ELEMENTARY PHYSIOLOGY 



sympathetic system are chiefly supplied to these organs and the 

 blood-vessels, thus controlling the involuntary movements, and 

 regulating the supply of blood to the various parts of the body. 



Nerve substance or nerve tissue is either cellular or fibrous. 



The cellular tissue is composed of branched cells together 

 with interlacing fibres, and, on account of its colour, is called grey 

 substance. It is found in the brain, the spinal cord, and in all 

 the other nerve centres. 



Fibrous nerve tissue is composed of fibres only, and is of two 

 kinds grey and white. The white fibrous tissue is an essential 

 constituent of the cerebro-spinal nerves : it also enters largely into 

 the structure of the brain and cord. The grey fibres are found 

 principally in the nerves of the sympathetic system. 



Fig. 162. Nerve-cells from the Grey Matter of the Brain. 



Each white nerve fihre is a minute semitransparent filament when fresh. 

 But a few hours after death it undergoes changes and is then seen to consist 

 of an outer membrane, in the middle of which is a coagulated central thread 

 (the axis-cylinder), surrounded by a white sheath. 



The grey fibres closely resemble the white, but the white sheath enclosing 

 the axis-cylinder is wanting. 



Each nerve is a bundle of such fibres, passing to or from a nerve 

 centre, generally bound together by a delicate sheath of connective 

 tissue called the neurilemma (Gr. neuron, a nerve ; and lemma, 

 rind). It may be easily distinguished in the body of an animal as 

 having the appearance of white or cream-coloured threads. 



We may pass on at once to notice the essential difference 

 between nerves and nerve centres. A nerve, composed of nerve 

 fibres, has no power to generate what we may call a nerve impulse, 

 but it can conduct an impulse already produced, along its fibres, 

 either to or from the nerve centre with which "it is connected. 

 On the other hand, a nerve centre, which consists partly of the 



