UNITS OF LIFE-CELLS 



59 



corpuscles are tiny discs, fitted by shape to 

 float in the blood stream. Smooth muscle 

 cells are spindle-shaped, which is the ideal 

 shape for a cell that must shorten or con- 

 tract. Cells that line the respiratory tract in 

 our bodies are small cylinders with minute, 

 vibratile, hair-like structures (cilia) on one 

 end, which function in carrying the mucus 

 along the tract by their whip-like action. 

 Other cells, such as the amorphous white 

 blood cells, resemble tiny bits of jelly that 

 move by rolling along, taking in and de- 

 stroying bacteria and other foreign par- 

 ticles in the blood and tissues. 



Number. A glance through a microscope 

 at a thin slice of tissue taken from any ani- 

 mal will demonstrate the fact that there are 

 a great many cells in even a small animal 

 like the mouse or spider. The larger the 

 animal, the more the cells, although swift- 

 moving and very active animals such as 

 insects and birds usually have smaller cells 

 per unit volume than do sluggish, slow- 

 moving creatures such as the salamander. 

 There is no correlation between the size of 

 the animal and the size of its cells. The 

 larger animals simply have more cells. 

 Every cubic millimeter of human blood 

 contains about 5 million red blood cells, 

 and the total number in the entire blood 

 stream approximates 30 quadrillions. The 

 human brain alone has billions of cells; the 

 number in the whole body thus takes on 

 astronomical figures. 



THE PHYSIOLOGY 

 OF CELLS 



As previously noted, cells function much 

 alike, irrespective of their situation. All 

 have certain needs which are satisfied in 

 much the same way. To be sure, certain 

 complications arise when they are grouped 

 together in great masses but that problem 

 will be the subject of a later discussion. Let 

 us now examine the processes common to 

 all cells whether they be independent or- 

 ganisms or members of large complexes. 



Fig. 3-2. Various kinds of cells. A. Red corpuscles. B. Flat 

 (squamous) epithelial cell. C. Columnar ciliated epi- 

 thelial cells. 0. Smooth muscle cells. E. Nerve cell 

 (neuron). 



