The precise nature of the sensations is not yet fully understood. It is pretty clear that the “minimal stimuli” here employed do not give rise to purely tactile sensations of low intensity. This seems to be established by the fact brought out by Dr. Louis Robinson that the parts of the skin having the most acute tactile sensibility, the tips of the fingers and the tip of the tongue, are “scarcely at all sensitive {52} to titillation”.[30] It has been pointed out by Wundt that the sensations in this case, as in that of some other skin sensations, tend to spread themselves out, other and even distant parts of the surface being engaged by means of the mechanism of reflex sensation.[31] This in itself suggests that the sensations of tickling are more allied to organic than to purely tactile sensations. It is supposed that the light stimuli set up in the skin certain organic changes, more particularly modifications of the circulation of blood in the small vessels.[32] It is well known that not all parts of the skin are equally susceptible of the effect of tickling. Certain areas, for example, the sole of the foot and the armpit, are commonly said to be “ticklish places”. In the answers to questions sent out by Dr. Stanley Hall we find the order, as determined by most frequent naming of the part, to be as follows: the sole of the foot, the armpit, the neck and part under the chin, the ribs, and so forth. The inquiries brought out the fact that there are considerable differences of experience here, some saying that they were ticklish in all parts, others only in one. The method adopted in this inquiry clearly affords no accurate measurement of comparative sensibility.[33] {53} A more scientific attempt to measure this was made by Dr. Louis Robinson, who carried out a large number of experiments on children from two to four years of age with the definite purpose of testing the degree of responsiveness by way of laughter. According to his results the order of decreasing sensibility is as follows: (1) the region in front of the neck; (2) the ribs; (3) axillae; (4) bend of elbow; (5) junction of ribs and abdominal muscles; (6) flanks; (7) region of the hip joint; (8) upper anterior part of the thigh.[34] A glance at these statements shows that the determination of the scale of ticklish sensibility over the surface is not yet completed. Dr. L. Robinson, by the way, mentions neither the sole, a highly ticklish spot in the popular creed, nor the palm, which, as we shall see, is decidedly a ticklish region.[35] It is highly desirable that more precise experimental inquiries should be directed to these local variations of ticklishness, and that, after the seats of the higher degrees of the sensibility have been ascertained, the question should be considered whether these are marked off by any definite peculiarities of structure.