Summary.

1. Table I. shows that the structures known as touch corpuscles are distributed on those parts of the skin where the stimuli of touch fall most and in propor­tion to the degree in which those parts are employed in tactile discrimina­tion; thus, most of all on the index finger (with the exception of the tip of the tongue) next on the thumb and less on the middle finger. There are 530 of these corpuscles to the upper and 317 to the lower extremity.

2. Table II. bears out the same conclusion, the average number of corpuscles to a square millimetre being twenty-one on the terminal, eight on the second and four on the first phalanges of the index finger, whereas on the foot there are seven on the great toe much exposed to stimuli and only two on the middle of the sole of the foot, which is little exposed. The absence of them from the cornea and conjunctiva, protected by quick and powerful reflexes from such stimuli, and from the (normally) covered glans penis is in accordance with the other results.

3. Table III. dealing with touch spots, shows that these are nearly twice as numerous on the flexor as the dorsal surface of the forearm; and nearly five times as numerous as on the scalp, where tactile stimuli are few, and that the distal phalanx of a finger contains about seven times as many as an area between the shoulders. The regions poor in touch spots are shown to be those where relatively few tactile stimuli can fall.

4. Table IV. gives cold and warmth sensations graded according to the delicacy with which they are perceived in many regions of the skin. The cold sensations are best distinguished on the parts normally most exposed to cold, as the tips of fingers, malleoli, tip of nose, chin, patella, wrist, and least on the protected areas, inner side of thigh, flank, loins and abdomen. The warmth sensations are best distinguished on the regions on which the stimuli of warmth has most frequently fallen, tips of fingers and toes, cavity of mouth, palm of hand, less so on the neck and loin. And the striking fact is noted that warmth sensations are not felt in the lower gums, the inside of the cheek at a certain level and the cornea, which again is protected from these stimuli by its efficient reflex, whereas to the gums and inside of the cheek most warmth stimuli have not been “stimuli” at all.

5. Table V. also gives results of the mode of distribu­tion of cold and warmth spots, examined with punctiform stimuli. The “local sign” for cold is higher than that for warmth spots, and two of these are distinguished as double when only 0.8 millimetres apart on the palm, cheek, chin and forehead, whereas on the upper arm, back and thigh, they are only distinguished as double when separated by two millimetres, and this distance is the minimum at which warmth spots are distinguished as two, that is 2 mm. on the palm, and five on cheek, chin, forehead and back. This tells the same story as Table IV., of past stimuli of cold and warmth.

6. Table VI. deals more elaborately than the others with double sensation in different areas of the skin, the tip of the tongue being the most accurate in this respect of all examined, and the tip of the index finger next, which is to the great toe as 2.3 to 11.3, the palmar surface of a finger half as accurate again as the dorsal surface, the palm of the hand twice as accurate as the surfaces of the forehead and back of ankle, nearly four times as much so as the dorsum of the foot and six times as the skin of the middle of the back.

There is here a very close relation between the amount of exposure of these various regions to tactile stimuli and their present equipment of ability to discriminate between two small objects.

7. Table VII. deals with the sensation of pressure in certain groups of areas, and shows that change of pressure is perceived about three or four times as accurately on the forehead, lips and tongue, as on the finger nail, back of forearm, hand, or fingers, and more than three or four times on the back of the foot, and sole, and surface of leg and thigh. In this group of observations also the rule is followed that the greater and more frequent in man’s ancestral past have been the exposure of his skin to variations of pressure, the greater is his present power of accurate discrimina­tion of them.

There are some scattered facts mentioned by Professor Sherrington which are in keeping with the line here taken, that the formation of receptors in the skin have their origin in accumulated stimuli. He refers to the vain endeavours of Goltz to evoke the reflex croak of the female frog by applying electrical stimuli to the skin, whereas non-nocuous mechanical stimuli were the only stimuli that proved effective.

He never was able to elicit the “extensor thrust” in the “spinal dog” by any form of electrical stimula­tion, but only by a particular kind of mechanical stimulus. This peculiarity was also found in the pinna reflex of the cat.

As to the scratch reflex in the dog it was only when it was easily elicitable that it could be evoked by electrical stimula­tion as well as mechanical, and when it was not easily elicitable electrical stimuli failed altogether while mechanical stimuli still evoked it.

He describes the receptor as a mechanism “attuned to respond specially to a certain one or ones of the agencies that act as stimuli to the body,” and points to the fact that electrical stimuli are not of common occurrence in nature and no chance for adapta­tion to evolve in the organism receptors appropriate for such stimuli has been afforded. Such negative facts are at the least suggestive in considering the question of the mode of origin of receptors and end-organs, electrical stimuli being rare in nature.

The subject of the innerva­tion of the skin and its receptors has been treated here in a great measure by the aid of imagina­tion, with some evidence, and a good deal of reconstruc­tion has been attempted, but perhaps this will be pardoned by those who are prepared to carry out a corresponding process with such as Pithecanthropus, Eoanthropus and Saurian monsters from somewhat scanty osseous remains. Any biological theory of the origin of these receptors than the one here put forward is faced with some formidable difficulties, which are probably insurmountable.

CHAPTER XXIV.
THE BUILDING OF REFLEX ARCS.

Assuming the foregoing origin of the innerva­tion of the skin, I submit that between this rudimentary process and the building of sensori-motor arcs in the spinal cord and brain there is a field, almost unlimited, for initiative in the construc­tion of new forms of animal life. The former is nothing without the latter. To leave it without proceeding further is to leave it “in the air” as military writers say. The formation of Receptors, then, both in the skin field and in the higher sense-organs, leads of necessity to the formation, multiplica­tion and co-ordina­tion of reflex arcs. As in an imperfectly organised telephone service after many a repeated stimuli or “rings” the messages begin to reach their destinations, and as by practice the operators better and better learn their business, so the impulses passing through receptors and nerve-fibrils become organized into more or less efficient systems of arcs, and response is secured to them by some effector of gland or muscle. It is not true of man alone that practice makes perfect.

A certain feature of higher animals which distinguishes them from lower must be remembered, and that is that among them the individual becomes increasingly important. Speaking generally, the latter are born and die in large groups, and their lives resemble those of their group more closely than in the former. The struggle of the individual is vividly pictured by Professor Woods Jones in his descrip­tion of the baby of the perfected arboreal animals. He shows how they and the roaming Ungulates and Pelagic Cetacea cannot indulge in large families, and that it is only those forms which have a safe retreat for their young which can avoid reduction of the size of their families, and how the higher apes still more resemble in these respects mankind, as we know it. For the proper study of the “synthesis of the individual” organism this essential fact must be kept in mind.