35-i 



THE POPULAR EDUCATOE. 



The skin consists of two layers. The outer one is called the 

 cuticle or scarf-skin, and the deeper layer the cutis. The cuticle 

 has neither blood-vessels nor nerves, but consists of cells which 

 are formed at its inner surface (where it lies on the cutis or true 

 skin), and are pushed outward as fresh strata are successively 

 formed below them. When first formed, these cells are filled 

 with fluid ; they are oval, and longer in the direction perpen- 

 dicular to the surface than in the other. As they are tlutust 

 outward, they become flattened in the contrary direction, so that 

 .at the surface they form dry, transparent layers, which are 

 capable of being shredded off and stripped away in scaly or 

 scurfy fragments by the ordinary wear and tear to which the 

 oater surface is subjected. 



The office of this part of the skin is simply protective; and in 

 relation to this office of clothing and defending the blood-bearing 

 skin, it is found thickest where there is the greatest friction, 

 >uid thinnest where there is least. It is, however, thin every- 

 -where, varying from ^ of an inch in the palm of the hand to 

 .-7*0 of an inch in less exposed parts. As, however, this scarf- 

 skin is in continual process of being rubbed away, it is not only 

 thicker in much-used parts, but is much more rapidly formed on 

 those parts. Moreover, if any peculiar employments make the 

 wear and tear excessive, unwearied nature still supplies the 

 demand, and an excessive manufacture of fresh cells is stimu- 

 lated from below. Thus, in the polishing of japanned articles it 

 is found that no other fabric but the human cuticle is sufficiently 

 delicate to produce the shining surface. The finest wash-leather 

 would scratch; and hence women are employed to scour trays, 

 etc., all daylong; and yet they never wear down to the true skin 

 so as to make the fingers sore, except during the first few weeks. 

 "The provision for the repair of this closely-fitting vestment is 

 even carried beyond this, for if the whole cuticle be stripped off, 

 o as to leave the cutis naked and sore, there is an immediate 

 outpouring of fluid from the blood, which forms at once into a 

 scarf-skin. 



As this scarf-skin has no blood-vessels running into its sub- 

 stance, it has no means of self -repair ; so that in proceeding 

 from the deeper layers to the surface, the cells go through all 

 the processes of birth, death, decay, and dissolution, though the 

 raembrane is so thin. Since, also, this skin has no nerves 

 entering it, it has no sensation, and the sensation of touch must 

 be felt through it in the same way though in a much more per- 

 fect manner as we feel anything which touches us through our 

 clothing. It will be seen, then, that it must fit very accurately 

 and closely to the sensitive skin beneath, or the sense would bo 

 dull and imperfect. The skin below has an immense number of 

 small hillocks, and each one of these is closely surrounded by, 

 and inclosed in, the inner layer of the cuticle which is moulded 

 upon them. When the cuticle is stripped off after being long 

 soaked in water, it shows an infinite number of small pits, out 

 of which the hillocks or papillae have been dragged. If the 

 whole be torn away before maceration, i.e., from the living akin, 

 it usually tears away the papillae with it, leaving a bleeding 

 surface. 



In providing at once for the protection of the cutis, and also 

 for the preservation of the acuteness of the sensation of touch, 

 there is this difficulty: those parts which are most used to 

 gain information by touch, are necessarily those which are most 

 subject to friction. In such situations, then, the cuticle must 

 be thick ; yet a solid thick sheet would be liable to make us 

 confound impressions made by two points near together which 

 were in contact with the skin. There is a beautiful arrange- 

 ment to obviate this difficulty, which is found in the cuticle of 

 the tips of the fingers, palm of the hand, etc. Here the surface 

 of the skin is seen to be thrown into small ridges and furrows, 

 which run in curved lines parallel to one another, so that an im- 

 pression made on the surface, or tops of the ridges, is only con- 

 veyed down to the papillae Immediately beneath it, and does not 

 press sideways on those of the other ridges. A more minute 

 examination of the tip of the finger with a lens, will show that 

 these wavy ridges are subdivided into square-shaped masses by 

 cross furrows, which occur at regular intervals, so as to leave 

 the thickened part between of the same width as the ridge. 

 Each one of the square-shaped masses has in its centre a little 

 pit, which is the opening of a sweat-gland. No such definite 

 arrangement of ridge and furrow occurs in other parts of the 

 body, where the sense of touch is comparatively obtuse, or 

 rather, not nicely distinguishing. 



