SENSATION. The brain is a soft pul py mass, of a whitish colour on the inside, occupying all the cavity of the skull. Minute differences are observable in the substance of the brain in different parts of it, but it is unnecessary to enter upon a statement of them here. (See Aitero XT.) The spinal marrow is the continua tion of the lowest part of the brain, which passes through the great opening of the skull down the hollow of the back-hone. The substance contained in the hollow of the backbone is the same with that in the cavities of the skull ; and it is sometimes convenient to comprehend both the por tions under the same general name of brain. From the brain proceeds the nerves, which at first are fine fibres, of the same substance with the brain : these fibres meet, and form soft white pulpy cords, which afterwards spread them selves over various parts of the body, by splitting into innumerable and exceeding ly minute branches. Anatomists count forty pairs of nerves, (for they come off in pairs, though they afterwards sepa rate.) and of those nine or ten only come from the brain at the bottom of the skull, and the rest from the spinal marrow. Those from the brain are distributed to various parts of the head : those from the spinal marrow are distributed over the trunk and extremities. The external or gans of sense, the nerves, and the brain, are the organs of sensation. All, as we are at present constituted, are necessary to sensation. If the external organ is destroyed, no sensation can be produced: where there are no nerves, there is no sensation: where the nervous branches are most numerous, there is moat sensa tion : if the nerve be destroyed, sensation cannot be produced from those parts to which the nerve belongs, which are fur therfrom the brain than the injured parts. The brain is the ultimate organ of sensa tion of which we have any knowledge. All the nerves terminate in the brain. If the brain is compressed, sensation is sus pended. If the brain is considerably in jured, sensation ceases —So, also, there is considerable reason to believe that the brain is the immediate organ of ideas. If the brain is diseased, many of the pheno mena of thought are altogether changed; if the brain is compressed, thought is sus pended; if the brain is injured, ideas cease.—So, also, the brain appears to be
the ultimate organ of all motions which are not produced by the immediate action of external objects upon the muscles. The muscles are the immediate organs of motion. The muscles consist of fleshy substances, and sometimes of tendons. The tendons fasten the muscles to the bones; and the fleshy part by its contrac tions produces the motions of the bones. Into the fleshy parts of the muscles nu merous nerves enter; they are diffused over its surface and within its substance. These nerves, as before mentioned, ter minate in the brain either of the head or back. They are the intermediate organs of voluntary motion between the brain and the muscles. If a nerve be compress. ed or punctured, motion is produced in the muscle over which that nerve is dia. tributed. If a portion of a nerve be cut, or otherwise destroyed, voluntary motion can no longer be produced in that muscle over %filch it was distributed. If the brain be touched with any instrument or caustic, applied to it, the muscular system undergoes the most violent contortions. If the spinal marrow be pierced with a probe, all the muscles of the trunk and limbs undergo violent contortions, parti cularly those of the back. If the brain be compressed, the whole body becomes pa. ralysed, and the power of voluntary mo. tion is suspended. If the spinal marrow be compressed, the power of voluntary motion is suspended in those muscles which receive their nerves from the back. If the brain is considerably injured, all power of voluntary motion ceases.
The external organs of sense are usu ally classed under five heads, those of sight, of hearing, of feeling, of smell, and of taste. The sense of feeling might pro bably be divided with convenience into two or three, because the classes of sen sations, which are referred to this sense, differ considerably in themselves and in the external causes producing them. But the common arrangement is sufficient for our purpose.