HO THE HUMAN BODY. 



protoplasm which formed the starting-point of each individ- 

 ual. With the physiological division of labor which 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 compared with a complex plant, like a pine or a hick- 

 ory. The higher animal possesses in addition to greatly de- 

 veloped 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 spontaneous, 

 irritable 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, enabling 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 ani- 

 mate world, and his power of so largely controlling and direct- 

 ing natural forces for his own good, while the plant can only 

 passively strive to endure and make the best of what happens 

 to it; it has little or no influence in controlling the happening. 



Amoeboid Cells. The simplest motor tissues in the adult 

 Human Body are the amoeboid cells (Fig. 15) already de- 

 scribed, which may be regarded as the slightly modified 

 descendants of the undifferentiated 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 them- 

 selves 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 undiffer- 

 entiated 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 remark- 

 able change, by which each cell differentiates itself into a nu- 

 tritive and a highly motile and spontaneous portion. Such 

 cells are found for example lining the windpipe, and are 

 represented in Fig. 50. Each has a conical form, the base of 

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



