issue the mandates by which the Will calls the muscles into action. It must be borne in mind, however, that size is not by any means the only indication of their comparative development. As we advance from the lower to the higher Vertebrata, we observe a marked advance in the complexity of the structure of the Cerebrum. Its surface becomes marked by convolutions, that greatly increase the area over which blood-vessels can enter it from the surrounding membranes; and in proportion to the increase in the number and depth of these, do we find an increase in the thickness of the layer of gray matter, which is the source of all the powers of the organ. The arrangement of the white or fibrous tissue, which forms the interior of the mass, also increases in complexity; and as we ascend even from the lower Mammalia up to Man, we trace a marked increase in the number of the fibres, which establish communication between different parts of the organ. It is, in fact, not merely from the different parts of the gray matter which forms the surface of the hemispheres, that these commissural fibres arise; but also from those isolated portions of vesicular substance, which are found in different parts of their interior; and an extremely complex system is thus formed, which is still but very imperfectly understood.

914. The most important group of commissural fibres, is that which connects the Sensory with the Hemispheric Ganglia; that is, which radiates from the Thalami Optici and Corpora Striata, to the stratum of gray matter which forms the convoluted surface of the Cerebrum. These fibres constitute, in fact, the principal part of the white substance of the brain; the remainder being made up by the commissures to be presently described, and by commissural fibres which (it is probable) connect the different parts of the Cerebral surface with each other. It was formerly supposed (and is still maintained by many Anatomists), that the radiating fibres which may be traced to the Corpora Striata and Thalami Optici, pass through these bodies, and are continuous with the Crura Cerebri, and consequently with the sensory and motor tracts of the Medulla Oblongata. But when the small size of the Crura Cerebri is compared with the relatively enormous bulk of the radiating fibres, it is obvious that the former can only contain but a very small proportion of the latter; and as no absolute continuity has been traced, it appears more conformable to Anatomical and Physiological probability, to believe that the fibres of the Crura Cerebri pass no further upwards than the Sensory Ganglia, and that the radiating fibres take a fresh departure from these bodies, to pass towards the surface of the Cerebrum.-Thus, then, we should be led to regard the Spinal Cord, Medulla Oblongata, and chain of Sensory Ganglia, as precisely representing the entire Nervous System of Insects, the character of whose action is essentially automatic; and to consider the Cerebrum as an organ superadded at its summit, receiving all its incitement to action from impressions transmitted to it through the Sensory Ganglia, and carrying into effect its volitional determinations and emotional impulses, not (as formerly supposed) by immediately exciting muscular movements through nervous communications passing direct from the convoluted surface of the Cerebrum, but by playing downwards upon the Automatic apparatus by which its mandates are

carried into effect (Figs. 155, 156). Of this view we shall presently find that there is strong physiological evidence.

Diagram of the mutual relations of the principal Encephalic centres, as shown in a vertical section:-A, Cerebrum; B, Cerebellum; c, Sensori-motor tract, including the Olfactive ganglion olf, the Optic opt, and the Auditory aud, with the Thalami Optici thal, and the Corpora Striata cs; D, Medulla Oblongata; E, Spinal Cord:-a, olfactive nerve; b, optic; c, auditory; d, pneumogastric; e, hypoglossal; f, spinal: fibres of the medullary substance of the cerebrum are shown, connecting its ganglionic surface with the sensori-motor

tract.

915. The two Hemispheres are united on the median line by several transverse commissures; of which the Corpus Callosum is the most important. This consists of a mass of fibres very closely interlaced together; which may be traced into the substance of the hemispheres on each side, particularly at their lower part, where they are connected with the thalami optici and corpora striata. It is difficult, if not impossible, to trace its fibres any further; but there can be little doubt that they radiate, with the fibres proceeding from the bodies just named, to the different parts of the surface of the hemispheres. This commissure is altogether absent in Fish, Reptiles, and Birds; and it is partially or completely wanting in the Mammalia with least perfect brain, as the Rodents and Marsupials.-The other transverse commissures rather belong to the Sensory Ganglia than to the Cerebral hemispheres. Thus the anterior commissure particularly unites the Corpora Striata of the two sides; but many of its fibres pass through those organs, and radiate towards the convolutions of the hemispheres, especially those of the middle lobe. This commissure is particularly large in those Marsupials, in which the corpus callosum is deficient. The posterior commissure is a band of fibres which connects the Optic Thalami; crossing over from the posterior extremity of one to that of the other.-Besides these, there are other groups of fibres, which seem to have similar commissural functions, but which are intermingled with vesicular substance.

