18 



ADVANCED PHYSIOLOGY 



the nucleus. This nucleus seems to be the center of the 



life and activity of the whole cell, for if deprived of it the 



cell cannot long continue 

 to live. There are other 

 parts of the cell, as is 

 shown in Figure 14, for 

 the cell is really a com- 

 plicated bit of machinery. 

 The chemical composi- 

 tion of protoplasm is not 

 definitely known, but cer- 

 tain elements carbon (C), 

 hydrogen (H), oxygen (0), 

 nitrogen (N), iron (Fe), 

 sulfur (S), calcium (Ca) 

 and phosphorus (P) enter 

 into the composition of all 

 kinds of cell protoplasm; 

 other elements may occur 

 in smaller quantities. 



Protoplasm, then, 

 should be remembered as 

 the only living substance 

 in the body. It cannot 

 live long without a nu- 

 cleus, neither can a nucleus 

 live without the support of 

 surrounding protoplasm. 



The living protoplasm 

 may be surrounded by a 

 layer of greater consistency. 

 This may be very thick, as 

 in cartilage (Fig. 4) ; it may 



be very thin, as in the nerve cell, or it may be entirely absent, 



as in the white blood corpuscles; Fig. 10. 





FlQ. 11. A NERVE CELL OR NEURON 



At a is shown the cell body; 6, the nu- 

 cleus; c, the axon of a nerve fibre; d, 

 branching of the fibre; e, muscle fibres 

 within which the fibre ends: /, a node 

 in the fibre. (Barker) 



