90 
A nerve-fibre is a microscopic element composed of a proper wall and 
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 gs'v5 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 different shapes. BL 
shows a stellate cell from a developing animal, magnified 4oo 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 to- 
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: 
are 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 
