114 THE HUMAN BODY. 



which they live. The lowest animals and plants are in fact 

 those which have undergone least differentiation in their 

 development, and which therefore resemble each other in 

 possessing, in a more or less manifest degree, all the funda- 

 mental physiological properties of that simple mass of pro- 

 toplasm which formed the starting point of each individual. 

 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 char- 

 acterized 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 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-devel- 

 oped 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 sur- 

 rounding 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 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 endure and make the best of what happens to 

 it ; it has little or no influence in controlling the hap- 

 pening. 



Amoeboid Cells. The simplest motor tissues in the adult 

 Human Body are the amoeboid cells (Fig. 12) 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 themselves and produce no movements in the rest of 

 the Body. Hence with regard to the whole frame they 



