PARTS OF ANIMALS, III. x.-xii. 



living for a long time. The reason for these pheno- 

 mena has been explained elsewhere.] 



We have now said what is the purpose for which 

 each of the viscera is present ; but also they have 

 been formed of necessity at the inner ends of the blood- 

 vessels, because moisture, i.e. moisture of a blood- 

 like nature, must of necessity make its way out there, 

 and, as it sets and solidifies, form the substance of 

 the viscera. That, too, is why they are blood-like in 

 character, and why the substance of all of them is 

 similar, though different from that of the other 

 parts. 



XI. All the viscera are enclosed in membranes. Membranes. 

 Some covering is needed to ensure their safety, and 



it must be a light one. These conditions are fulfilled 

 by a membrane, which is close-textured, thus making 

 a good protection ; does not consist of flesh, and 

 therefore does not draw in moisture or retain it ; is 

 thin, therefore light, and causes no burden. The 

 biggest and strongest membranes are those round 

 the heart and the brain, which is natural enough, as 

 it is always the controlling power which has to be 

 protected ; therefore the heart and the brain, which 

 have the supreme controlling power over the life of 

 the body, need the most protection. 



XII. Some animals possess a full complement of Variationa 

 viscera, some do not. We have already stated what '^"^^5^^. 

 animals have less than the full number, and the 

 reason. But also, the same viscera are different in 



the various animals that have them. For instance, 

 the heart is not identical in all the animals which have 

 a heart ; nor is any other of the viscera. The liver 

 illustrates this : in some it is split into several parts, 

 in some almost undivided. This variation of form is 



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