THE NERVOUS SYSTEM. 439 
We have now to consider the prime mover of all these several agents, the 
nervous system, which may be compared to the fuel that heats the water 
of the steam-engine, and converts that apparently most simple and inno- 
cent fluid into the powerful agent which is capable of developing almost 
any amount of force. This fuel, however, is itself inactive until it is en- 
dowed with life by the agency of fire ; and, in the same way, the nervous 
system of the animal being must be provided with the living principle, of 
whose nature we can only judge by its effects when present, and ky the 
cessation of all action when absent. There are many processes which are 
carried on in the animal as in the vegetable without the necessity for any 
direct stimulus from a nervous centre, such as the growth of each separate 
tissue throughout the body, which takes place in the former, just as it 
does in the latter, by a species of cell-development and metamorphosis 
independent of nervous energy ; but though this growth is thus accom- 
plished, yet it would soon be starved out for want of pabulum, were it 
not for the supply of food to the stomach, which requires the mandate of 
the nervous system for its performance, and so on with every corresponding 
action of the body. 
THE NERVOUS SYSTEM is made up of two distinct substances, one grey 
in colour, and granular in structure, which is the seat of all nervous power; 
the other white and fibrous, which is the telegraph wire by which this 
power is communicated. Sometimes the grey matter envelops the white, 
and at others it is inclosed within it, but in every case each has its pecu- 
liar office as above mentioned. ach collection of grey matter is called a 
ganglion, whatever its shape may be ; but the white fibres may be either 
in the form of commissures for connecting the ganglia together, or they 
may be agents for communicating with other organs, and are then called 
nerves, 
CHIEF DIVISIONS OF THE NERVOUS SYSTEM. 
In THE HORSE, as in all the vertebrata, the nervous system is made up 
of the following parts. 1. The ganglia, which are intended to subserve 
what are called the reflex actions of the organs of locomotion, &c., and which 
occupy the whole length of the spinal cord, one on each side. 2. The 
respiratory ganglia, situated higher up towards the brain, constituting the 
part called medulla oblongata, and placed in superintendence over the 
functions of respiration, mastication, and deglutition. 3. A series of 
canglia controlling the organs or special sense, situated at the base of the 
brain. 4. The cerebellum, which seems specially intended to combine 
and balance the several muscular actions of the body. 5. The cerebrum, 
which is the seat of intelligence and wili. 6. The sympathetic system 
of ganglia, which specially controls the vital organs of circulation, diges- 
tion, and depuration. The first five divisions are generally included under 
the head of the nervous system of animal life, the last being considered 
to be peculiar to organic life. The diagram on the next page will show 
at one view the chief component parts of the two systems. 
THE SPINAL CORD. 
THE SPINAL CORD may be considered to be the primary division of the 
nervous system, because it represents the lowest development of this 
organ in the animal kingdom. But instead of consisting of a series of 
locomotive ganglia, as in the articulata, it is here found in the shape of 
