THE VITAL UNITS CALLED CELLS 



193 



All other cells present the same structural pattern in so 

 far that a cell wall enclosing liquid material may be dis- 

 tinguished. The nucleus and the mitochondria are likewise 



o ^o^-T^o 



Cell membpdne 



tlitochondpiA 



-NucleAP membrane 

 Naclcap contents 

 ■Nucleolus 



Fig. 2. Diagram of mucus-secreting cell of stomach. (Magnified about 1500 



times.) 



invariably present. Various special components are found, 

 in certain kinds of cells. Among these may be mentioned 

 secretion in gland cells, droplets of fat, contractile fibrils 

 in muscle cells, and pigment in the cells of the eye and often 

 in those of the skin. 



THE CELL IS BUILT LIKE AN ENGINE 



We can liken the cell to an engine although it is in every 

 respect a more efficient mechanism. Despite its small 

 size it is able to bring about chemical and physical changes, 

 the majority of which it is impossible to repeat outside the 

 body even with the aid of the most delicate and complicated 

 apparatus. 



The cell takes in crude materials and makes them into 

 finished products (e.g. adrenalin) which influence other 

 industries or tissues, themselves composed of cells. As a 

 great engine is organized in space so is the cell. Gland cells, 

 for instance, devote a special part of their circumference to 

 the reception of substances from the blood stream, just as 



