The Body-Machine and Its Work 321 



built into the frame. We can, if we like, do this literally to-day. 

 We can mix an opaque powder, e.g. of bismuth, with a mouthful 

 of easily digested food (such as meal), and then by means of X- 

 rays we can photograph its progress along the alimentary canal 

 until what remains of it is dumped in the waste-chamber. This 

 is useful for some purposes, but in the main we rely on the minute 

 studies of the anatomist and the experiments of the physiologist 

 for our knowledge of that part of man's body that is concerned 

 with the utilisation of the food. 



The receiving office, so to say, or the mouth, is itself so deeply 

 interesting and full of ingenious contrivances that a whole section 

 of this work might be devoted to it. Above the mouth are the 

 sentinels, the eyes and nose, which we shall consider later. Then, 

 on the upper surface of the tongue, there are myriads of sensitive 

 little organs, taste-buds, ready to apply a final test to the food. 

 The juices of the food penetrate the thin skin which covers and 

 protects them, and probably have a chemical action on the little 

 nerve-endings in them. If the message which they automatically 

 dispatch to the brain is "O.K." other nerves set in motion the 

 muscles of the lower jaw, and the grinding of the food begins. 



In modern times a volume could be written on the teeth alone, 

 and it would be a remarkable story. Teeth began ages ago in 

 fishes of the shark and dog-fish type. Their rough scale-covered 

 skin was drawn in to form J, lining of the mouth, and long prac- 

 tice in crunching shell-fish, and so on, led to the evolution of the 

 hard scales on the skin into teeth. In the course of time the teeth 

 on the ridges of the jaw were particularly developed, and the 

 others disappeared. But we must not suppose from this brief 

 hint of their origin that teeth are simple things. Each tooth is a 

 remarkable structure. Coats of dentine and enamel are built 

 round a pulpy cavity into which nerves and blood-vessels run, 

 and roots, coated with cement, fix the tooth in its socket. It is 

 strange how few people wonder why our jaws do not ache and 

 jar from the crunching of a hard crust. In mammals there is a 



