SWEAT-PORES, RIDGES AND FURROWS
[Illustration: SECTION OF SKIN, SHOWING SWEAT-GLANDS, DUCTS AND PORES
_a_. pore open.
_b_. pore closed.
_d_. sweat duct.
_e_. sweat gland.]
The front or palmar surface of human hands, and the corresponding
solar or plantar surface of the feet, are marked with alternate ridges
and furrows, lying for the most in nearly parallel rows, but often
again at certain points on palm or sole, curving, splitting, twisting,
or joining to form patterns of much intricacy. The ridges, called
technically _rugæ_ (sing. _ruga_), are punctuated at very frequent
intervals with small openings, which are the mouths or pores of the
sweat ducts connected with certain glands which lie deep in the lower
strata of the skin. The furrows or _sulci_ (sing. _sulcus_) are almost
devoid of any such apertures. There are probably some two or three
millions of those tiny sweat pores in a human body, which afford an
evaporating surface, according to the anatomist Krause, of about
eight square inches. The sweat is a watery, slightly saline fluid,
with slight--very slight--traces of grease, some small cell-like
particles, and some carbonic acid and other gaseous matters, which
exhale from the skin. The more oily secretion of the skin comes
from a different set of openings with their associated glands, the
sebaceous glands, which are associated with the hairy surfaces of
the body. In Ludwig Hopf’s work, _The Human Species_, the subject is
discussed fully. When the palmar surface leaves a distinctly greasy
impression, this greasiness must have been acquired from outside or
from transmitted exudation from the back or dorsal surfaces, or other
parts of the body.
Those skin ridges, apart from any relation they may have either to the
sweat-pores or to the special nerves of touch and temperature which lie
near them, serve a useful purpose in helping the horny hands of toil
to grasp its tools firmly. They occur in a few other parts of animals
somewhat near to us in the scale of being. A striking example is that
on the palmar surface of the prehensile or grasping tail of the Spider
Monkey (_Ateles ater_), which it uses in climbing almost like a hand.
When the ridges in human fingers are well softened with water, and
are then rubbed along the surface of a tumbler or wine-glass, musical
sounds may be elicited, which are caused by the alternate resistance
and yielding of the softened ridges. This was the principle of the
“musical glasses” of Goldsmith’s time. The navvy often begins his
labours by moistening his loof. After his efforts make him perspire,
he has no further need in this way for his salivary resources. Hence
Nature, too, has placed the openings of the sweat-pores on the crests
of his ridges, and not, as Herbert Spencer on one occasion is said
to have supposed she had done, in the troughs of the furrows, where
they are very seldom to be found, and would not be nearly so useful.
Curiously enough, our modern makers of indiarubber tyres work a
trademark pattern or title in ridges on their wares, so as to secure a
good grip on the road--and on the market. In a similar way the carriers
of Manchuria adorn their clumsy wheels with studs to prevent their
skidding.
There are, as has been mentioned, two kinds of minute glands in the
skin: one, to secrete that complex excretion, the sweat; the other, to
provide a certain greasiness to hair. The latter are found chiefly in
other parts than the palms, and serve to secure that slight oiliness of
the surface of our bodies which is very well seen in taking one’s bath.
However thoroughly that thin film of surface greasiness is removed with
the use of soap and vigorous scrubbing, in a moment or two water is
seen to act on the cutaneous surface as it would on a slightly greasy
platter or a duck’s back. The importance of this point will become
apparent when we come to deal with some practical applications of
dactylography in searching for invisible greasy finger-marks, which may
be made visible.
Looking carefully at the visible texture of the fingers and palms, we
see, then, that the cutaneous ridges lie, for the most part, closely
and evenly, like furrows in a well-ploughed field. But just as in some
fields the ploughman has perforce had to swerve and veer round some
fast embedded boulder or old tree stump, varying his intended pattern,
so, too, in our fingers curious divergent lineations are found to
occur, and we cannot very well tell the reason why. Coloured patches
may be designed like so many pretty wall-paper designs, to enclose
these patterns in books on finger-prints, but I, for one, cannot see
that they throw any light on their genuine nature and origin. We find,
under purely mechanical conditions, similar patterns produced in the
ripples of a sub-aerial sand-drift and on a tidal shore. While writing
this chapter, I saw to-day similar deltas, junctions, forks, and the
like, on a lake whose frozen surface was thinly sprinkled with fine dry
snow. The lines were mostly parallel, but where certain gusts or eddies
had occurred they had been broken up into patterns not unlike those of
finger-tips.
