This vesicular surface with the fibrous matter which connects it with the optic thalami and corpora striata forms by far the largest portion of the encephalon in the higher classes of animals. This fact alone ought to stamp it with great physiological importance. But, further, it is a well-proved fact, that the com plexity 0. the convolutions in the animal scale is in the direct ratio with the advance of in telligence. At the same time it must be re membered that the complexity of the convo lutions is in part determined by the size of the head and the capacity of the cranium. If, for example, the habits and mode of life of the animal require a small head and at the same time a certain degree of intelligence, the brain would exhibit a greater number and complica tion of convolutions than would be found in an animal of corresponding intelligence, but which required and possessed a larger head. Hence neither the size nor the weight of the brain, whether absolute or in relation to the body, affords any certain criterion of the extent of the convoluted surface. Highly complicated convolutions may exist along with a brain both absolutely and relatively small. Thus, the ferret, as shown by Leuret, whose habits require a small head, has several well-marked convo lutions on each hemisphere, and a brain no larger than that of the squirrel, which has no convolutions at all, and which wants even the few fissures which mark their first develope ment in the rabbit, the beaver, the agouti, Sze. And the last-named animals have the brain both absolutely and relatively larger than that of the cat, the pole-cat, the roussette, the unau, the sloth, and the pangolin, all of which possess convolutions.
At the early periods of human life, in in fancy and childhood, the convolutions of the brain are very imperfectly developed, but their increase of size goes on simultaneously with the advance of mental power. If the former be arrested, or if some congenital fault pre vent the further growth of the convolutions, the mental powers are of the lowest and fee blest kind, but little Of not at all above those of the brute with imperfect convolutions. In all idiots the brain is not only small, but its convoluted surface is extremely limited.
Anatomy points to the conclusion that the office of the convolutions is connected with the functions of the mind. Perception, memory, the power of abstraction, judgment, imagina tion, all possess, as instruments of corporeal action, these folds of vesicular matter. And it seems not improbable that the phrenological view which assigns to certain convolutions a special office connected with some particular faculty or faculties is true. This is strongly supported by the fact of a regular disposition of certain primary convolutions in the various classes of animals, so that each form of brain has its proper convolutions, and that in tracing the convolutions from the most simple to the most complex, indications are found of the persistence of the primary and fundamental convolutions in the midst of many secondary and superadded ones.
It may be here rnentioned that Gall *as by no means the first to assign this function to the convolutions. Our countryman, Willis, in the seventeenth century, distinctly advanced this opinion, and conjectured that the various gy mtions were intended for retaining the animal spirits " for the various acts of imagination and memory" within certain limits.
It is important to ascertain the endowments of the fibres which connect the vesicular sur face of the convolutions to the corpora striata and optic thalami. They might be supposed to possess similar endowments to those of sensi tive and motor nerves, if we adopted the views of those who hold that all the nerves are con tinued up into the brain. This point, however, has been settled in the most decisive manner by experiments, dating as far back as the time of Lorry.* Mechanical injury to them excites neither pain nor disturbance of inotion. Even the electric current passed through them pro duces no sensible effect (Matteucci). We are led, therefore, to the conclusion that these fibres have endowments quite distinct from those of sensitive arid motor nerves, (a fact, by the way, quite irreconcileable with the doctrine which makes the brain the concourse of these fibres,) and that they are internuncial between parts which are beyond the immediate influence of the ordinary physical agents, and which have no direct connection with muscular organs. The proper stimulant of these fibres is the mind on the one hand, or the nutrient changes in the brain on the other. But, under the influence of a continued morbid irritation, they nnay excite either pain or convulsion, or both, as is frequently the case in disease of the cerebral meninges ; this, however, is effected through a change produced in the corpom striata and optic thalami, and propagated thence to the origins of motor and sensitive nerves, or through irritation of the nerves of the meninges, which affect the centres of motion and sensa tion, just as the nerves of other parts do.
The experiments of Flourens and of Hertwig show that removal of the cerebral hemispheres produces a state of stupor, and, to use Flourens' expression, as it were condemns the animal to perpetual sleep, but deprives it even of the fa culty of dreaming. There is, however, no para lytic state produced by these mutilations. It is evident, then, that the effect of these experi ments is psychical, and it may be adduced as confirmatory of the view which associates the functions of the cerebral convolutions with the operations of the mind.
Pathological anatomy affords interesting con firmation to this view. Inflammation of the membranes of the brain, more especially of the pia mater, is invariably attended by dis turbance of the mental faculties, as manifested by more or less delirium. It appears that any material alteration of the circulation in the grey matter of the convolutions is capa ble of giving rise to delirium ; in the in stance above quoted, the circulation in this part is affected in consequence of the inflam n-iation of the pia mater, the bloodvessels of the one being distinctly continuous with those of the other ; but in other instances of violent delirium, such, for example, a.s delirium tremens, the vesicular matter of the convolutions is found after death to be bloodless, as if its wonted supply of blood had been cut off or abstracted from it. We find this state in the delirium after great operations, after puerperal floodings, in the delirium of rheumatic fever, and in that of gout, and likewise in that which occurs in the more advanced stages of fever.