Such are the soft commissure, which also extends between the Thalami; the Pons Tarini, which extends between the two crura or peduncles of the Cerebrum; and the Tuber Cinereum, which seems to unite the optic tracts with the thalami, the corpus callosum, the fornix, &c., and to be a common point of meeting for several distinct groups of fibres.

916. The anterior and posterior parts of the hemispheres, moreover, are connected by longitudinal Commissures, of which some lie above, and some below, the corpus callosum; and of these, also, a part belong to the Sensory Ganglia. Above the transverse fibres of the corpus callosum, there is a longitudinal tract on each side of the median line, which serves to connect the convolutions of the anterior and posterior lobes of the brain. And above this, again, is the superior longitudinal commissure, which is formed by the fibrous matter of the great convolution nearest the median plane on the upper surface of the brain, and which connects the convolutions of the anterior and middle lobe with those of the posterior.-Beneath the great transverse commissure, we find the most extensive of all the longitudinal commissures, namely, the Fornix. This is connected in front with the optic thalami, the mammillary bodies, the tuber cinereum, &c.; and behind, it spreads its fibres over the hippocampi (major and minor), which are nothing else than peculiar convolutions that project into the posterior and descending cornua of the lateral ventricles. The fourth longitudinal commissure is the tænia semicircularis, which forms part of the same system of fibres with the fornix; connecting the corpus mammilare and thalamus opticus with the middle lobe of the cerebral hemisphere. If, as Dr. Todd has remarked, we could take away the corpus callosum, the gray matter of the internal convolution, and the ventricular prominence of the optic thalami, then all these commissures would fall together, and become united as one and the same series of longitudinal fibres.-It is curious that there should be no direct communication between the Cerebral hemispheres and the Cerebellum; the only commissural band between them being the processus a cerebello ad testes, which passes onwards, through the Tubercula Quadrigemina, to the Thalamus Opticus on each side. This would seem to confirm the idea of the complete distinctness of their functions.

917. The Cerebrum appears to be the instrument of all those psychical operations, which are superadded, in Man and the higher Vertebrata, to mere sensations. The impressions which are merely felt in the sensorium, give rise, when they pass upwards into the Cerebrum, to Ideas, which then become the material (so to speak) of all the higher mental processes. These processes may be ranked under two distinct heads, namely, the Emotional and the Intelligential; the former being most intimately connected with the sensations which prompt them, whilst the latter are commonly of a much more abstract character. The Emotions may, in fact, be considered as feelings of pleasure or pain associated with particular classes of ideas; and it is this association which gives them the character of the moving or active powers of the mind, and which makes them, either directly or indirectly, the springs of the greater part of our actions. When strongly excited, the Emotions may produce movements which the Will may not be able to re

strain; as when we burst into laughter at some ludicrous image presented to the mind, either by a present sensation or by an act of the Memory or Imagination, notwithstanding the strongest inducements presented by "time, place, and circumstance" to a preservation of our gravity. The distinctness of the character of Emotional and Volitional movements is further evident from this, that cases of paralysis not unfrequently occur (especially in the facial nerves, through which most of the muscles of "expression" are excited to action), in which the muscles are obedient to one class of impulses, while the other exerts no power over them. Thus, in one instance, the muscles of one side of the face were palsied in such a manner, that the patient could not voluntarily close his eye, nor draw his mouth towards that side; yet when any ludicrous circumstance caused him to laugh, their usual play was manifested in the expression of his countenance. And in another case, the muscles were obedient to the will; but when the individual laughed or cried under the influence of an emotion, it was only on one side of his face. To these may be added another case, in which the right arm was completely palsied, so that the individual had not the least voluntary power over it; yet it was violently agitated, whenever he met a friend whom he desired to greet.-The influence of an undue tendency to Emotional excitement, is remarkably seen in what are ordinarily termed Hysterical states of the system; in which violent convulsive paroxysms are frequently brought on by the most trivial causes, if these should call the passions or affections of the mind into undue activity. There can be no doubt that many of the peculiar actions performed by the subject of what is termed Mesmeric influence, are the result of a condition of this nature. There appears to be, in such persons, a proneness to activity of the consensual and emotional parts of the nervous centres, which manifests itself most strongly when the control of the will is withdrawn; and thus very slight impressions produce very powerful involuntary movements, especially when this response is favored by the strong desire, on the part of the patient, to exhibit any particular manifestation that is known to be expected by the bystanders.

