As the nervous system grows, the axons of its neurones acquire their fatty (myelin) sheaths in the order in which they come into functional activity. The passage through them of impulses is the stimulus which leads to the deposition of fat. The study of the progress of myelination enabled the anatomist Flechsig to ascertain the situation within the brain of the tracts of fibres related to the several senses, and hence the traffic of the areas of the cortex to which they go. Glistening white streaks appear successively in the pulpy yellowish-pink substance of the interior of the brain. At the time of birth all the fibres which enter or leave the cerebral hemispheres have acquired their myelin sheaths. In the baby’s brain the sense-organs have established all their connections with the cortex. No new fibres will appear in the nerves of the eye, the ear, or the other sense-organs, nor will their end-stations in the cortex be further multiplied. (The use of the expression “end-stations” is legitimate so far as sensations are concerned; notwithstanding that all sensory impulses are retransmitted by neurones in the cerebro-spinal axis.) But the cortex is very far from having finished its growth. It contains a large amount of embryonic tissue, which gradually spreads outwards from the developed areas into the surrounding unoccupied zones. The taking up of new territory, and the consequent increase in the size of the brain, is continued into adult life. The study of progressive myelination enabled Flechsig to divide the cortex into “sensory centres,” and intervening “association-zones”; although, doubtless, the difference in function between the portions which receive sensations direct and the portions in which the products of sensation are worked up is one of degree, and not of kind.
Fig. 24.—Vertical Sections of the Cortex of the Cerebrum—
A, of the Visual Sensory; B, of the Visual Association Area.
Between the two sections are shown the principal types of cell, at the levels at which they are severally found: a, small pyramid; b, medium-sized pyramid; c, large pyramid. The size of a pyramid is an indication of the distance to which its axon extends before branching; the longer its traject, the more widespread, it would seem, is its terminal arborization. The axon of c, one of the very large pyramids found in this association area, passes to the front of the cerebrum, where it breaks up in an association area of the tactual sense of the hand, or of sensations concerned with the regulation of gait, or in a centre for movements of the eyeball. d, a tangential cell of the surface; e, a Golgi cell with ramified axon; f, a polymorph cell, with its axon directed towards the surface. In sensory areas, tangential fibres and granules are more numerous; in association areas, small and medium-sized pyramids.
The structure of the cortex is not quite the same in sensory and association areas; but it is everywhere so far from showing the diagrammatic simplicity which characterizes the cortex of the cerebellum as to make it difficult to summarize the modifications which distinguish its various regions. To a considerable extent its elements shade one into the other, differing in size and in orientation rather than in form. Commonly it is described as divisible into five layers: (1) A thin superficial layer, containing cells of various forms and fibres derived from the cells of the deeper strata. Some of the cells are pluripolar, possessing several axons which run parallel with the surface. Their destination is unknown. They do not appear to form baskets like the cells of the molecular layer of the cerebellum. The dendrites of pyramidal cells extend into this layer. (2) The layer of small pyramids; cells with a branching apical process, root-like dendrites from the basal angles of the pyramid, and an axon which sinks into the white matter. (3) Granules. Carmine or other nuclear stains show that small cells are present in very large numbers, especially in the sensory areas; but since they are not, like the granules of the cerebellum, coloured by the chrome-silver method, their form and the disposition of their axons are unknown. (4) Large pyramids exactly similar in form to the small ones. Their apical processes are very thorny. Their axons give off several collaterals. Pyramids are the most conspicuous elements in the cortex. Properly speaking, they do not occur in layers, but are scattered throughout its whole thickness, although their cell-bodies are not seen in either its most superficial or its deepest strata. The largest are those of which the axons either descend into the spinal cord or pass to a very distant region of the cortex. They are found singly or in small clusters in the deeper levels. (5) Polymorphous cells, some of them pyramids lying on their sides, or even directing their axons towards the surface; some fusiform or irregular cells; some Golgi-cells ([p. 340]). The axons of pyramids enter the white matter, and many fibres from the white matter radiate towards the surface between the pyramids; but the way in which afferent, sensory fibres are connected with the collecting processes, dendrites, of the pyramids is not known. We have already referred to thorns, and to the possible nerve-net ([p. 301]). Sheets of tangential fibres also occur in the cortex. A particularly distinct sheet divides the granules in the visual cortex into two strata. In sections of this region the sheet of fibres appears as a white line, distinctly visible without a lens.
