Some Neural Phenomena.

Among numerous phenomena of nervous reactions discovered by the research of physiologists certain have a close bearing on the formation of receptors, afferent fibres and reflex-arcs, especially those of Delay, Summation, Fatigue, Block or Resistance, Localiza­tion, Facilita­tion and Inhibi­tion.

Facilitation.

But of all these important reactions in nervous tissues none bears so closely on the problem of the formation of reflex-arcs as that of Facilita­tion. This is equivalent to the Law of Neural Habit of the physiological psychologist, and is bound up with the highly important Law of Forward Direction, which Professor Starling says might as well be spoken of as the Irreciprocal conduc­tion of nerve-arcs. The Law of Forward Direction of sensori-motor arcs is too well known to need here any descrip­tion. But when this law is taken into account the phenomenon of Facilita­tion is seen to throw a strong light upon the earliest and rudimentary formation of specialized nerve-fibres, reflex-arcs and Final Common Paths leading to the effector glands or muscles. Facilita­tion is described shortly by Professor Starling as follows. If the passage of a nervous impulse across a synapse or series of synapses in the central nervous system be too often repeated, fatigue is produced, and there is an increase of the block at each synapse. If, however the stimulus be not excessive and the impulse not too frequently evoked, the effect of a passage of an impulse once is to diminish the resistance, so that a second applica­tion of the stimulus provokes the reaction more easily, and he adds that the result of summation of stimuli is in fact in the direction of removal of block. When an impulse has passed once through a certain set of neurones to the exclusion of others it will tend, other things being equal, to take the same course on a future occasion, and each time it traverses this path the resistance in the path will be smaller. Education then is the laying down of nerve-channels in the central nervous system, while still plastic, by this process of Facilita­tion along fit paths, combined with inhibi­tion (by pain) in the other unfit paths. He makes the important statement that Facilita­tion is of great interest in connec­tion with the development of “long paths” in the central nervous system and, more especially with the acquirement of new reactions by the higher animals. (Italics not in the original).

Raw Materials of the Central Nervous System.

The raw materials of higher central nervous systems are furnished even in lowly Vertebrates by the neurones and their processes, and the pathways into the grey matter by the “canalizing force of habit” in the receptors and afferent fibres. Facilita­tion, discovered in higher Vertebrates, such as dogs and cats, throws backwards a light on the earliest struggles towards success and integra­tion among phyla, sub-phyla and smaller groups, and here again the well-known may lead to the less-known. We may then frame a legitimate hypothesis, or at least an ideal construc­tion of trials and errors and success, if those of lower levels were ever to be introduced to the career of progress and achievement. But to make good this claim it is necessary that it be based on the important doctrine taught by Hughlings Jackson of the three (or more) levels of sensori-motor arcs—those of the spinal or lowest, of the sensory or intermediate, and those of the third or highest level, in which the associa­tion-areas of the Primate brain are at once the means and the title to his primacy, or headship of the sentient world. The light of this doctrine guides the mind backwards to the frog-stage of animal evolution with its highly organized congenital system of arcs of the spinal level, so efficient for its life that, even when the brain is removed, the frog can execute under certain stimuli a purposeful complicated movement such as that of trying to wipe away with its foot an irritant drop of acid applied to its head or back; or, still more, if touched lightly between the scapulæ, will “lower its head at the first touch, and again more so at a second, and at a third will, besides lowering the head, draw the front half of its trunk slightly backwards; at a fourth the same movement with stronger retrac­tion; at a fifth give an ineffectual sweep with its hind or fore-foot; at a sixth a stronger sweep; at a seventh a feeble jump; at an eighth a free jump, and so on.” Probably such an animal as the frog has all its reflexes congenitally organized, whereas a dog, reaching the sensory level, has added countless reflex-arcs to those inherited from its early ancestors of the Insectivores which had long emerged from the spinal level, retaining its old, perfecting its new inheritance, and eliminating the unfit. Perhaps a faint picture of this long process may be afforded by watching an experienced mountain guide ascending an ice-slope with the aid of ice-axe, hand and foot.

Integration of Raw Materials.

Every group of animals in the higher ranks has its own entailed property of innate reflexes, for example, the reflexes which subserve the reflex functions of the cord: those of locomo­tion, muscular and vascular tone, micturi­tion, defæca­tion, impregna­tion and parturi­tion. These exist in an animal of the spinal level whether or not it remains purely aquatic, partly aquatic, partly terrestrial, arboreal or terrestrial. As the progressive groups ascend the ladder of life they add to this inalienable heritage, gained we need not here ask how, fresh reflex-arcs by response to new initial stimuli, forging them by the incident of use. So, the original acquirements in the past levels serve as starting points for raising the degree of their nervous integra­tion with growing control over their environments. The long story from the simple central nervous system of a fish, with a few or no associa­tion-areas, to that of man with his extensive frontal, parietal, parieto-occipital associa­tion-areas, could never be deciphered, even with the light of the laws of genetics turned on full, without a protracted process of construc­tion of fresh arcs. A common illustra­tion of such a series of changes and results may be seen in the building of a house. Bricks, founda­tion-stones, walls and a roof may serve some of the elementary requirements of a house and much less than these were of use to early man for his shelter. Without them we cannot call any structure a modern house; but also without floors, staircases, windows, chimneys, division into rooms, some degree of decora­tion by paint or paper, and a supply of water, we should refuse in these days the name of house to that rough structure, apart from beauty of design, decora­tion, within and without, and some addition of modern appliances of comfort and convenience. In the history of house-building the stages of supply of raw materials, adapta­tion to needs guided by selection, initia­tion, trial and error have their counterpart in the construc­tion of higher animals.

Evidence.