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This commissure is pierced in its interior by a canal, which is more or less distinct in different animals, and is especially visible during the earliest periods of life. In the human fætus this canal is generally obliterated about the fifth month ; but some authors mention having seen it in advanced old age. It seems to be a continuation of the aqueduct of Sylvius.
The corresponding parts of each half of the spinal cord are voluminous in the ratio of their implanted nerves.
This fact is strikingly exemplified by comparing the spinal cord of animals whose superior are larger than their inferior extremi. ties with those of others whose inferior are of greater size than their superior limbs.
The interior of each of the halves of the spinal cord contains a mass of grey matter, always proportioned to its size and to the nervous roots issuing from it. This pulpy or grey matter is disposed in a crescent-form on either side of the apparatus of union.
Besides this division of the cord into vertical halves, it is also divided, although much less distinctly, as it were from side to side, into an abdominal or anterior surface, and a dorsal or posterior, and all the nerves issuing from the spine derive a root from each of these halves. The posterior or dorsal roots, and the corresponding or dorsal half of the spinal cord, are larger than the abdominal or anterior. The intervertebral ganglions are proportioned in size to the dorsal roots. The latter frequently inosculate, and filaments from one pair of nerves often run to join the fasciculi composing another. The anterior or abdominal roots communicate with the nerves of the thorax and abdomen.
Drs Gall and Spurzheim have for many years maintained, that the spinal cord of man and animals is enlarged, and contains a larger quantity of grey matter at those places where the great nerves of the extremities are detached than at any others. Some anatomists still deny this; but it may be more easily seen in the ox or horse than in man. Even in man, however, if the spinal cord is stripped of its dura mater and
arachnoid covering, and held profile-wise against the light, the undulated line of its edges will be abundantly obvious. For some other particulars we are obliged to refer to the book itself.
The medulla oblongata is generally but erroneously confounded with the rest of the spinal cord. It bears no proportion to the latter in size. Its volume varies greatly in different classes of animals, and is determined by the nerves that arise from it, and by the bundles that proceed to the brain and cerebellum. In many of the mammalia it is larger than in man, from the great size of the nerves issuing from it.
Properly speaking, no part of the nervous system can be said to be derived from another. Each is in truth independent, but connected. We shall not enter into any details about the origin of the nerves, farther than to notice, that the optic nerves arise from the anterior pair of quadrigeminal bodies, and not from the optic thalami, as is still commonly thought in this country, although not on the continent. The anatomical, physiological, and pathological evidence of this fact adduced by Dr Spurzheim is quite incontrovertible and demonstrative.
We come now to the fourth section, viz. the best mode of dissecting the brain. Cuvier tells us, that “the most “ accredited method of the schools, and that usually recommended “ in books of anatomy, is to take away successive slices of the organ “ (the brain,) and to remark the appearances offered by each. This “ is the easiest in practice for the demonstration, but it is the most “ difficult for the imagination. The true relations of parts, which
are always seen cut across, escape not the pupil alone, but the “ master himself." This is much the same as examining the anatomy of the thigh by cutting off successive transverse slices, and noticing the appearances of each slice. Accordingly, many authors before Gall and Spurzheim exposed its faultiness, and tried to pursue a better method; but to these distinguished men is unquestionably due the credit of proposing the only complete method yet known, and now so successfully practised by the best continental anatomists. The opponents of their physiological doctrines no doubt contest their claim to originality in this as in many other things, and ingeniously refer to the works of Vieussens, Varolius, Willis, Santorini, &c. for proofs of their assertion ; but they take special care not to inform their readers that these very authors are his torically quoted by Drs Gall and Spurzheim themselves, and that to all appearance it is in reality to the pages of these gentlemen that their opponents are indebted for a knowledge of the writings referred to, seeing that their now valuable contents were utterly unnoticed by the best anatomical writers and teachers till after the publication of Gall and Spurzheim's works. We hope soon to take a historical view of the matter, and therefore for the present willingly pass on to the consideration of the subject more immediately before.us.
