Some Aspects of the Nervous System.
It has been said with some truth that the telephone has struck a mortal blow at such serenity of life as the Juggernaut Car of modern progress has left us. But if it has done nothing else it has furnished the physiologist with a good illustration when he sets out to expound the functions and arrangement of the elements of the central nervous system and its peripheral expansion. In addition to this general light upon a great matter the vivid experience of many an Englishman during the recent years of war adds point to a subordinate phase of the general story of the telephone, for it represents my contention as to the origin or initiative of the sensorial areas of the mosaic under consideration. Modern persons may be divided into two classes, those who want and those who do not want the telephone, and the former may be sub-divided into A, those who can, and B those who cannot get it (or could not). A and B from the present point of view may be termed Receptors, though to call the B people by that name is to speak Hibernically. With this war-time experience in our minds, we may picture a vast period of time during which the stimuli of pain, cold, warmth and touch were hammering on the skin both before it began to lose its chief hairy covering, and after that process had left man still a hairy animal, but with much-diminished amount of his ancient heritage. These stimuli fell upon the skin very much as the class A, among telephone receptors, spent numerous fruitless stimuli on Postmasters-General, Ministers in Parliament and in “short” bitter letters to our bright little Daily Pope, and who yet found themselves not “connected up,” as the saying goes. There is no knowing how long it was before they had enough effect on the delicate nerve fibrils struggling up into the epidermis and produced receptors or were “connected up” to the exchange or central nervous system. I am inclined to liken the pain stimuli to the short letters referred to, the cold and warmth stimuli to those addressed to the Postmasters-General and the touch stimuli to those which fell upon Ministers at question time.
Another comparison of the peripheral portion of the nervous system to common things has at times forced itself upon my mind when reflecting on the stimuli which are continually assaulting the skin, as I have watched on the Needles’ Downs a flock of sheep on a summer evening returning to their fold. As the sun begins to set they are scattered over the western end of the Downs, still cropping the short grass clothing those chalk and flint slopes which from immemorial time has alone flourished there. They wander singly or in small groups on such parts of the slope as the intrusive golfer still allows, and gradually fall into larger groups which follow somewhat indefinite paths. As they move further and further towards home they are seen to follow one another in single file on some score or more of clearer paths, and finally converge into one well-beaten and broad path until they descend the northern slope and pass out by a single roadway into which a gate opens, and so reach the haven where they would be. Here one has a simple picture of the common stimuli of the skin, at first indefinite and ineffectual, by their cumulative action producing an individual receptor and its nerve connection with the central system.
Professor Leonard Hill[82] also gives a view of the general action of the nervous system and compares it to control of the police force. He supposes a murder to have been committed in a village, and that the local policeman telegraphs to the local town ordering the roads to be searched. The policeman is the tactile sense-organ, the telegraph wire is the sensory nerve, the telegraph office in the local town is the spinal cord, from this office a message is sent to the town police-station by another wire and the police are set in motion. The police are the muscles, the wire that sets them in motion in the motor nerve. The message is also sent to neighbouring towns and to London, that is to say, other local offices (parts of the spinal cord) and the head office (the brain) are informed of the crime or sensory impulse. The central office in London directs the operations controlling the local police office. The whole order of events need not be here described because it goes beyond my immediate purpose, but it is enough to say that attached to the head office are the cleverest detectives (higher sense-organs) and in these are kept records of past crimes, lines of action of the police, and success or non-success of their investigations.
Following on this picture he speaks of the way in which conscious actions become automatic and makes a statement to the effect that “There is evidence to show that the axons (or processes of the nerve-cells which extend unbroken from nerve-cell to its termination) become covered with a adulated coat as each new tract is formed. Thus the structure, like the habit, becomes fixed”—and—“It would appear as if, by repeated experiences, tracts and pathways must be beaten through the nervous system”[83] (Italics not in original).
Beside this I place a statement from Professor Graham Kerr as to his view of the development of peripheral nerve-trunks. He is reviewing the “outgrowth” theory of His, the “chain cell” theory of Balfour, and the “Primitive Continuity” theory of Hensen, and expresses himself as follows: “It is suggested that the development of the actual nerve fibril is simply the coming into view of a pathway produced by the repeated passage of nerve impulses over a given route.”[84] (Italics not in original.)
A passage from Professor McDougall’s Physiological Psychology may also be referred to at more length than it was in Chapter III., page [25]. Speaking of the automatization of voluntarily acquired actions which have been explained by the view that purely reflex actions carried out by mechanisms of the spinal level were also originally acquired by our original ancestors as voluntary actions, he says, “This view is usually associated with the name of Wundt, who has forcibly advocated it. It implies, of course, the assumption that acquired characters are in some degree transmitted from one generation to another, a proposition which most biologists at the present time are inclined to deny because they cannot conceive how such transmissions can be effected. Nevertheless, the rejection of this view leaves us with insuperable difficulties when we attempt to account for the evolution of the nervous system, and there are no established facts with which it is incompatible. If, therefore, we accept this view we shall regard the congenital neural dispositions, both those that determine pure reflexes and those that determine instinctive actions, as having been acquired and consolidated under the guidance of individual experience, with the co-operation, to a degree which we cannot determine, of natural selection.”[85]
These three statements from a physiologist, a zoologist, and a psychologist, all of great eminence, though they differ in particular problems studied, tell very strongly in favour of the position here put forward as to initiative in the production of specialised innervation of the skin.