[Illustration: RIPPLE MARKS IN SAND (_After Lyell_).]
In human skin, and in the anthropoid apes, those scroll-like patterns
present almost infinite varieties of detail, and they often resemble
a condensed railway plan, showing junctions, blind sidings, loops,
triangles, and curves. There is one important distinction to be
observed. The lineations of skin ridges are not always quite uniform
in breadth, but broaden out sometimes or dwindle away. Again, they are
dotted with sweat-pores and do not always, when printed from, show
those pores in the same degree of patency or openness. Hence a little
variation is inevitable when the same finger is several times impressed
under varying conditions. It is not to be forgotten that, to a limited
extent, this is true of a rigid box-wood engraving or steel plate, or
lithographic stone, which give somewhat divergent results with varying
degrees of pressure in printing, moisture of atmosphere or paper, and
other conditions.
In this country the feet do not afford a favourable field of study
to the dactylographer. So far as identification is concerned, little
use could be made of them practically. In the East, however, it is
different, and many years’ residence there gave me opportunities to
observe that the toes, unrestrained by the use of stiff leather boots,
are mobile and powerful, grasping as fingers do. The carpenter in
Japan, for example, uses his toes to grip and steady the board he is
sawing or hewing, while many of my readers must be familiar with the
extraordinary agility of Japanese acrobats in the use of their feet
and toes. In those cases the ridges are often varied in grouping, and
well defined in development. A European baby generally begins life
with similar simian-like powers. But so far as my own observations go,
the patterns in the hands usually show a somewhat higher degree of
evolution, a more complex and intricate network of lines, than those
exhibited by the feet of the same person. Hence, apart from the greater
convenience of inspecting them, the finger-prints have greater value
for the purpose of identification. Cases, however, of crime, might
readily occur even in this country, where the imprints of naked feet
might yield important and irrefutable evidence of one’s presence at a
scene of evil-doing.
But there are other important points of scientific interest besides
their evidential value for identification. An important problem in
evolutionary development, on which a considerable amount of literature
begins to accumulate, is the serial relation of the limbs. Professor
Bowditch, the distinguished biologist, of Harvard University, U.S.,
wrote me, of date November 18th, 1880, thus:--
“Dear Sir,--I have just read in _Nature_ of October 28th, your
article on the skin-furrows of the hand. The subject interested me
because it so happened that fourteen years ago, at the suggestion
of the late Professor Jeffries Wyman, I made some prints of the
finger and toe tips with the hope of throwing some light on the
question of the antero-posterior symmetry of the body. Since
reading your article I have made some new impressions from the same
individual, and it is interesting to notice the unchanged character
of the cutaneous furrows.”
Some additional particulars are added in the letter, and a fine finger
imprint was enclosed.
It is well to remember that the comparison of the ridges to those of a
ploughed field does not always, and in every way, hold good. As I have
elsewhere said:[C]
“The lines are not of uniform width. Ofttimes they may be likened
rather to the mountains and valleys in a good survey. The ridges
sometimes split or send little spurs down into the neighbouring
valleys; at other times a ridge seems to cleave, giving rise to
a form like a tarn or lake in a limestone range: here and there
solitary islands rise in the valleys, and sometimes quite an
archipelago takes the place of some of the commoner patterns.
Indeed, the ordinary nomenclature of an ordinary physical geography
map may be found quite helpful in laying a case clearly before a
magistrate or a jury. And just as we find in the case of mountains
and valleys in a map, every variety of shape may occur in a
finger-pattern.”
[C] _Guide to Finger-Print Identification_ (p. 11).
Here it may be as well to state, as we shall see more precisely further
on, that an English jury is well enabled to judge of the conformity of
two patterns, one of which is suspect only, and the other officially
printed from the fingers of some one in custody--by great photographic
enlargement of the exhibits in the case, used as evidence.
The ridges, as may be seen by an enlarged photograph (as on
frontispiece), do not always continue to be of quite uniform width
throughout. Sometimes they taper away sharply like a railway point, or
trickle off in diminishing dots; or again, especially where something
like triangles occur, called deltas (after the Greek letter, Δ
_delta_), they flatten out in breadth considerably. In old age they are
found usually to have partaken of the general drying up and shrivelling
of the tissues.
