50 ESSENTIALS OF BIOLOGY 



tissue that binds together other tissues and organs, that acts as 

 a packing between organs and parts of such, that suspends organs 

 in the body cavity ; that joins muscles to their bones, etc. 



14. ON ANIMAL STRUCTURE IN GENERAL 



There are animal forms, each form being typical of a category 

 (see Section 77). Such forms are indefinitely numerous. So 

 also there is an indefinitely great number of patterns of animal 

 behaviour. 



But if we regard the structure of the body of an animal as that 

 of a system of parts, or a physico-chemical mechanism that 

 subserves the activities of behaviour, we find that : 



The number of essential structural patterns is very much less 



than the number of animal forms. 

 Thus the alimentary system (mouth and teeth, salivary glands, 

 oesophagus, stomach and gastric glands, intestine and intestinal 

 glands, liver, spleen, pancreas, etc.) is very much the same, and 

 does very much the same things in all vertebrate animals. 



[The analogy is with the multitude of makes of automobiles 

 that one sees on the roads. These differ in size, accommodation, 

 general elegance, finish, etc. But as mechanisms their essential 

 structures are far fewer than the number of " different " makes 

 of cars]. 



In the animal body there are organs — brain and nervous system, 

 muscular organs, glandular organs, etc. But the number of 

 essentially different organs (different in the way that the fish-gill, 

 as a respiratory organ, differs from the mammalian lung) is 

 relatively small. 



Smaller still is the number of different kinds of tissues and 

 tissue-cells. In the last resort the characteristic activity of an 

 organ is the activities of its characteristic tissue-cells. 



14^. Structure in Relation to Functioning. There is 

 not a close one-to-one correspondence between the structure of 

 an organ and the things that that organ does. Thus the result 

 of activity of a respiratory organ is the oxygenation of the blood, 

 but the fish-gill does that as adequately (from the point of view 

 of the fish) as do the lungs of a mammal. That is, mechanisms 

 that are externally or superficially different in structure may do 

 the same things, in the functional sense. 



