STRUCTURE OF THE HUMAN BODY. 



37 



soft and pulpy, and is called medulla, or pith. The colour 

 of the hair varies in different individuals, and isfgenerally 

 supposed to depend on the fluids contained in the pith! The 

 hair, it is said, has been known in several instances to have 

 changed from black to grey in the course of a single night, 

 from the effects of grief, fear, or some other great mental 

 agitation. Vauquelin supposes that as the colouring matter 

 of the hair is destroyed by acids, this phenomenon is owing 

 to the production of some acid in the system. Others sup- 

 pose that the effect depends upon the sudden stagnation of 

 the vessels which secrete the colouring matter, while the 

 absorbents continue to act and remove that which already 

 exists. There are probably two causes which act in chang- 

 ing the hair to grey ; (the first is a defective -secretion of a 

 colouring fluid, and the second, the canals which convey the 

 fluid into the hair become obliterated) In the first case, the 

 hair will remain ; in the last, it inevitably dies and drops 

 out. 



