74 THE HUMAN BODY 



takes place in the higher forms we find that, speaking broadly, 

 plants especially develop nutritive tissues, while animals are 

 characterized by the high development of tissues with motor and 

 irritable properties; so that the preponderance of these latter is 

 very marked when a complex animal, like a dog or a man, is com- 

 pared with a complex plant, like a pine or a hickory. The higher 

 animal possesses in addition to greatly developed nutritive tissues 

 (which differ only in detail from those of the plant, and constitute 

 what are therefore often called organs of vegetative life) well- 

 developed irritable conducting, and contractile tissues, found 

 mainly in the nervous and muscular systems, and forming what 

 have been called the organs of animal life. Since these place the 

 animal in close relationship with the surrounding universe, en- 

 abling slight external forces to excite it, and it in turn to act upon 

 external objects, they are also often spoken of as organs of relation. 

 In man they have a higher development on the whole than in any 

 other animal, and give him his leading place in the animate world, 

 and his power of so largely controlling and directing natural forces 

 for his own good, while the plant can only passively strive to en- 

 dure and make the best of what happens to it ; it has little or no in- 

 fluence in controlling the happening. 



Amreboid Cells. The simplest motor tissues in the adult Human 

 Body are the amoeboid cells (Fig. 99) already described, which 

 may be regarded as the slightly modified descendants of the un- 

 differentiated cells which at one time made up the whole Body. 

 In the adult they are not attached to other parts, so that their 

 changes of form only affect themselves and produce no movements 

 in the rest of the Body. Hence with regard to the whole frame they 

 can hardly be called motor tissues, and are classed in the group of 

 undifferentiated tissues. 



Ciliated Cells. As the growing Body develops from its primitive 

 simplicity we find that the cells lining some of the tubes and 

 cavities in its interior undergo a very remarkable change, by which 

 each cell differentiates itself into a nutritive and a highly motile 

 portion. Such cells are found for example lining the windpipe, and 

 are represented in Fig. 41. Each has a conical form, the base of 

 the cone being turned to the cavity of the air-tube, and contains 

 an oval nucleus with a nucleolus. On the broader free end are a 

 number (about thirty on the average) of extremely fine processes 



