The Body-Machine and Its Work 351 



areas on his arm. There are other nerve-endings again for the 

 sense of pressure. 



Other sensitive bulbs, which line part of the mouth, are the 

 receiving organs for the sense of taste. Little oval bodies stand 

 up like a close regiment of diminutive soldiers on the upper sur- 

 face of the tongue. Each of the internal cells of these "taste- 

 buds" ends in a hair-like process, and these processes touch the 

 nerves which convey their particular stimulation to the brain. 

 Probably different flavours are perceived by different nerves. 

 The tip of the tongue is richer in the little bulbs that appreciate 

 sweet things, while the back part is richer in the means of recog- 

 nising bitterness. 



Substances must be in a liquid form to announce themselves 

 to taste. For the sense of smell, on the other hand, they have to 

 be broken up into very fine particles like a gas. Nerves from the 

 olfactory centres in the brain branch out in the membrane which 

 lines the upper part of the nasal cavities, and this membrane in- 

 cludes numerous sensory nerve-cells which act as sentinels against 

 dangers which announce themselves in the air. An odorous body 

 is one which gives off minute particles of its matter into the air. 

 The sense of smell was once of the gravest importance in the 

 animal economy, and even in men it is so highly developed that 

 they can detect a speck of musk diluted in eight million times as 

 much air. A very strong offensive substance like mercaptan can 

 be "sensed" even if there is only one grain to twenty-five trillion 

 times as much air. In man, however, the sense of smell is degen- 

 erating, and many individuals have it very feebly. 



The Sense of Vision 



Most important of all the senses is that of vision, for nearly 

 all the ideas of things in the mind of an ordinary person are visual 

 images. The essential part of the mechanism of this sense is the 

 eyeball and the nerve which goes from this to the sight-centre in 

 the brain. The eye is a camera of a most remarkable description. 



