68 PHYSIOLOGY OF THE HUMAN BOPY. 
number of nerves, or nervous fibres, spread over the surface of the body, 
being especially abundant in the organs of sense. 
What is the nature of the connection that exists between the mind and 
the action of the brain, no one has yet been able to tell; we only know 
that there is a connection. Nor can the nature of nervous influence be 
explained. The nearest approach to an explanation that can be given, is 
to say that it acts like electricity ; and, indeed, the similarity between the 
action of the nervous system and that of the electric telegraph is greater 
than at first might be supposed. When treating of the muscles, we saw 
that they have the power of contracting. The principal agent in pro- 
ducing contraction of the muscles is the stimulus conveyed to them 
through the nerves. . 
The substance of which the nervous system is composed is of two 
distinct kinds, called, from their appearance, white and gray matter. The 
white matter consists of straight fibres, lying side by side, and bound into 
bundles; these bundles are again bound together into larger bundles ; 
and in this way are formed the nervous trunks which are spread throughout 
the body, and which are entirely formed of this white or fibrous matter. 
The gray matter is found, to a great extent, in the ganglia,} or nerve- 
centres, which are masses of nerve-matter occurring at intervals on the 
trunks. This gray matter is not of a fibrous texture, like the white, 
but consists of cells; and it seems, that in some way or other, the nerve- 
fibres of the trunks communicate with the cells of the gray matter in 
the ganglia. The office of the nerve-fibres in the trunks is to convey the 
influence of something going on in one part of the system to another 
part. These influences, as has been indicated, are sent in two directions 
—the impressions made on the sensitive parts of the body are conveyed 
to the nerve-centres, and give rise to what are called sensations ; and when 
these sensations cause certain emotions, or an exercise of the will, an 
influence is sent outward to the muscles, which are thereby excited to 
‘contraction. And it has been proved that these different messages are 
conveyed by two different sets of fibres: those which carry an influence to 
the nerve-centres being called sensitive or afferent; * and those which carry 
an influence from these to the muscles, motor (because for the most part 
giving rise to motion) or efferent. 
The nervous system consists of two parts—a central part, called 
the cerebro-spinal axis, which consists of the brain and the spinal cord, 
lodged within the skull and the vertebral column ; and an outside part, 
consisting of nervous trunks proceeding from the central part, and 
distributed to all parts of the body. The brain is composed of a number 
of ganglionic masses, each of which is at the head of a special department ; 
1 Plural of ganglion, Greek, a knot. 2 From Latin ad, to, and fero, to bring. 
3 From Latin ez, out from, and fero, to bring. 
