THE BODY-CAVITY 89 



mouth to anus and is the same length as the body, in more 

 complex forms the existence of this body-cavity enables the 

 intestine to grow to a length greatly exceeding that of the 

 body. The intestine of man is just under 31 feet in length, 

 five or six times the length of the body. Thus the digestive 

 and absorptive capacity of the alimentary canal is greatly 

 increased. 



But the existence of a body-cavity has a further advantage. 

 It frees the alimentary canal from the control of the sur- 

 rounding tissues. It is no longer dragged hither and thither 

 as the contractions of the surrounding tissues sway it. The 

 intestine in a body-cavity can exercise movements of its 

 own, movements independent of the surrounding body- wall ; 

 and this freedom of action is rendered possible by the exist- 

 ence of a space — the body-cavity — in which the intestine 

 lies free. 



Animals with an alimentary canal open at both ends — 

 and they are the vast majority — might also be compared 

 "svith a thermos-flask, if only the inner tube of the flask were 

 fused to the outer tube at the lower end as it is at the top end 

 and opened externally. Between the two tubes of the flask is 

 a space, a vacuum in the thermos-flask, which represents the 

 body-cavity of an animal. This space in animals is almost 

 completely filled up with various organs, muscles, excretory and 

 reproductive organs, glands, etc. If you were to put a foreign 

 body such as a pebble into the inside or lumen of the inner 

 tube of the thermos-flask, it might still in a sense be regarded as 

 outside the body, the body being in reality the tissues which 

 take the place of the vacuum of our flask. Now the food that 

 passes from the mouth down the gullet into the stomach and 

 thence into the intestine is also in a sense outside the body, 

 and it has to be changed in many ways before it can soak 

 through the lining of the intestine in the higher animals into 

 the blood and be carried by the blood to the various active 

 and hungry cells that constitute our body. 



Animals eat every kind of food, and in all of those foods 

 which are nutritious we find certain foodstuffs. In building 

 up their proteins animals must have nitrogen compounds at 

 least as complex as amino-acids. Nitrates are excreted un- 

 changed. You might fill up the bodies of the unhappy, 

 starving Russians with nitrates but not for one moment would 



