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chanters. Pressure epiphyses are the first to ossify, their centres appearing
in the order of their functional importance ; they are always fitted to the
shaft by a species of dovetailing to withstand dislocating forces. The




isch. tub



A.



B.



0.



Fig. 479, A. — The Epiphyseal Cartilage of the Pubis and Ischium, which arises from
the Median Cartilage of the Pelvic Girdle. (Parsons.)
B. — Traction and Pressure Epiphyses on the upper extremity of the Femur.
C. — The Epiphyses of the Olecranon : a, the usual Epiphyses ; b, occasional
Epiphyses : both a and b may be present. (Fawcett.)

upper extremity of the shaft of the humerus projects as a three-sided pyra-
mid within the epiphysis ; Professor Arthur Thomson has shown that the
lower end of the shaft of the femur is fitted within its lower epiphysis by a
number of projections not well marked in the human bone but pronounced
in those animals which maintain the knee in a flexed position. Epiphyses
are mammalian characters ; their rudiments are to be seen in reptilia.

The great trochanter is the traction epiphysis of the gluteus medius
and minimus ; the small trochanter, of the psoas and iliacus ; the third
trochanter, in which a centre appears in the 20th year (Dixon), that of the
gluteus maximus.

As examples of atavistic epiphyses. Parsons cites the following : those
of the ischium and pubis (Fig. 479, A) from the median pelvic bar (Figs.
442, 479) ; the coracoid process ; the epiphysis on the os calcis, the scale-

^ The account given by Parsons has been followed. See Journ. Anat. and Physiol.
1903, vol. 37, p. 315 ; 1904, vol. 38, p. 248 ; 1908, vol. 42, p. 388. R. L. Moodie,
Amer. Journ. Anat. 1907, vol. 7, p. 443 (Reptilian Epiphyses). A. Kirchner, Anat.
Hefte, 1907, vol. 33, p. 513 (Epiphyses of Os Calcis and 5th Metatarsal). T. Walmsley,
Journ. Anat. 1919, vol. 53, p. 326.



MORPHOLOGY OF THE LIMBS 461

like epiphysis of the olecranon (Fig. 479, C). The internal and external
condyles of the humerus may be derived from sesamoid ossifications,
such as are now seen in the patellae, in the tendons of the popliteus, outer
head of gastrocnemius (occasional), peroneus longus, tibialis posticus and
at the metacarpo-phalangeal joints of the thumb and great toe. The
patella is usually regarded as a sesamoid, but Mile. Bertha Vriese collected
evidence to show that it is really a true morphological skeletal element.^

Lines of Pressure and Tension oJ Bones.— The trabeculae, in which
the bony matter is deposited by the osteoblasts, are arranged so as to
withstand the forces to which the body is subjected. When a bone, such
as th'e astragalus, rib or neck of the femur, is laid open by a section, the
trabeculae appear to form straight lines or septa which converge and meet
at various angles ; when, how^ever, such bones are examined stereoscopic-
ally with the X-rays, the trabeculae are seen to be arranged in a double
spiral — one system twisting from right to left, the other from left to right
(Haughton and Dixon). ^ By this means, the greatest strength is obtained
with the least expenditure of material.

Split Hand and Foot. — The extremities are subject to a remarkable
series of malformations, which apparently represent arrests of their
development. The digits may be abnormally short (brachy-dactyly),
owing to an arrest in the differentiation of the blastema of the phalanges,
the terminal phalanx being unseparated from the middle.^ Besides errors
in the separation of the phalanges, there is an arrest of growth — usually in
th^ middle phalanges, while, as Dr. Drinkwater has shown, extra phalanges
may be intercalated. This is of frequent occurrence in the fifth digit of
the foot. In another series of cases the hand or foot appears as if cleft
— an appearance due in some cases to the fact that three or more of the
digits on the ulnar side of the hand or fibular side of the foot have remained
joined or webbed, as in the embryo of the 2nd month, while in others the
condition is due to a splitting or dichotomy of the terminal plate of
the limb bud. The condition is hereditary.* In more extreme cases the
digits on the radial, or more rarely, those on the ulnar side of the hand,
may be absent ; the corresponding bone of the forearm or leg is also
undeveloped. Such cases lead one to suppose that the two distal segments
of the limbs are developed from a radial and ulnar or tibial and fibular
buds, and in such cases only one of these has been affected. Both may
be arrested, the extremities terminating at the proximal segment. In
extreme cases the limb buds are undeveloped.

