As an appendage to the system of the brain, or rather as a detached nervous apparatus of considerable importance, we shall shortly describe the course and formation of the great sympathetic or intercostal nerve; the brain of the vital and involuntary motions, its succeda-neum in the lower classes of animals, and, indeed, in human monsters where the brain is absent.

The great intercostal is styled a portion of the sixth pair of nerves, another portion of which is distributed to the muscles of the eye. It is, however, more properly a nerve from the medulla spinalis, and is sent rather to the brain, than proceeds from it. This we collect from the parts to which it is distributed being wholly independent of the will; and from the nerve between the neck, and that part where the nerves of the muscles of the eye are sent off, being larger than that between the brain and the same point. This almost insulated, nearly independent, nervous system, is connected with the brain, not only by this branch, but by a slight twig from the fifth pair; but its substance is formed by branches from all the different vertebrae. As soon as it appears in the neck it forms a ganglion, a medullary enlargement, in which the different filaments are intermixed, with generally the addition of some cineritious substance. In the neck only, there are three of these ganglia, to which nerves from almost every vetebra can be traced. In the chest it receives additional nerves, forming several smaller ganglia. From hence it escapes with the aorta, and reaches the sacrum; forming, with the branches from the spinal marrow of that part, other ganglia. It then turns inward; and at the hollow of the os coccygis, meets its fellow from the opposite side, which has traced the same course with similar additions.

When not united in a hard body like a ganglion, smaller nerves are often intermixed, and form, by their mutual decussation, a net work. In these a similar interchange of fibres seemingly takes place; and, in the course of the intercostal nerve through the trunk, nine distinguished plexuses have been described. To illustrate the advantages of this arrangement, we may now, though not in its proper place, remark that if an organ essential to life was supplied by one nerve, any injury to that, either in its origin or course, would be fatal. If, by uniting in a ganglion or a plexus with another nerve the fibres of both are intermixed, the injury to one nerve would be attended only with half the injury to the function of the organ, but the chance of injury would be increased in the same proportion. If, then, fifty nerves are mixed, the chances of injury are augmented, but the real injury to the organ is very inconsiderable. In this way has nature guarded those us on which life depends; and we see that animal life may exist with a very small brain, or without any. But, after a perfect animal has existed, such is the connection of excitability through the whole nervous system, that, independent of the loss of blood, the destruction of the brain must destroy the excitability in the nerves. It may be diminished to a very inconsiderable degree; and, by the arrangement described, life may for a time be continued till the cause is removed.

The great sympathetic nerve is also the connecting link, between the two parts of which the human body consists. All our organs are doubled; and one half may be diseased or dead, with a partial injury only to the vital organs; and, in the first case, if the brain is not affected, without any apparent inconvenience to the unaffected parts. The great sympathetic, we shall find, supplies, in a great degree, the lungs, the heart, the stomach and intestines, the urinary and genital organs. The action of these, it must be obvious, are too important to be entrusted to the influence of one or several nerves: they must be raised above common accident: and, above all, must be independent of the will.

Such is the substance of the descriptions of the most correct anatomists; and we should now proceed to consider the functions of this organ, if we did not find it necessary to attend to the doctrines of an author who has begun to excite considerable attention on the continent; we mean Dr. Gall of Vienna. Dr. Gall has published no account of his system, and the substance must be collected from the reports of his pupils. We have now before us two, the one by Dr. Arnemann, and the other by M. Boisjames (Journal de Physique, vol. 5 5.), in which there is a view of the cranium, where the different organs are distinctly marked. Our chief object, at present, is his account of the brain and medulla spinalis.

We laid the foundation for this explanation very early, by inserting the arrangement of the animal kingdom by M. Virey (see Animal). In this scheme, animals arc distinguished as possessing a cerebral and intercostal, or sympathetic system: an intercostal system alone; or separate nerves with nervous molecules only. Dr. Gall considers the lower classes of animals as possessing nervous filaments only; the next, a spinal marrow; the superior animals possess a double spinal marrow; and the more perfect, a brain. In this lsst class, the spinal marrow, he thinks, exclusively forms the brain, and all the other nerves. The eight pair of nerves, supposed to arise from the brain, are derived, therefore, by Dr. Gall, from the spinal marrow, and seem to become a part of the brain before they are sent off in separate bundles. In this rank, the first are the nervus accessorius, and nervus occulo motorius; the common ganglion of which is supposed to be the corpus olivare, at the upper part of the medulla oblongata. Nearer the centre of the medulla are the nerves of the cerebellum; those medullary processes formerly supposed to be sent from the latter to the former: the ganglion of these is a substance situated in the body of the cerebellum, called, by anatomists, corpus ciliare.

The corpora pyramidalia, which we have mentioned as protuberances on the medulla oblongata, are supposed, by Dr. Gall, to be the origin of the cerebrum; for he expressly remarks, that the size of the hemispheres is always proportional to that of the pyramidal bodies. The ganglia of the pyramids are the pons varolii; and a part we omitted to mention, the ganglion cerebri, near the fossa sylvii. These nervous strings are distinctly marked at the base of the skull by gyrating lines.