in a few minutes, but will then react to a new odor; all recoveries 

 are rapid since odors are diffused through the air. The smaller, lighter par- 

 ticles spread most readily, and as they are scattered farther apart the chance 

 of inhaling them lessens. The aroma of coffee thins quickly; the scent fades 

 on yesterday's rabbit tracks; the odor of last night's cigarette lingers and 

 changes. 



Equilibrium — Statoreceptors 



The great majority of animals, grasshoppers, fishes, or cows, have an up- 

 side and a downside, and it is very important that the owner be informed of 



