178 PRINCIPLES OF PHYSIOLOGY 



the central organs, which are by far the most 

 complicated and most difficult to understand. 

 They consist of the brain, the spinal cord or 

 marrow, and ganglia. Ganglia are small 

 masses of nerve matter found in many parts 

 of the body, and they abound in many organs, 

 as in the heart and in the mesentery. Such 

 small nodules of nervous matter consist of 

 supporting tissue, nerve cells, and nerve fibres. 

 From one point of view, the spinal marrow and 

 the brain may be regarded as masses of ganglia 

 fused together during countless ages of evolu- 

 tion. All ganglia, be they simple or complex, 

 are known to be composed of certain morpho- 

 logical elements or structural units. These 

 units are supported and protected by a 

 specialized form of tissue, the neuroglia, 

 which, however, is not ordinary connective 

 tissue, although that may also enter into the 

 composition of nervous structures. The units 

 are known as nerve-cells, or neurones. These 

 vary much in general form and size, but 

 they have certain general characteristics. 

 They are composed of protoplasm in which 

 there is a well-defined nucleus, and both in 



