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far as the frontal extremity of the hemisphere; and that they have their special termination in that band of convolutions which extends along its superior border, and of which the high development is peculiarly characteristic of the human brain. Now in the Monkeys, the optic nerve is very large; but the portion of it which proceeds to the cerebrum is relatively small, a much more considerable part of its fibres proceeding to the corpora quadrigemina. In the inferior Mammalia, it is very difficult to discover the cerebral expansion of the optic nerve at all, owing to the deficiency of the posterior portion of the hemispheres, to which it may most readily be traced; still M. Gratiolet states that he is satisfied himself of its existence in the Carnivora, whilst in the Ilerbivora, and especially in the Rodentia, its presence is more doubtful. In the Marsupials, however, the roots of the optic nerve pass directly to their large tubercula quadrigemina and thalami optici, the cerebral expansion being altogether deficient. Here again, therefore, we have a case not merely of suppression but of substitution; for as the cerebral roots of the optic nerve diminish, the roots proceeding to the sensory ganglia augment not merely in proportionate but in absolute amount—a fact which (if M. Gratiolet's statement of it should prove correct) is of remarkable physiological import, as we shall presently show. And there is another circumstance to wliich we may refer in the same connexion; viz., that as we descend the Vertebrate series, the portion of the roots proceeding to the thalami optici diminishes, whilst that which passes to the tubercula bigemina (or proper optic ganglia) increases, and this not merely relatively but positively.
Now let us apply the same method of inquiry to the Cerebral hemisphere taken as a whole; and consider what evidence we have in regard to the homology of its parts in the higher and in the lower Mammalia. In the first place, has the ordinary division of the hemisphere into lobes, anterior, middle, and posterior, any real existence? The anterior lobe is, it is true, bounded inferiorly by the fissure of Sylvius, whilst the extension of the anterior cornu of the lateral ventricle into its substance seems a definite mark of its distinctness. But along the inner and outer faces of the hemisphere, and on its superior aspect, there is not the least mark of separation; on the contrary, an absolute continuity in the convolutions between the anterior and the middle lobes exists in every part but that into which the fissure of Sylvius extends; and when the anatomy of the Cerebrum is studied from the developmental point of view, it becomes obvious that the fissure of Sylvius, so far from constituting a division between the anterior and middle lobes, marks the real termination of the anterior lobe which is as it were folded back upon itself. A very simple illustration will render this apparent. Let the reader merely double his fingers down towards the palm of his hand, as if not quite clenching his fist; the palm will then represent the middle lobe, the fingers the anterior lobe, the line where the ends of the digits approximate the palm will be the fissure of Sylvius, and the space between the doubled phalanges will be the anterior cornu of the ventricle. It is obvious, then, that as the real continuity of the fingers and the palm is at the bases of the former, there is no true division between them where the bending of the fingers brings their points into proximity with the palm; aud further, that the proper anterior termination of the member is not at the forward-projecting knuckles, but at the doubled-back tips.
Between the middle and the posterior lobes, there is no external semblance of a division; but the presence of the posterior cornu of the ventricle and that of the hippocampus minor are regarded as its characteristics. It may fairly be questioned, however, whether the presence of these parts can be regarded as establishing the fact that the posterior is altogether a superadded organ in Man; since the posterior cornu may be considered, like the anterior, merely as an infolded space; and the hippocampus minor is not a special organ, but merely a part of the ganglionic mass which is peculiarly shaped-out by the inward projection of one of the fissures dividing the convolutions at the inner side of the posterior lobe, where it is covered by the posterior fibres of the longitudinal commissure (Solly).
