GENERAL ORGANOLOGY. I 19 



which have the purpose merely of supporting or ren- 

 dering possible the function of the principal tissue. In 

 the muscle of the vertebrates we find, besides the mus- 

 cle-fibres, connective substance which, like a kind of 

 cement, unites the bundles of muscle ; also blood-vessels 

 which serve for nourishment; finally, nerves by which the 

 muscles are aroused to action. In the human liver also, 

 besides the functionally most important part, the liver- 

 cells, there are present blood-vessels, nervous and connec- 

 tive tissues. These kinds of accessory tissues are usually 

 found only in the highly developed organs ; in the case of 

 the lower animals they may be absent ; thus the digestive 

 tract of Ccelenterates has only an epithelial lining; their 

 nervous system consists merely of a cord of nerve-fibres 

 and ganglion-cells. 



Effect of Use and Disuse. It is of the greatest im- 

 portance for the permanency of an organ that it be con- 

 stantly in function. Living substance is distinguished from 

 the non-living by the fact that, if it is destroyed by use, it 

 is immediately replaced by a quantity more than sufficient 

 to make good the loss. Functioning tissues and organs 

 increase in volume; on the other hand, parts becoming 

 functionless undergo a gradual decrease, which finally 

 leads to their disappearance. 



Change of Function of Organs. The two factors 

 mentioned, that the permanence of the tissues depends 

 upon continued use, and that usually several tissues enter 

 into the structure of an organ, are important for the under- 

 standing of the principle of change of function which plays 

 a prominent role in the metamorphosis of animal form. It 

 may happen that an organ is brought under changed con- 

 ditions, and no longer has an opportunity to function as 

 before. In that case, the functioning tissue from lack of 

 use gradually degenerates, but yet it may exist longer by 

 means of its accessory tissues; if the new conditions make 

 it possible for one of the accessory tissues to attain to func- 

 tional activity, the organ may assume a new physiological 

 character. 



