96 



A nerve-fibre is a microscopic element composed of a proper wall anct 

 contents. The wall is the sheath we mentioned, and it is a thin elastic 

 membrane. The contents comprise in the centre a solid core, called the axis 

 cylinder, along which the nerve current passes. In many fibres, between the 

 axis cylinder and the wall, is found a viscid substance called the medulla. 

 Those fibres which do not contain the medulla, and which are specially 

 characteristic of the sympathetic system, are called non-medullated. 



The majority of nerve-fibres measure about ^^ of an inch in diameter,. 

 The nerve-cells are large nucleated bodies of very variable shape, and they 

 have one or more prolongations extending from them. These prolongations 

 or poles establish relations with the nerve-fibres, and constitute the origin of 

 the nerves. 



In the above figures, A shows some nerve cells of dilTerent shapes. I> 

 shows a stellate cell from a developing animal, magnified 400 diameters. 

 When a cerebro-spinal nerve is irritated by pinching, there is either pain 

 manifested, or there is twitching of one or more muscles, to which the nerve 

 distributes its fibres. From various considerations, it is certain that pain is- 

 always the result of change in the nerve cells of the brain. Therefore, in 

 such experiments as those referred to, it seems to the experimenter that the 

 irritation of the nerve-fibre is conducted in one of two directions, either ta 

 the brain the central termination of the fibre, when there is pain, or to a 

 muscle when there is movement. The effects of these simple experiments 

 arc the types of what always occur, when nerve-fibres are engaged in the 

 performance of their functions (Kirke). The brain of the horse and of the 

 other higher animals is formed of a central white part composed of fibres, 

 and an outer convoluted portion of grey matter composed of nerve-cells and 

 fibres. In the horse it weighs about 23 ounces, in the ass 12 ounces; and it 

 is formed of a front portion called the cerebrum, and a hind part called the 