The limits of the several areas can be determined by examining the structure of the cortex; but the individual peculiarities of the various regions are not so marked as to indicate that they have different kinds of work to do; if by kinds of work we wish to imply that one part is “sensory,” another “motor,” a third concerned with “intellectual processes.” On the contrary, its relative uniformity shows unmistakably that all parts are engaged in the same work. Nevertheless, certain broad conclusions can be drawn with regard to the form of the neurones more immediately concerned with sensation, with motion—that is to say, with the discharge to the grey matter of the cerebro-spinal axis of the impulses which call its neurones into activity—and with the secondary processes, called collectively “association,” which occur within the cortex. Granules, as everywhere throughout the nervous system, are receivers and distributors of sensory impulses; although a study of the cerebral cortex does not justify the conclusion that they are necessary links in its sensori-motor arcs. Large pyramids are occupied with the nutrition of fibres which have a long traject through the system. Hence they are “motor.” They constitute a marked feature of the area which is susceptible to stimulation. They occur also in the visual area and elsewhere. Small pyramids are associational; that is to say, their axons do not leave the cerebral hemispheres. They distribute impulses from sensory areas to association-zones, and from one part of an association-zone to another. The layer of polymorphous cells is relatively thicker in animals in which the cortex of the brain exercises less control over action than in animals in which the cortex is supreme—in a rabbit thicker than in a monkey; in a monkey thicker than in Man. This layer is therefore said to be concerned with the lower functions of the cortex, whatever this expression may mean. Since the relative abundance of small pyramids is a test of the supremacy of the cortex, we may speak of them vaguely as concerned with its higher functions. But a surer test of the capacity of the cortex for the elaboration of the raw materials of thought which sensory nerves deliver to it is the relative abundance of the tissue which intervenes between its cells. The number of cell-bodies to be counted in a square millimetre of a section of a given thickness is smaller in Man than in a monkey, in a monkey than in a dog, and in a dog than in a rabbit.
A comparison of the brains of various mammals in which particular sense-organs are either deficient or exceptionally well developed affords the clearest proof of the localization of sensory areas. This, if it were possible to make satisfactory measurements, would be by far the best class of evidence as to the part played by the several senses in an animal’s mental life. Unfortunately, measurement appears to be out of the question; but a glance at a rabbit’s brain, placed by the side of a mole’s, shows that vision is localized in the occipital region. All marine mammals are destitute of the sense of smell; the brain of a dog, compared with that of a porpoise or a whale, shows that the sphenoidal region ([cf. Fig. 25]) is associated with this sense. The brain of an otter exhibits very clearly the area into which impulses arising in the nerve-endings of the sensory bristles of the cheek are poured.
“Nihil est in intellectu quod non prius in sensu fuerit.” The organ of the intellect is the cortex of the great brain, a sheet of grey matter which has developed in connection with the various sense-organs. The cerebral hemisphere of an infant is merely an extension of the nerve-tissue associated with its sense-organs. Such it remains in a microcephalous idiot. In the lower animals its capacity of growth after birth is very small. But in a normal child the inflow of impressions through sense-organs, the experience acquired regarding itself and its surroundings, education, whether accidental or directed, causes the extension of nerve-tissue from the sensory areas into the expansible intervening zones.
There is still some uncertainty as to the nature of the sensations received in the excitable area. They may be termed “kinæsthetic” (sensations connected with movement) without more exact definition. Some physiologists consider that tactile sensations, as well as the obscure sensations, originated in the nerve-endings in muscles, around tendons, or on joint-surfaces, are distributed to the areas, which, when stimulated, are shown to represent fingers, hand, arm, and other parts of the body. Others have sought, though with doubtful success, for a tactile area, independent of the kinæsthetic centres. When first discovered, these centres were termed “motor,” and still this term may be retained, on the understanding that it does not imply that the exchanges which occur in the kinæsthetic centres are of a different nature to those which take place elsewhere. The region which they occupy has become the motor area of the cortex because voluntary movement is possible only under the guidance of sensations of movement. A sound or a retinal image may prompt the movement; but the part of the temporal region, or of the occipital region in which the sound-movement exchange or sight-movement exchange occurs must act through the motor area by opening kinæsthetic-movement arcs. Destruction of a part of the kinæsthetic cortex causes in Man and the higher apes permanent paralysis for the movements directed by the spot destroyed. In lower animals the definition of the movement centres is vague, and their removal produces only temporary results. Their mastery over the muscles is less complete than in the higher apes and Man.