The scratch-reflex in the dog, which like the tendon-reflex in man was in my youth a subject for schoolboy tricks, has received a vast amount of attention and research from physiologists to whom it has brought valuable fruit. It is a familiar phenomenon in a familiar friend of man. There is a saddle-shaped area on the back of the dog over which it was found empirically that even a light stimulus when applied rhythmically, produces the “scalptor-reflex” or a reflex rhythmical action of the flexor muscles of the leg on the same side, calculated to remove the irritating causes of the stimulus. This includes a series of receptors in the skin leading to a spinal segment in the region of the shoulder, a long neurone in the cord, then a motor neurone, the axon from which activates the flexor muscles of the leg and produces scratching. It is described as an efferent arc from receptor to the motor neurone, from which the Final Common Path supplies the motor apparatus or effector. Professor Sherrington says that in this reflex a single stimulus which is far below threshold intensity is found on its fortieth repetition and nearly four seconds after its first application to become effective and provoke the reflex and that its frequency is about 4.5 per second. The reflex movement remains rhythmic and clonic under the strongest as under weaker stimulation. When it is easily elicitable the scratch-reflex can be evoked by various forms of electrical as well as mechanical stimulation, but, when not easily elicitable, electrical stimulation fails whereas rubbing or other mechanical forms of stimuli still evoke it, though less vigorously than usual. This reflex can also be set aside by the “nociceptive arc from the homonymous foot” or, in other words, a nocuous stimulus to the leg of that side produces “interferences which amounts to inhibition.” Empirically it is easy to notice also that if the “scalptor-reflex” can be elicited on both sides of the body, the dog when standing will momentarily lose the power in the hind legs.
Note.—The rhythm of this reflex act is so special even to the layman that lately I had a singular confirmation of its stereotyped character, when lying awake at night and being puzzled by a curious rhythmical scratching sound coming from my next door neighbour’s back yard. It might have been taken by a wakeful person for some mechanical work on the part of a burglar, but after listening repeatedly to the apparently familiar sound I found that it came from the kennel of a fox terrier kept by my neighbour.
Purposes of Reflexes.
All reflexes being purposive this particular innate reflex is acknowledged to have for its purpose the grooming or cleaning of the skin over its hereditary territory. This introduces its connection with initiative here propounded, and the justification for its introduction is contained in Professor Sherrington’s statement that “In the analysis of the animal’s life as a machine in action there can be split off from its total behaviour fractional pieces which may be treated conveniently, though artificially, apart, and among these are the reflexes we have been attempting to decipher”—scratch-reflexes and others. There seems to be no reason for the existence and stereotyped character of this reflex except the need or rather the desire (if one may use a convenient but inaccurate term) on the part of the dog to remove an irritant which disturbs its comfort when at rest. Some “minor horrors,” probably fleas moving across the skin-receptive field of its shoulder and back, must be assumed to be the irritant in question. This touches the great question of the initiative of this remarkable reflex, which seems more fixed and powerful in the dog as we know him than that other reflex which leads him to turn tail and flee immediately he sees a boy stoop down as if to pick up a stone. I dare say a clever advocate on the opposite side might impress a jury by building up a case under which an adaptation to a protective need would be conceived as responsible for the rapid flight at the sight of the threatening attitude of the boy. Such a reconstruction is not required, for it is perfectly clear that in the history of the domesticated dog the selection of such an adapted reflex could have no place. The survival-value of this reflex would be nil, for the number of dogs killed by a stone or maimed for life would be so negligible that the production of a specialised reflex for the purpose by selection or survival of the fittest would not arise. Obviously the danger would be intermittent and rare; and dead dogs tell no tales. On the other hand it would be highly unpleasant for dogs to be hit by stones and educability would lead them to avoid the stooping attitude associated with missiles.
We are told on high authority that not education but educability is transmissible, and yet this humble reflex appears in very young dogs that could hardly if ever have known the impact of a stone. Incidentally we are compelled to remember how in past battles of our youth the aim both of “ourselves and the enemy” was deplorably poor, and not from want of practice. This school-boy-stone reflex is either an example of educational effects transmitted or of a minute bit of the unpacking of an original complexity which it would require the brain of a de Quincey to work out. But if we suppose the initial stages of such a stimulus as the occasional impact of a stone in many generations to be slowly ingrained in the skin-receptors, reflex-arcs and receptors we do not need opium either for the acceptance of orthodox dogma or to aid us in the Mendelian alternative to a very simple ideal construction.