In the cold or shivering stage of ague and fevers, and in the affection
called Reynaud’s disease, in which the fingers may tend to become pale
and bloodless, some slight shrinking of the ridges also takes place,
a point which might be of importance in the measurement of enlarged
exhibits in the trial, for example, of an old Indian soldier or
traveller who had been subject to fits of ague.
I have heard Sir A. Moseley Channel, who has informed himself well
about finger-print matters, in a charge to a jury in a murder case,
refer to the doubtful and unsatisfactory nature of evidence from a
print done by a sweaty finger.
The fact that sweaty finger-marks have been adduced in evidence of
crime makes it important for lawyers, police officials, judges and
jurymen, to understand what is meant by such natural records. A mark
from pure sweat would necessarily be excessively transient, as it
consists chiefly of water and salines, and should properly contain no
greasy matter whatever. Dr. Reginald Alcock, of the North Stafford
Infirmary, in a recent paper read at Stoke-on-Trent, and since
republished in _The British Medical Journal_, described his researches
into the relation of the sweat-pores to practical surgery, and to the
recognized difficulty in sterilising the skin for subsequent operation.
Dr. Alcock shows that there may often be found remaining, after the
best efforts to cleanse the surface, a stubborn residue of live and
obnoxious matter in those tiny invisible ducts, matter which had
insidiously gained entrance from without. Now such decaying or dead
particles of foreign protoplasm would, I think, readily enough account
for the very faint traces of oily matter sometimes observed, which
oiliness makes sweat from a skin, fair and clean in the ordinary sense,
leave slight but somewhat persistent traces on such substances as glass
and the like.
In a case reported some time ago, in _The Birmingham Post_,
Detective-Sergeant Charles Munro, on cross-examination as to a sweaty
smudge left on glass, said: “The impressions on the window-pane were
sweat-marks. They had conducted experiments in Scotland Yard, and
ascertained that sweat-marks lasted on glass for a week if not exposed
to the wind.” Here, I suppose, the distinction between a sweat-mark
proper and a somewhat greasy sweat-mark was not discerned. Even a
deliberately designed greasy mark is volatile to a certain extent just
as the oil of new paint dries in a day or two according to the weather.
In the _Guide_ (p. 65) I have alluded to the fact of coloured sweat or
_Chromidrosis_, thus:--
“A blackish ooze takes place in some hysterical cases. More
striking is the class of cases in which the colouring matter is
derived, like the bright colours in the plumage of parrots, from
copper, and in some cases from iron. Workers in copper have been
found subject to it. The sweat is generally of a bluish colour in
those cases. Red sweat has been observed in lockjaw. A kind of
saffron colour I have found to be not very uncommon in some classes
of malarious cases. One lady I attended had an extraordinary
temperature during some of the attacks, the thermometer recording
110° Fahrenheit. With a temperature of about 104° Fahr. she did not
seem to be really unwell. I took good impressions at one of those
times, with the yellow-coloured sweat. Ordinarily, however, sweat
does not help, but hinder, impressions from being made. A case of
blue sweat came under my treatment quite recently. There was no
history of copper poisoning.”
Since writing the above, I have met with other cases of coloured
sweat. My teacher, the late Sir Thomas McCall Anderson, in his work,
_Contributions to Clinical Medicine_, mentions some very interesting
facts in this connection in the chapter on “Hemidrosis.”
Herbert Spencer, in the May number of the _Nineteenth Century_ (1886),
discussing the Factors of Organic Evolution, explains the origin of the
ridges in a passage which I must quote in full:--
“Continuous pressure on any portion of the surface causes
absorption, while intermittent pressure causes growth: the one
impeding circulation and the passage of plasma from the capillaries
into the tissues, and the other aiding both. There are yet further
mechanically produced effects. That the general character of the
ribbed skin on the under-surfaces of the feet and inside of the
hands, is directly due to friction and intermitten pressure, we
have the proofs: first, that the tracks most exposed to rough usage
are the most ribbed; second, that the insides of hands subject to
unusual amounts of rough usage, as those of sailors, are strongly
ribbed all over; and third, that in hands which are very little
used, the parts commonly ribbed become quite smooth.”