Viewing the brain not as a single organ, but as an assemblage of particular apparatu ses destined to special and determinate functions, Dr Spurzheim begins the dissection at the place where the proper cerebral masses are added to the nervous parts already described, (at the medulla oblongata), and traces them in their continuations and in their connexions with each other, and with the nerves of the five senses and of voluntary motions, in a manner precisely analogous to that which is followed in the anatomical demonstration of the other parts of the body. On account of the great delicacy of structure of some parts, Dr Spurzheim prefers scraping to cutting, as better adapted for the purpose. ị The brain should be removed from the cranium, care being taken not to tear the crura at the superior edge of the annular protuberance (an accident which is very apt to occur), nor to injure the medulla oblongata at the lower edge of the 'same' part, and to cut the spinal mass so low down as to obtain, besides the entire medulla oblongata, the upper part of the true spinal cord. The brain, thus freed from the skull, is then to be put into a plate with the basis: uppermost. In this position all the appearances presented by the base of the brain are visible. The first part we proceed to examine, after the nerves already noticed, is the cerebellum.
: Before examining the cerebellum, Dr Spurzheim lays before his readers numerous proofs of the brain being an aggregate of many distinct parts or organs, and shows very beautifully that it is not by anatomy or physiology separately, that we can ever arrive at the determination of the number and functions of these parts either in man or animals, but solely by combining physiological with anatomical observation. He is, we believe, the first who has insisted on this very important point, and in all his works he has clearly kept it in view.
The cerebellum exists in all vertebral animals; but its form and size vary exceedingly. The anatomical principle laid down in regard to the regularity of proportion between the pulpy or cineritious and fibrous or white substances, and to the occurrence of pulpy matter at the origins of nervous masses, is confirmed by the structure of the cerebellum. At the place of its attachment to the medulla oblongata, there is always an accumulation of pulpy substance proportioned in quantity to the size of the cerebellum. In man it composes an irregularly-shaped mass serrated at the edges; hence its name of corpus dentatum. Dr Spurzheim calls it the ganglion of the cerebellum. This ganglion is very distinct in man, in many of the mammalia, and in birds. In some of these animals its small size and paler colour have caused it to be overlooked; but we have already said pulpiness and not colour is its true characteristic..
In forming the cerebellum, nature has in all animals pursued the same plan. Two bundles constantly bring it into connexion with the two sides of the medulla oblongata: these are of variable size; they meet a greater or smaller quantity of grey substance, and proceed strengthened in proportion; they then regularly compose a primary portion, which, in the lowest tribes, is smooth superficially, but which, as we said, appears furrowed transversely, or divided into lamellæ, and becomes complicated by the addition of lateral masses, laminated in like manner. From the ganglion, or corpus dentatum, numerous bundles proceed; one, with its
fellow of the opposite side, forms the vermiform process, or primary portion of the cerebellum ; others proceed backwards, upwards, downwards, and outwards, and expand into layers disposed horizontally.
In the mammalia the cerebellum is augmented by the addition of a mass known by the names of Pons Varolii, tuber annulare, &c., the transverse fibres of which evidently belong and are proportioned in size to the lateral portions of the cerebellum. The merit of discovering this relation is exclusively due to Drs Gall and Spurzheim.
To demonstrate the connexion of the cerebellum with a bundle of the corpus restiforme, the medulla oblongata must be pushed to one side, and the auditory nerve and a thin layer interposed between the medulla and the cerebellum scraped off with the handle of the knife. The second bundle of the restiform body, reckoning from the posterior pyramid, will then be seen to plunge into the cerebellum. By entering the point of the knife at the insertion of this bundle, and cutting the cerebellum vertically, so that about two-thirds of its substance may be left externally, and the other third remain internally, the communication of the cerebellum with the medulla oblongata ; its ganglion, or corpus dentatum, from the entrance of the connecting bundle of the restiforme body to about its middle; the ramifications of the white substance, and the peripheral extremities of the various branches universally covered with cineritious matter constituting the appearance denominated arbor vitæ will be exposed.
To see that the uniting fibres of the cerebellum composing the annular protuberance or commissure are distinct from those of the bundle that connects it with the medulla oblongata, the last-named part must be turned aside, and the vocal, glossopharyngeal, facial, and auditory nerves removed with the handle of the scalpel ; the fibres of union will now be seen gathering themselves from the peripheral parts, and lying over the bundle that springs from the medulla oblongata, and plunges into the cerebellar ganglion.