GENERAL ORGANOLOG7. 99 



blood-corpuscles, leucocytes), or in addition to these also red 

 blood-corpuscles. 



21. Red blood-corpuscles occur, in the main, only in verte- 

 brates and cause the redness of the blood ; they are absent in most 

 invertebrate animals. 



22. When invertebrate animals have colored blood (red, 

 yellow), this is usually due to the color of the blood-plasma. 



23. The red blood-corpuscles are nonnucleated in mammals, 

 nucleated in all the other vertebrates. 



III. THE COMBINATION OF TISSUES INTO ORGANS. 



An Organ Defined. Organs are formed from the tissues. An 

 organ is a tissue complex, marked off from the other tissues, which 

 has taken on a definite form for carrying on a special function. 

 Thus a single muscle is an organ which consists of a certain 

 amount of muscular tissue; with scalpel and scissors it can be 

 removed from its environment as a connected whole and can still 

 accomplish a definite movement. 



Principal and Accessory Tissues. In each organ there is a 

 tissue which determines the function of the organ, and therefore 

 its physiological character. This may be called the principal 

 tissue, for there may be other accessory tissues present, which 

 merely support or render possible the function of the principal 

 tissue. In the muscle of the vertebrates we find, besides the 

 muscle-fibres, connective tissue which, like a kind of cement, 

 unites the bundles of muscle; blood-vessels which provide nourish- 

 ment; finally, nerves by which the muscles are aroused to action. 

 In the human liver also, besides the functionally most important 

 part, the liver-cells, blood-vessels, nervous and connective tissues 

 are present. These 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 importance for 

 the permanency of an organ that it be constantly in function. 

 Living substance is distinguished from the non-living by the fact 

 that, if it be destroyed by use, it is immediately replaced, often by 

 more than sufficient to make good the loss. Functioning tissues 

 and organs under favorable conditions increase in volume; on the 



