Observed Facts.
Briefly stated the facts of the innervation of the skin are of two orders, anatomical and physiological; the former examined by the aid of the microscope, the latter by physiological experiments of a varied kind. The chief aspect in which these are viewed here is the mode of distribution of these two groups of fact, and it is held that this strongly suggests without proving it, the alleged mode of origin of both.
Table I.—Distribution of Touch Corpuscles:—
In the deep connective tissue of the dermis there are:—
In the thumb about 70.
" " index finger 105.
" " middle finger 60.
" " whole hand 500.
They are numerous over finger joints and front of elbow joint.
In all 530 about the joints of the upper extremity.
317 about those of the lower extremity.
Table II.—(From Schäfer’s Text Book of Physiology):—
Average of Meissner’s corpuscles to each square millimetre, which is approximately one five hundredth part of a square inch:—
| Palmar surface of distal phalanx of index finger | 21 |
| Palmar surface of second phalanx of index finger | 8 |
| Palmar surface of first phalanx of index finger | 4 |
| Palmar surface of metacarpus of little finger | 2 |
| Plantar surface of distal phalanx of great toe | 7 |
| Middle of sole of foot | 2 |
| Flexor surface of forearm in each sq. mm. | 1 |
| Distal end of flexor surface of forearm in each sq. mm.:— | |
| 1 to each 6 sq. mm. approximately. |
* Absent from the cornea, and conjunctiva of the upper eyelid and from the glans penis.
Table III.—Distribution of Touch Spots:—
These must be distinguished, of course, from the touch corpuscles of the preceding list and the subjective element in the study of them must be borne in mind.
If an area, as of the calf of the leg, be prepared, by cutting short the small hairs, and examined, it is found that there are about 15 touch spots in each square centimetre, which is about one-fifth of a square inch.
In another area so treated the hairs are counted and the following result is given:—
1.
On the dorsal surface of the forearm 78 touch spots are found in an area containing 15 hairs.
2.
On the flexor surface of the forearm 147 touch spots are found in an area containing 22 hairs.
3.
On the scalp 66 touch spots are found in an area containing 38 hairs.
Schäfer says: “An area of the dorsum of the distal phalanx of a finger contains about seven times as many touch spots as an equal area between the shoulders. Regions poor in touch spots are the flexor surface of the upper arm, the upper third of the thigh, the leg above the inner malleolus, the neck, and in general the skin over subcutaneous surfaces of bone.”[81]
Table IV.—Distribution of Cold and Warmth Sensations.
The Scale includes twelve grades of sensation in cold, and eight in warmth sensations, and commences with the regions which yield the maximal intensity of sensation.
Cold Sensations.
1.
Tips of fingers and toes, malleoli, ankle.
2.
Other regions of digits, tip of nose, olecranon.
3.
Chin, palm, gums, glabella (a small central area just above bridge of nose).
4.
Occiput, patella, wrist.
5.
Clavicle, neck, forehead, tongue.
6.
Buttock, upper eyelid.
7.
Lower eyelid, popliteal space, sole, cheek.
8.
Inner aspect of thigh, arm above elbow.
9.
Intercostal spaces along region of axillary line.
10.
Areola of mamma.
11.
Nipple, flank.
12.
Certain areas of loins and abdomen.
Warmth Sensations.
Absent from lower gums, mucosa of cheek at second lower molar and cornea.
1.
Tips of fingers and toes, cavity of mouth, conjunctiva, patella.
2.
Remaining surfaces of digits, middle of forehead, olecranon.
3.
Glabella, chin, clavicle.
4.
Palm, buttock, popliteal space.
5.
Neck.
6.
Back.
7.
Lower eyelid, cheek.
8.
Nipple, loin.
Table V.—Distribution of Cold and Warmth Spots.
By stimulation of cold or warmth spots there is shown, not only the quality and quantity of the stimulus, but the locality. When punctiform stimuli are applied to pairs of cold spots and pairs of warmth spots marked “local sign” is found. This Goldscheider showed to be higher for cold than warmth spots.
| Cold Spots. | Warmth Spots | |
| Palm | .8 mm. | Do. 2 mm. |
| Cheek, Chin and forehead | 0.8 mm. | Do. 5.0 mm. |
| Upper arm | 2 mm. | Do. 3 mm. |
| Back of hand, leg, thigh | 3 mm. | Do. 4 mm. |
| Forearm | 3 mm. | Do. 3 mm. |
| Back, chest, abdomen | 2 mm. | Do. 5 mm. |
Thus on the palm of the hand two pairs of cold spots .8 mm. apart are distinguished by this punctiform stimulation, whereas on this surface two pairs of warmth spots are only distinguished when they are 2 mm. apart on the cheek, chin or forehead and cold spots are distinguished when .8 mm. apart on the same surfaces warmth spots when 5 mm. apart.
Table VI.—Average lowest distances in millimetres on different areas of skin where two points are felt as two or minimal distances from which double sensation is obtained.
Table VII.—(According to Weber’s Law.) Average differences in different regions of skin of sensation of pressure.
| Forehead |  | 1/30 to 1/40 |
| Lips | ||
| Dorsum of tongue | ||
| Cheeks | ||
| Temple | ||
| Finger nail |  | 1/10 to 1/20 |
| Dorsal surface of forearm, leg, and thigh | ||
| Dorsal surface of hand | ||
| Dorsal surface of 1st and 2nd phalanges of fingers | ||
| Palmar surface of finger | ||
| Palmar surface of hand | ||
| Flexor surface of forearm | ||
| Dorsum of foot | | More than 1/10 |
| Dorsal surface of toes | ||
| Plantar surface of toes | ||
| Sole of foot | ||
| Surface of leg and thigh |
Thus on the forehead differences of pressure are distinguished when they are increased by 1/30, whereas on the dorsum of the foot they have to be increased by 1/10 to be distinguished. This is carried out by impact of little balls of a light substances such as pith.
It may be remarked of these tabulated results that on the one hand they are the results of work extending over some seventy years and numerous observers, and on the other that, broadly looked at, they all tell the same story of stimuli in their incidence on the skin—those of pain, cold, warmth and touch. There is also one thread of origin running through all, and that is that the regions most exposed to the four stimuli show the highest development of specialised function and structure.