The cutis, or blood vascular skin, is tough and elastic, and 

 consists in its deeper layers of interlaced fibres which hold in 

 their interspaces little masses of fat, sweat-glands, oil-glands, and 

 hair-bulbs, with hairs proceeding from these last to rise above the 

 surface. It is also permeated with nerves, arteries, and veins. 

 This, therefore, is a structure having all the endowments of life, 

 and with the faculty of self-sustenance and sensitiveness. The 

 true seat of the sense of touch is, however, its external portion, 

 that which lies immediately under the cuticle. Towards tho 

 surface the fibres become closer and denser, and the various 

 glands and fatty masses cease, while the blood-vessels and 

 nerves are more numerous. In order to increase tho touching 

 surface, and to bring the nerve-threads closer to the exterior, the 

 outer surface of the true skin is, as we have seen, raised at in- 

 tervals into papillae. Each of these is well supplied with vessels 

 and nerves. Under the ridged surface of the palmar side of 

 the hand, these papillae run in lines corresponding to the ridges, 

 there being two rows to each ridge, and sometimes smaller ones 

 between. In other parts they are scattered irregularly, and are 

 much fewer in number. That these papillae are the true seats 

 of the sense of touch, appears not only from the fact that nerves 

 are traced into them, but because there is a strict relation 

 between their number in a given space and the delicacy of the 

 sense of touch in those parts. Thus in the space of one square 

 line (jij of a square inch) there are 108 on the tip of the finger, 

 40 on the second joint, and only 15 on the last; and this de- 

 crease in number is in direct proportien to the sensitiveness of 

 the surface to touch. Where the sense of touch is most acute 

 and discriminating, little oval-shaped bodies have been found, 

 one lying in the centre of each papilla, and these have been 

 called the " little bodies of touch." It must not be supposed, 

 however, that each of these papillae is capable of transmitting a 

 separate impression to the brain, or that their office is simply 

 tactile. Nerves do not enter all of them, and they are concerned 

 in secreting the substance to form the cuticle. It would seem 

 as though each nerve which conveys a single distinct impression 

 to the mind, had a certain definite space of surface of skin, over 

 which its final branches spread themselves ; so that if two 

 objects touch the skin at two different points within this area, 

 they feel like one. In order to be felt as two separate contacts, 

 they must be placed one on one special nerve-surface, and one 

 on another. The size of the special spaces allotted to each 

 nerve-unit is very different in different parts of the body. The 

 determination of the size of these areas, and, by conseqiience, 

 tho accuracy of the sense of touch in various parts of the body, 

 was effected by Weber. His method was at once so ingenious 

 and so simple, that it is curious it should not have been adopted 

 before. He took a pair of compasses, and having placed upon 

 their points very small globules of sealing-wax, opened them 

 to a small distance, and applied them to the surface of the body 

 where the sense of touch was to be tested. The impression 

 produced was as of a single point. He then opened them 

 more and more until two distinct impressions were felt; and 

 then measured the distance on a scale of inohes and lines. He 

 thus arrived at very definite and very interesting results. Among 

 many other measurements of the least distances at which two 

 points could be distinctly felt, we quote the following: 



in. lines 

 . 0| 

 . 



Back of the hand .... 



Scalp of the head .... 



Breast .......... 



Middle of thigh, arm, 



and back ....... 2 6 



in. lines 

 1 2 

 1 3 

 1 8 



Tip of the tongue . 



Tip of the forefinger . . 1 



Second joint of forefinger 2 



Back of the fingers ... 3 



Palms of the hands ... 5 



End of the great toe . . 5 



The reader may verify these estimates for himself, but it 

 better to try them on some other person, because the impressions 

 produced upon the eye and the mind by the sight and knowledge 

 of the open compasses, have a tendency to bias the information 

 received from the sense alone. The legs of the compasses must 

 be applied both at the same instant, and not moved before the 

 estimate is given. If they are moved, very different results will 

 be given. From these statistics it will be seen that the tip of 

 the tongue is the most discriminating part of the whole body. 

 An easy verification of this will occur to every one when they re- 

 member how small a flaw in the teeth the tongue can detect a 

 flaw which is quite unnoticed by the tip of the finger, if that bo 

 applied to it. At first thought, it may seem strange that such 

 acutenesa of touch should be bestowed on an organ which ia 