It seems to us, then, that each Cerebral hemisphere must be looked-on as a single organ, unless a truly natural division can be shown to exist in it, either by the evidence of its developmental history, or by the comparison of its convolutions in different groups of ammals. Now if we compare the Cerebrum of a Rabbit (for example) with that of Man, either of three modes of development might be theoretically conceived possible. 1. The organ may enlarge as a whole, every part of it extending itself in equal or nearly equal proportion. In this case it is obvious that, notwithstanding the difference in relative size and configuration, the cerebrum of the Rabbit would be homologous as a whole with that of Man. 2. The organ may extend itself chiefly by outgrowth at its two extremities, just as a tree grows at the ends of the ramifications of its branches and of its roots; the anterior lobes budding forth first, and then the posterior, so as really to constitute new and distinct parts. In this case, the cerebrum of the Rabbit would in its enlarged form correspond only with the middle lobe of that of Man. 3. The principal growth may really take place in the central portion, new substance being interposed there, so as to separate the anterior and posterior halves, carrying the one forwards, and the other backwards, just as the epiphyses of a long bone are separated by the elongation of the shaft. In this case, the cerebrum of the Rabbit would really represent the anterior and the posterior lobes of that of Man; and the part peculiar to him would be the middle lobe. Between these possible modes, a more careful study of development than has yet been made may enable us to ascertain the true one. The study, however, is rendered extremely difficult by the softness of the embryonic cerebrum, which makes it difficult if not impossible to track the course of its fibres; and by the absence of any such definite indications, either on the surface or in the substance of the organ, as may afford a guide to the distinctness of its parts. The descriptions that have been given of the development of the posterior lobe, which is the last part to be formed, certainly favour the conclusion that this buds forth from the posterior extremity of the middle lobe; but we do not consider them by any means incompatible with the idea of such a shiftingback of a part of the substance of the middle lobe as our third hypothesis involves. And all that is known of the development of the anterior lobe is quite in harmony with the idea that this is an extended part of the original cerebrum, and not an addition to it.
Now if we inquire into the evidence afforded by the convolutions, we shall find that it rather favours the third hypothesis than others of the preceding. It may be true that the convolution termed by Foville VourUt or the hem, which is in immediate proximity to the corpus callosum, represents the same part of the cerebral hemisphere in all Mammals in which it can be distinguished. But it seems equally true that the anterior convolutions in the cerebrum of a Ruminant correspond with the anterior convolutions in the cerebrum of Man, and that the posterior convolutions which are continuous with them in the former are separated from them in the latter by a set of interposed convolutions of whose existence we have clear evidence in the elephant. "Suppose," says Leuret, "all the superior convolutions, and the prolongation of the great internal convolution which is united with them, to be obliterated, then the fourth anterior convolution might be united to the fourth posterior, the third to the third, and we should have one of the groups of convolutions of the brain of ruminants and solipedes." Now it is this series of interposed convolutions that especially distinguishes the brain of Quadrumana from the brains of other Gyreneephala excepting the Elephant; and its development is still more remarkable in Man. Hence we cannot help surmising that the principal extension of the Cerebrum in the anteroposterior direction is due rather to the introduction of a sort of wedge-shaped addition into the middle of the hemisphere, which not only separates its two extremities but causes them to bend downwards, than to the outgrowth of a new lobe at either end. At present we can only throw out these ideas as little better than speculations; our object is chiefly to show that they are at any rate as worthy of consideration, and have as much claim to acceptance as the views at present commonly entertained; the time may perhaps not be far distant, when more certain data for the determination of this question may fall within our reach.
The advance in the development of the Cerebrum, however, which is so obvious in the ascending series of Mammalia, does not consist only in augmentation of size, and in extension of surface by the convoluted arrangement of the vesicular stratum; there is also a vast increase in the complexity of the arrangement of the fibrous tracts connecting the different parts of each hemisphere with each other; and connecting either hemisphere with its fellow as well as with other parts of the encephalon. Much attention has been bestowed by M. Gratiolet on the disposition of these tracts; and he gives an elaborate description of their course. Proceeding from the internal towards the external surface of each hemisphere, he enumerates the following as its constituent parts in the posterior part of the Human cerebrum: —1. The cortical layers of the internal surface. 2. The system of commissures proper to the internal surface, all of which attach themselves to the fibrous riband or Vourlet. 3. The expansions of the corpus callosum which pass to the convolutions of the internal surface. 4. The internal wall of the occipital cornu of the lateral ventricle. 5. The external wall of this cavity. 6. Outside this wall, the expansion of the corpus callosum which Reil has designated the tapis. 7. Externally to the tapis, the stratum of the cerebral expansion of the optic nerve. 8. Beyond this, the stratum of the fasciculus which springs from the internal geniculate body. 9. Then the stratum of fibres directly proceeding from the peduncular fan. 10. The stratum of fibres expanding from the anterior commissure. 11. The lamina which, from the corpus callosum, descends into the inferior convolutions at the fissure of Sylvius. 12. The system of commissures proper to the external surface of the hemisphere. 13. The cortical layei's of the external surface.—In the fronto-parietal region, the interlacements of these different strata are so complicated that their course cannot be traced with the same certainty as in the part of the hemisphere that lies behind the posterior duplicature of the corpus callosum.
