ELEMENTS OF PHYSIOLOGY 



FIG. 28. Cells forming a Membrane. 



(Mucous membrane of intestines.) A few cells (dark) that from 

 secrete mucus are shown. 



46. Membranes. Sometimes the cells lie side by side, 

 packed closely together, and form a thin skinlike structure, 



called a mem- 

 brane (Fig. 28). 

 47. The Varied 

 Life of the Cells. 

 Some cells, as 

 the red blood 

 cells, move about 

 place to 

 place, but always 

 keep in a definite channel. There is, however, one kind of 

 cell, called the white blood cell, that can change shape and 

 pass from one tissue into another (Fig. 29). 



48. In having a wall, soft contents, and nucleus, the cells 

 of the body resemble minute one-celled animals sometimes 

 found in water. Figure 30 shows the appearance under 

 the microscope of the ameba, one of these small animals. 

 The ameba seems to be hardly more than a minute drop of 

 jelly, yet it can move by pushing out a part of its body. 

 It can absorb food from a tiny particle by rolling itself 

 around the par- 

 ticle (Fig. 30). 

 Afterward the 

 ameba pulls it- 

 self away and 

 leaves behind the 

 indigestible part. 

 If the ameba is jarred or struck, it shows that it has feeling. 

 It absorbs oxygen and gives off carbon dioxid. When it 

 attains a certain size, it divides into two parts, which lead 

 independent lives (Fig. 31). The white blood cells are 

 more like independent one-celled animals than any of the 

 other cells. Some animals are larger than others because 

 their bodies contain more cells, not larger cells. 



FlG. 29. Successive Forms assumed by a Colorless 



Corpuscle of the Human Blood. 



At intervals of one minute. 



