The Body at Work: A Treatise on the Principles of Physiology

by Alex Hill

The cerebrum which has eventually become, as the seat of consciousness, and hence the apparatus of mind, the dominant factor in the nervous system, was in the first instance the part of the brain concerned with the distribution to the muscles of impulses generated in olfactory organs. There is scarcely any indication in a fish’s brain of the representation in the cerebral hemispheres of any other sense, even that of vision.

A bird’s brain presents a striking contrast to the brain of a fish. With the exception of the apteryx and other ground-birds of New Zealand, all birds are apparently destitute of the sense of smell. Vision is the sense upon which their activity depends. It has invaded the cerebrum, converting it into an organ in which sensations of sight are worked up into “mind-stuff.” The optic lobe connection is restricted to the production of reflex actions in which vision is immediately followed by movement.

All the senses are represented in the great brains of mammals. The cerebrum, which owes its existence to its connection with the favourably-situated sense-organ of the nose, and grew in importance when vision invaded it, has now taken in the senses of hearing, taste, and touch. Only what may be termed in general visceral sense, and the sense of orientation, are excluded.

Looking back to the starting-point, we see a segmented animal; its segments of equal value; its nervous reactions unisegmental, although linked in functional sequence. If it starts to walk, owing to stimulation of one of its sense-organs, the impulse to walk spreads from segment to segment. Comparing the latest product of evolution with the earliest, we find that nervous tissue has concentrated at the anterior end of the body. The double chain of ganglia, now condensed into the axis of the brain and the spinal cord, still contain all the effector neurones by which muscles are called into action. Sensory nerves still arborize in the axis, providing the mechanism for actuating motor neurones. But the vast majority of intermediate or intercalated neurones have been attracted to the two huge brain-masses—the cerebellum and cerebrum. In the former all sensations (not conscious) connected with tone, position, orientation and equilibrium are worked into appropriate impulses for the regulation of the muscular system. In the latter all sensations which convey information regarding the relation of the environment, including the body, to the ego—the not-me to the me—are transformed into motor discharges which set a-going the movements (and the thoughts) by means of which the purposes of life are fulfilled; for in the cortex of the great brain alone is the passage of nerve-currents accompanied by consciousness. Concentration of nerve-tissue allows of the combination of sensations. It also facilitates the no less important effect of mutual influence, interference. Sensations are suppressed, and therefore the multitude of reactions to which they would give rise are inhibited, in the interests of restricted and sustained movement or thought.

The Cerebellum.—Sharks and other swift-swimming fishes have large, deeply fissured cerebella, for the cerebellum is the part of the brain which has gathered into itself most of the grey matter associated with balancing, attitude, posture. The cerebellum is in birds large and deeply folded. Developed from the ganglia to which the auditory nerve distributes impulses from the semicircular canals, it has established connections with all the other nervous tissues concerned with sensations of position, strain, or pressure, including the eyes, which afford information regarding the position of our limbs relatively to the trunk, and of the whole body relatively to external objects. Morphologically it is a median growth. The adverb is one of those qualifying terms, convenient in science, which direct thought without confining it. As used above, it implies that anyone who passes before his mind the cerebella of all animals from fishes to Man, and in all stages of growth, from their earliest appearance in the embryo to their condition in the adult, sees the organ as a median prominence surmounting the medulla oblongata. The bulgings of its sides which, in human anatomy, are termed hemispheres, do not disturb its central, unpaired plan of structure. It has, it is true, a lateral appendage on either side (the combined flocculus and paraflocculus of mammalian anatomy), but this lobe, although of great historic interest, is so small, as compared with the median growth, as not to affect our general conception of the form of the organ. By transverse fissures the cerebellum is divided into a series of lobes.

In appearance the cerebellum varies greatly in the different classes and orders of Vertebrata. Yet underlying this variety there is marked unity of plan. A sagittal section of the organ of a shark, of a bird, of a kangaroo, of a dog, of a whale, of Man, shows that it is divided, from before backwards, into the same number of lobes in animals occupying every position from the bottom to the top of the vertebrate scale. A very little effort to grasp the significance of this mystic number, nine, convinces one of the hopelessness of any attempt to correlate the form of the cerebellum with the muscular development or sensory endowments of vertebrates as a sub-kingdom. It is the same for animals with limbs and animals without; animals with well-developed noses or eyes, and animals destitute of one or other of these sense-organs. This uniformity is extremely significant, when contrasted with the wide differences exhibited by the cerebral hemispheres. It shows that, unlike the great brain which mediates between the several senses and the muscular system, the little brain is concerned in bringing about adjustments to the environment which are equally important to all animals, no matter how far they may depart from the common type. The cerebellum is crossed by deep fissures, dividing it into narrow convolutions or folia. The folia are grouped in nine lobes. If the reader has secured as an illustration the brain of a sheep, he will notice that the lateral regions of the cerebellum present a complicated appearance owing to the contortion of the folia, which results from the unequal development on its sides of the several lobes. In its total size the cerebellum keeps step with the cerebrum, the right side of one organ being associated with the left side of the other.

Fig. 23.—Vertical Section of the Cortex of the Cerebellum,
cut Parallel with the Long Axis of a Folium.

A shows three cells of Purkinje, their espalier systems of dendrites being seen in profile. A “mossy fibre” enters the granular layer from the white matter. About a dozen of the granules are shown, each with four or five dendrites and a single axon. The axon bifurcates in the molecular layer, its two branches running for a considerable distance to left and right along the folium. B shows the other nervous elements which are found in the cortex: a cell of Golgi with a ramified axon, a climbing fibre, a basket-cell, of which the axon divides into four branches, and a small stellate cell.