As we have already adverted to M. Gratiolet's account of the double connexion of the optic nerve, we shall here only stop to mention the results of his inquiries into the connexions of the corpus callosum; as to which there has been, as many of our readers are aware, an important difference of opinion amongst some of the most able neurologists,—Arnold and Reil, followed by most British authorities, affirming that its fibres pass directly from the convolutions of one hemisphere to those of the other, whilst Foville has adopted the other idea of Willis that it is a commissure between the peduncles and not between the convolutions of the hemispheres, which has also been supported by Tiedemann and Rolando. According to M. Gratiolet it is not difficult to produce, by a particular mode of preparation, each of the appearances on which the above conclusions are based; so that each may be accepted as having a measure of truth, but not as being the whole truth. How, then, are these two opposing views to be reconciled? According to one possible hypothesis, the corpus callosum contains two distinct systems of fibres, one passing between the convolutions of the hemispheres, the others uniting by transverse arches the coronae of the peduncles. According to another, the corpus callosum is formed of fibres proceeding from the corona of one peduncle to the convolutions of the opposite hemisphere. It is very positively affirmed by M. Gratiolet that the second of these hypotheses must be the correct one; for that in no instance has he been able to trace any fasciculus continuously from the convolutions of one hemisphere to those of the other; whilst he has found every one which he could trace so far, arching downwards into the peduncle of the opposite hemisphere. This solution appears by no means improbable in itself; and it has this great claim upon our admission of its high probability, that it reconciles the apparently contradictory statements of most experienced and trustworthy anatomists, without invalidating the real basis on either side.
In his chapter on the Development of the Encephalon, M. Gratiolet warns us against being led astray by the analogies which unquestionably present themselves between its transitional forms in Man and its permanent conditions in the lower Mammals; and remarks that Tiedemann and those who have followed him have done wrong in attaching themselves too exclusively to these analogies, to the neglect of the differences. And he lays great stress on the fact that there is no necessary parallelism between the order of the appearance and that of the development of the different parts, or (as we presume that he means to say) between the morphological and the histological transformation of the organ. Thus notwithstanding a certain general resemblance in plan between the cerebrum of Man and that of the Quadrumana, there are very marked differences in their respective courses of development. For in the ascending series of Monkeys, the completion of the general form is found to precede the first appearance of convolutions; whilst in Man the convolutions appear and multiply a long time before the general form is complete. The order in which the several convolutions appear is almost opposite in the two cases; the alphabet being recited (so to speak) from alpha to omega in the one case, and from omega to alpha in the other. Thus it happens that at no epoch in its development can the human brain be said to resemble that of any monkey; and that if its development be arrested in any inferior stage, the organ still presents characters which are peculiarly anthropic. It is somewhat remarkable that the two hemispheres do not seem to develope themselves in an absolutely symmetrical manner: the frontal convolutions most frequently appearing earlier on the left side, whilst those of the occipito-sphenoidal lobe are first seen on the right. We do not attach much importance to this observation, except from its concurrence with the well-known fact that there is a less perfect symmetry between the convolutions in the completely-developed hemiKpheres of Man, than there is in the Mammalia generally.—Without wishing to undervalue the information it affords, we feel constrained to say that we are far from being satisfied with this portion of M. Gratiolet's work: and have no hesitation in affirming that the true developmental history of the Human Encephalon has yet to be worked out. In its earlier stages there will doubtless be found a close conformity to the general Mammalian type ; and up to a somewhat advanced condition its development will probably be found to run tolerably parallel with that of the higher Gyrencephala; but those later stages which mark its distinctive characters, require to be worked out in far more minute detail than that of which we find evidence in M. Gratiolet's book; though we meet with sundry hints, that lead us to surmise that he has accumulated a mass of material relating to this subject, which for some reason or other he has not thought it suitable to bring forwards.