This digression bears on the initiative of the more important scratch-reflex, and it is profitable to ask “are not both of these reflexes in dogs examples of Evolution of the Indifferent?” Is it possible to imagine that from its inception to its fully-formed state, with a specialised territory of skin-receptors accurately mapped out, with receptor neurones, reflex-arcs and adapted effectors, this scratch-reflex can have arisen through Germinal Selection or selective processes within the germ? At no stage can anything more than a contribution to more or less comfort to the animal be held to result from its operation. It is strangely reminiscent of the proceedings of an elderly man after lunch on a hot day when he protects his head against house-flies with a handkerchief. I am aware that it is but one of a large number of reflexes produced for the purpose of grooming the trunk head or limbs of animals as low down in the scale as the house-fly or grasshopper, many of which were beautifully described a few years ago by Miss Frances Pitt in the National Review in an article dealing with small mammals, chiefly rodents. But I have availed myself here as elsewhere, of the liberty of doing what Professor Sherrington says we may do, and consider this scratch-reflex as split off from the rest of the animal’s behaviour for the purpose of analysis. He also says in discussing the subject of parasites moving across the receptive surface of the skin that the ulterior purpose may be the removal of what “would confuse its function as a receptive surface to more significant environmental stimuli.” This statement is hypothetical and the problem obscure; but at any rate we know this that the removal of the parasite must conduce to the greater comfort of the dog without any more recondite purpose. The one suggested by Professor Sherrington would in some possible but very vague manner be referable to selection, but, whether the suggestion be valid or not, it is almost impossible to suppose that a saddle-shaped area of the kind described could be under the guidance of selection. The law of Parcimony forbids. There is a close similarity between this saddle-shaped area in the dog and that on the cow’s trunk described in Chapter X. It is difficult to believe that from man downwards to grasshoppers relief from mild irritating causes such as this is not enjoyable to the particular animal, and yet indifferent altogether as to its survival in the struggles of life for food and mates. The “scalptor-reflex” only reaches the limits of the receptive field of the scratch-reflex and it is contrary to observed facts that parasites confine their depredations just to the region where the formidable scalptor-reflex can reach. The wicked flea knows better than that. The initiative of this reflex can well be pictured as taking place in domesticated dogs and their wild ancestors whose habitats in prehistoric times were probably infested with these irritants to such a degree that no modern mind can conceive, and the adequate stimuli, leading to receptors after ages of impact and consequent hammering out pathways through certain reflex-arcs until the required weapons of offence or effectors were organised into a defensive-offensive system—were there in profusion. But a great and fundamental principle of the evolutionary process such as Selection is not honoured by being dragged in, even for forensic purposes, to account for results which owe to the search for comfort their perfection of organisation. I have personally seen in some professional invalids of the softer sex nearly as perfect adaptations to their comfort which in no way contributed to their length of life. This may be put aside as irrelevant but it is at least suggestive.
I submit the statement as to the scratch-reflex in the dog that from beginning to end it is an indifferent mechanism and the probability is immense that its initial stages were governed alone by repeated stimuli from parasites which produced receptors, conducting fibres afferent neurones and efferent neurones, leading into the Final Common Path controlling the flexors of the hind limb. It would then come under the Law of Subjective or Hedonic Selection formulated by Professor Stout in the words: “Lines of action, if and so far as they are unsuccessful, tend to be discontinued or varied; and those which prove successful to be maintained. There is a constant tending to persist in those movements and motor attitudes which yield satisfactory experiences, and to renew them when similar conditions recur; on the other hand those movements and attitudes which yield unsatisfactory experiences tend to be discontinued at the time of their occurrence, and to be suppressed on subsequent similar occasions.”
In this connection a statement from Professor McDougall’s work may be advantageously quoted. He says that “It is characteristic of those (arcs) of the higher or third level that their organisation, their interconnections, by means of which the simpler neural systems of great complexity, is congenitally determined in a very partial degree only, and is principally determined in each individual by the course of its experience. The arcs of the higher level thus constitute the physiological basis or condition of docility, the power of learning by experience.”[88] (My italics)