918. It has been supposed by some, that the Emotional movements of Man and the higher animals may be ranked in the same category with Instinctive actions of the lower; and that the Desires of the former are comparable to the instinctive Propensities of the latter. But this comparison is erroneous; for what we term propensities (among the lower animals) are nothing else than tendencies to perform particular movements in respondence to particular sensations, without any idea of the purpose of the movement or of the object which has excited it; whilst an Emotion involves an idea of the object which has called it up, and a Desire involves a conception of the object to be obtained.-The imitative actions afford a good example of the difference between a propensity and a desire. The former is manifested in such imitative movements as are purely consensual; the sensation, which is the mainspring of the action in each case, exciting a respondent automatic movement, as when we yawn involuntarily from seeing or hearing the action performed by another, or as when children learn undesignedly to perform many of the movements which they witness in adults. This propensity to in

voluntary imitation is much stronger in some individuals than in others. On the other hand, imitative actions may be voluntarily performed, as the result of a desire to execute them, which involves a distinct idea of the object; and the moving force of this desire is derived from the pleasure which the individual derives from the performance, and which he finds either in the act itself, or in the enjoyment which it affords to others, or in its prospective benefits (pecuniary or otherwise) to himself. Thus we see that the Mind (properly so called) is concerned in all Emotional actions; whilst there is no evidence of the participation of any higher attribute than sensation in the purely Instinctive acts; and even this is not a requisite link in the chain, by which many of the movements are excited, that are usually grouped together under that designation.

919. Again, the Emotions may be excited by operations of the Mind itself, as well as by sensations immediately received from without. Thus, involuntary laughter may result from a ludicrous idea, called up by some train of association, and having no obvious connexion with the sensation which first set this process in operation; and the various movements of the face and person by which Actors endeavor to express strong emotions, are most effectual in conveying their meaning, when they result from the actual working of the emotions in the mind of the performer, who has by an effort of the will, identified himself (so to speak) with the character he personates. A still more remarkable case is that in which paroxysms of Hysterical convulsion, in themselves beyond the power of the Will to excite or control, are brought on by a voluntary effort; this being exerted, not in the attempts to perform the movements, but in "getting up," so to speak, the state of feeling, from which, when it is once excited, the movements spontaneously flow. In all these instances, and others of like nature, it would seem as if the agency of the Cerebrum produced the same condition in the Automatic. centres, as that which is more directly excited by sensations received through their own afferent nerves.-But on the other hand, the Emotions, by their influence on the Reasoning processes, are largely concerned in many actions which are strictly voluntary; in fact it may be questioned whether there are any of our actions, the power necessary for whose performance is not derived, directly or indirectly, from emotional states of mind; all our motives to any kind of exertion being found, if carefully analysed, to have reference to pleasure to be derived, or pain to be avoided, either in the very performance of the action, or in the consequences which our reasoning processes connect with it. And it will be found that the difference between those persons who are said to act from feeling, and those who are said to be guided by reason, is not precisely what these terms imply; for the actions of both are equally determined by the motives supplied by emotional states; and the difference rather lies in this, that one class act on their first impulses without considering the consequences, whilst the other calculate the remoter results, and weigh the future pain against the present pleasure, the ultimate enjoyment against the immediate distress.-The Emotional states are peculiarly liable to be influenced by the condition of the corporeal system; thus a very slight depravation of the blood may produce an irresistible tendency to take a gloomy view of everything to which the mind may