We have therefore no hesitation in concluding, that, in The nervous system consists of four principal parts, the brain, the spinal cord, the nerves, and the ganglia. The brain is an organ of a pulpy consistence, resembling a soft coagulum, lodged in and filling the hollow bone called the skull, which forms the main bulk of the head. It is of an extremely irregular figure, having a great number of projections and depressions, corresponding partly to the irregularities of the skull, and partly formed by convolutions and cavities in the brain itself. There are also some considerable cavities in the interior of the brain, which are called ventricles ; they are lined with a serous membrane, secreting an albuminous fluid, which is generally absorbed as rapidly as it is produced, but in certain morbid states is accumulated in the part, and gives rise to the formidable disease of hydrocephalus. With respect to the use of the different projections and depres sions of the brain, nothing certain is known ; but there are two points connected with its form, which it is important to attend to, as they appear to throw some light upon the physiology of the organ ; first, the division of the ence phalon, or cerebral mass into the brain, properly so called, or cerebrum, and into the lesser brain or cerebellum ; and, second, its division into the two hemispheres. By much the greatest part of the nervous matter within the skull composes the proper brain or cerebrum : it occupies the whole of the upper part of the head, and is separated from the cerebellum by a dense membrane, called the tentorium, except at the common basis of both, where they are united. There is also a dense membrane pro jecting downwards to a considerable depth from the upper part of the skull, called the falx, which stretches from the fore to the back part of the head, and divides the brain into its two hemispheres; there is also the same kind of division in the cerebellum.
When we cut into the brain we find it to be composed of two substances, that differ in their colour and consis tence ; the one external, called the cortical or cineritious matter, the other occupying the central part, named the proper medullary matter. The cortical is of a brownish colour, and contains a great number of blood-vessels that are visible to the eye, while many more are brought into view by means of the microscope. Ruysch went so far as to conceive that it is entirely composed of vessels, toge ther with the connecting membrane, an opinion which is obviously incorrect. Malpighi, probably influenced rather by his hypothesis than by actual observation, announced the existence of a glandular structure in this part of the brain. The medullary matter is less vascular than the cortical part, and may be regarded as consisting of the nervous matter in a purer or more perfect state.
When we minutely examine the medullary part ofthe brain, it will be found to exhibit a fibrous texture, or to be composed of a number of longitudinal striae, and these striae are disposed in such a manner as that they meet in the centre of the brain, and form what are styled the corn missures. It has been supposed that they proceed still far ther in the same direction, actually decussating or crossing each other, and a variety of pathological facts are adduced, which seem to favour this opinion ; as where an injury or disease exists on one side of the brain, and manifests its effects on the opposite side of the body. With respect
to the fact itself of the decussation of these fibres, it may be doubted how far it is established by dissection, but un doubtedly the morbid phenomena are satisfactorily ex plained by this supposition.
The brain is covered with two principal membranes, an external one, thick and dense, which lines the skull, and the other thin and delicate, which is more immediately attached to the brain itself, and follows it through all its convolutions ; these membranes are called respectively the dura and pia mater, from a whimsical notion of the older anatomists, that they were the origin of all the mem branes of the body. Between these two membranes lies the third, of a texture still more delicate than the pia mater, which, from its fineness, is termed the arachnoid membrane.
From the lower part of the brain, and connected with it by the intervention of a medullary mass, called the me dulla oblongata, proceeds the spinal cord, or, as it has been improperly termed, the spinal marrow ; this is a quantity of nervous matter, filling up the hollow which is continued through the centre of the chain of bones composing the spine. Like the brain, it is inclosed in membranes, and possesses both cortical and cineritious matter, but the re lative situation of the two is reversed. It has a depression on its back part, analagous to the division of the brain into the two hemispheres, but the effect of injuries of the spinal cord is different from those of the brain, in as much as the loss of power is on the same side of the body.
From the base of the brain, or the parts immediately connected with it, proceed a number of white cords, called nerves, composed of medullary matter, contained in mem branous sheaths, and apparently possessing ‘fibrous struc ture, which are sent to the different organs of sense. The same kind of bodies pass from the spinal cord to all the muscles, and to the different viscera of the thorax and abdomen ; both the cerebral and the spinal nerves are dis posed in pairs, and proceed in corresponding directions to the opposite sides of the body. There arc originally nine pair of cerebral, and thirty of spinal nerves, but soon after their commencement they branch out in various di rections, and form numerous anastornoses ; these are in some cases so complicated, as to compose a complete net work, to which the name of plexus has been applied. Front these plexuses new nerves arise, which often seem to be quite independent of those which produced them, and are some of them evidently formed by the union of the two nerves, which probably originated from a different quarter. When the nerves arrive at their ultimate desti nation, they ramify into small branches, which become more and more minute, until at length they are no lenge: visible to the eye.
the present state of our knowledge, contractility ought to be regarded as the unknown cause of known effects, a quality attached to certain forms of matter, possessed of specific properties, which we cannot refer to any more general principle.