170 PRINCIPLES OF PHYSIOLOGY 



removed by special arrangements, while it is, 

 as far as possible, protected from injury. 



94. Essentially, the nervous mechanism 

 consists of centres, nerves, and nerve-end 

 organs. The centres are in great masses 

 constituting the brain and spinal cord, and 

 in smaller masses found scattered here and 

 there, known as ganglia. The nerves are 

 found almost everywhere, as whitish cords, 

 varying in calibre from the largest nerves, 

 such as the sciatic in the back of the thigh, 

 down to minute filaments invisible to the naked 

 eye and requiring the use of the microscope 

 for their detection. Each nerve is composed 

 of minute fibres, all of microscopic dimensions, 

 and each showing a central rod or axis, 

 surrounded by a sheath, called the white 

 substance, and this, in turn, usually covered 

 by a thin membrane, the neurilemma. These 

 matters are all of soft consistence and are 

 apparently structureless, but, by special 

 methods, details of structure may be seen. 

 Thus the central axis is sometimes composed 

 of fine fibrils, and the surrounding matter, 

 the white substance, is composed of elongated 