We have carefully examined the Second Part, which treats of the Experimental History of the Nervous Centres, and of the Intelligence, for some addition to our knowledge, in the way of facts, inferences, or suggestions; but we are bound to say that we have found it singularly barren of novelty, and so far behind the state of opinion in this country, that we should have to go back a quarter of a century at least to bring ourselves down to its level. It will be sufficient to indicate, by way of example, that the psychology of vision is discussed without any reference to Professor Wheatstone's researches, and that under the head of the effects of imagination the phenomena of Mesmerism are discussed without the least reference to the experiments of Mr. Braid.
We now turn to Mr. Dunn's Essay, which contrasts most favourably with M. Gratiolet's more pretentious treatise in the intelligent appreciation it displays of the fundamental questions of physiological psychology, and in the value of the suggestions it throws out with a view to the elucidation of them. Avowedly taking his stand on the views which we recapitulated at the commencement of this article, the author looks at the Cerebrum as an organ superimposed upon the centres of the sensorial consciousness, to serve as the instrument of the faculties both of the Perceptive and of the Intellectual consciousness; and he considers its type through the whole Vertebrate series to be essentially the same, its evolution taking place from before backwards, in a series of ascending planes of development. The longitudinal convolutions, from their first marking-out, increase in number, volume, and extension backwards as the animal rises in the scale of being, and as the range of its perceptive intuitions or faculties widens. The primitive basement convolutions of the hemispheres,—the great internal, or Pourlet of Foville,—form in Mr. Dunn's opinion the broad boundary lines of demarcation between the sensory and the perceptive ganglia, between the sensational and the ideational apparatus; being, in fact, as the central organs of the perceptive consciousness generally, the common portals to intellectual action and volitional power. Now with these basement convolutions, all the other longitudinal convolutions of the hemispheres are directly connected and associated; and Mr. Dunn is strongly impressed with the conviction that the entire series forms an aggregate or whole, constituting the nervous apparatus of our perceptive consciousness,—the instrument of all our immediate cognition, not merely of external existences, their sensible qualities and physical attributes, but also of those purely ideational activities which constitute so large a part of our mental operations, and which, together with the emotional conditions that are closely associated with them, make up the essential part of the character of each individual.
In its lowest grade of development, the Cerebrum is regarded by Mr. Dunn as consisting essentially of the basement or internal convolution, and of its anterior and basilar connexions; and looking to the faculties of the animals which are characterized by this grade of cerebral development, he thinks that we should justly consider the function of the anterior portion of their hemispheres to consist in the acquirement of a perceptive knowledge of external things, whilst the posterior or basilar minister to those elementary feelings and appetites which may be supposed to be common to all Vertebrata, such, for example, as the love of life, the alimentative propensity, the sexual appetite, and the tender regard for offspring. Mr. Dunn thinks that it can scarcely be denied that the intuitions (whether original or acquired) which are called forth by the activity of the special senses, and with which are allied the feelings, appetites, and instincts that have reference to the demands of our corporeal nature, constitute the chief and prominent part of the mental life of the inferior Vertebrata, while at the same time they constitute the inferior region of the more elevated mind of Man, and enter largely into the complicated web of his conscious existence.
Upon this foundation there is erected in the superior Mammals a higher plane of cerebral development, in which the perceptive apparatus presents itself in a more extended form. For the longitudinal convolutions are carried upwards above the lower perceptive organs, and are prolonged backwards even beyond the median lobe; and with this mark of elevation Mr. Dunn thinks that we may associate that higher exercise of the perceptive faculties, in which there is a recognition not merely of the sensible qualities and physical attributes of external things, but of their differences and relations, of their order, arrangement and number, of the phenomena of their action (or events), and in which the higher individual or personal affections, such as love of self and love of others, are called into play.
In the highest plane of cerebral development, which is exclusively human, the towering longitudmal convolutions receive the completion of their development both anteriorly and posteriorly; and with this are associated in Mr. Dunn's idea that more elaborate and complex action of the perceptive consciousness, and that operation of the moral and religious feelings, which are the distinctive prerogatives of Man.
So far we have been concerned with those forms of cerebral activity alone which minister to the various kinds of perceptive activity that directly result from the contact of the perceptive faculties of the Mind with the external world or Nature. But to perceive and to think are distinct mental processes, though indissolubly connected, since ideation is the pabulum of thought. All our representative knowledge is the product or creation of the mind's own power of intellection ; and of this power Mr. Dunn holds that the transverse convolutions of the Cerebrum serve as the instrument, these being anatomically a distinct series, and having no direct connexion with the basement or great internal convolutions. The appeal