SECTION III.
DISEASES OF THE HEAD AND SPINE, &c.
The classes of disease treated of in this section, are affections of the nervous system and of the organs of special sense. The nervous system consists of nervous tissue (white and graif} collected into central masses (brain and spinal cord}; of nerves, by which the central organs are brought into relation with parts at a distance ; of numerous small masses (ganglia), which are connected with one another and the larger masses just noticed by nervous filaments (sympathetic system) ; and of gray matter at the peripheral extremities of certain nerves.
The gray matter, in which nervous force or power originates, is composed of cells, many of which are directly connected with the axis cylinder of the white or conducting fibres.
The white matter consists of minute tubes, which convey or conduct impressions to and from the gray matter.
The gray matter at the peripheral extremities of certain nerves plays an important part in defining the kind of impres- sion that is to be conveyed to the nervous centres.
In the encephalon, the gray matter is for the most part super- ficial, the white matter being within it ; in the spinal cord, the v* white matter is near the surface, and the gray matter the most deeply placed. Decussations or crossings of the fibres of the white matter take place in certain situations, viz. in the anterior pyramids of the medulla oblongata, and in the gray matter of jthespjnaljjordj and hence in diseases of the brain we^Te^abfe to explain loss of motion and sensation upon the side of the body opposite to the side of the brain which is diseased ; and in affections of the spinal cord, loss of sensation on the oppo- site, and loss of motion on the same side, as that which is diseased.
