nerves, go through ganglia, but rather avoid the ganglia of the diverging nerves. d.) That they meet together from the homogeneous congeries of nerve on both sides, and form commissures. The commissures, which Gall has hitherto been able to exhibit anatomically are: 1.) The commissure of the converging fibres of the auditory nerve. It lies immediately behind and before the pons Varolii, and in men it is covered by it, but in other animals, as they have a smaller cerebellum, and consequently a smaller pons Varolii as its commissure, it is perfectly free and distinct. 2.) The commissure of the converging fibres of the olfactory nerve. 3.) The commissure of the converging nerves of the cerebellum. This, as already observed, is formed in the pons Varolii. When the brain is reversed, the converging nerves of both hemispheres of the cerebellum are to be seen very distinctly running across and meeting on the pons Varolii. These, and the diverging nerves which run along from the pyramids, and are destined for the hemispheres, succeed each other in distinct layers, as already stated. (4. The : 4,) The commissures of the converging nerves of the cerebrum. a.) The largest and most important of these is the corpus callosum. In this are united, not merely most of the converging nerves of the whole hemispheres, but also the remaining particular commissures of the converging nerves of the cerebrum. b.) The commissura anterior, or the union of the converging nerves of the front and middle lobe of the brain above the optic nerve. The Septum pellucidum is a part or continuation of this commissure. In animals whose middle lobes are smaller, the commissura anterior is weaker, and in these the olfactory nerve furnishes the same with converging nerves. c.) In like manner, the converging nerves of the back lobes of the cerebrum form together a commissura posterior. d.) Besides these commissures, the converging nerves of the cerebrum form both before and behind some other particular commissures on the corpus callosum, for constituting a sort of covering round it. Besides the above mentioned nerves and congeries of nerve, there also proceeds a tender nervous mass from betiveen the two halves of the spinal marrow, upwards through all all the double organs which are formed by the nervous fascicles of the spinal marrow. This nervous mass is, as it were, the instrument of connection between the double organs, and appears on the great commissure the corpus callosum, as the Raphe Lancisii. It may be proper to observe, that Gall was first led to that contemplation and study of the brain which ended in the doctrine above stated, by observing the phænomena of the Hydrochephali interni, in whom the whole brain is often stretched out into a membrane scarcely a line thick; hence he inferred that the brain cannot be, as is commonly fancied, a pulpy substance, but must be a mem→ brane. About the same time certain pathological appearances, for instance, that the extremities are lamed by the hemispheres of the brain being wounded, evinced to him that an uninterrupted connection must take place between those hemispheres and the spinal marrow. He accordingly directed his attention to an anatomical exhibition of that membranaceous quality of the brain which he suspected from physiological reasons: and he was enabled, in opposition to all the anatomists of antiquity, and before all modern anatomists, to make this anatomical discovery, by pursuing a mode of anatomical research research contrary to the practice hitherto generally observed; that is, he traced the connection of the nerves and brain, not from the summit downwards, but from the spinal marrow upwards. In doing this, he followed the course which nature itself takes; as in the higher and more elaborate organisation of animals, the commencement is, as it were, in the spinal marrow, and the brain is gradually and more subtly formed, according to the kind and rank of the animal in the order of creation In the simplest animals, viz. the polypus, we see only scattered nerves; in the next order of animals, we meet with a kind of stem, from which diverging nerves issue in more highly organised beings. In animals, still further advanced, the nerves springing out of both halves of the spinal marrow (for the spinal marrow, as well as the brain and all organs of animal life, is double) form, partly the brain, partly nerves; all of which in fact spring from the spinal marrow, though they seem to have their origin in the brain, as has been already stated. CHAP. CHAP. II. OF PHYSIOLOGY IN GENERAL. THE anatomist is contented, when he detects a distinction of parts which is constant and invariable. This he marks and proclaims to his scientific brethren, and they not unfrequently, in grateful memory of his service, immortalize his name, by affixing it to the thing he first saw and made known (pons Varolii, membrana Schneideri, &c.). But it so happens, that the name of the discoverer is applied to the object, not because of the importance and value of the discovery, but, on the contrary, because for the present it is the mere detection of a thing, without the least insight into its functions and uses. It is impossible to look upon merely a picture of the brain, whether we take a section of it vertically, or survey its different layers horizontally, without being struck with the nice. complexity |