1 Bertha de Vriese, Bull, de VAcad. Roy. de Sc. Belgique, 1909, March 27th.

2 Dixon, Journ. Anat. and Physiol. 1910, vol. 44, p. 223.

3 A. Fischel, Anat. Hefte, 1909, vol. 40, p. 1 ; J. D. Fiddes, Anat. Anz. 1912, vol. 40,
p. 544 (Supernumerary Hallux) ; J. Symington, Journ. Anat. and Physiol. 1906,
vol. 40, p. 100 (Hyper-phalangism in Cetacea) ; H. Drinkwater, Journ. Anat. 1916,
vol. 50, p. 177.

4 See T. Lewis, Biometrika, 1908, vol. 6, p. 25.



CHAPTER XXVIII.

SKIN AND ITS APPENDAGES.

Stages in the Evolution of the Skin. — We have already seen that the
structures which are developed in the human embryo can be best explained
by supposing that at one stage of evolution the ancestry of mammals
lived and breathed in water. The skin of the human embryo until the
end of the 2nd month of development is translucent, and has many points
in common with that of the lowest gill-bearing vertebrates. It then
consists of two layers — a deep or germinal, consisting of cubical epithelium
and a superficial, made up of fattened cells (Fig. 481). In the 3rd month
this superficial layer, known as the epitrichium or periderm, becomes
horny in nature, recalhng a stage which represents the evolution into a



EPIDERMIC SCALES
HAIR QROUP

(5)



HAIR OROUP

(3)

Fig. 480. — Showing the Arrangement of Hair Groups in the Human Foetus and their
Relationship to Hypothetical Dermal Scales. (Stohr.)

terrestrial form of life. The appendages of the skin — its hair and glands
— appear later ; they seem to be modifications of glandular and sensory
structures seen in the soft skin of amphibia. The hairs are developed in
groups and lines.^ Their arrangement can be best explained, according
to Dr. Max Weber, by supposing that the skin of primitive mammals was
covered by scales, and that the hairs sprouted out in groups at their
tessellated junctions, as in certain living edentates (see Fig. 480). The
human hairs are arranged in irregular series, but in most instances only
the chief hair of a group is developed. In later period of foetal life, however,
the chief hair has one or two subsidiary hairs planted on either side of it
— making one of a group of three or five hairs.

The skin of man, compared to other primates, is comparatively hairless.
We must regard his nudity as a lately acquired character. At the 7th

1 Stohr, VerJiand. Anat. Gesellsch. 1907, p. 153.
462




SKIN AND ITS APPENDAGES 463

month of foetal life the chimpanzee and gorilla have hair only on the
scalp, eyebrows and lips ; the rest of the body is nude, except for fine hairs
or lanugo. This is also the condition in the human foetus at a corresponding
period ; in man, although the foetal crop of lanugo is succeeded by a
general outgrowth of fully developed hair, yet we may regard the human
condition as representing an arrest of hair development at a stage seen in
foetal apes. The human skin is also more sensitive and more richly
supplied with sensory nerves than is the case in other primates. In
Professor Elliot Smith's opinion the rich sensory supply to the skin must
have been a factor in bringing about the large size of the human brain.
In the distribution and " lie " of the hair on his body and limbs man also
resembles the hairy anthropoids.

There are on record a number of cases of men and women, in whom the
whole surface of the body was covered with a close covering of hair. The
development of hair on the face is certainly regulated by a secretion
derived from the sexual organs, for in eunuchs the beard is never developed.
It is also well known that the thyroid has a direct influence on the develop-
ment and growth of hair. Desquamation from the epidermis begins in
the 3rd month of foetal life, and never ceases until death. In a certain
disease of foetal life, named Ichthyosis, desquamation does not take place ;
the unshed epidermis forms cracked cakes on the surface of the child at
birth.

Development of the Skin. — Considerable assistance in the under-
standing of the diseases to which the skin is liable and of the nature of the
growths which arise from the epidermis, such as corns, bunions, and
cancer, is to be obtained by studying the manner in which the skin is
developed. At first the human embryo is covered by a single layer of
epithelium (epiblast or ectoderm), as is the case in the adult amphioxus.
By the end of the 1st month there are two layers, the lower representing
the germinal or basal layer ; the upper the epitrichimn, so named because
it was supposed that hairs are developed beneath it, and when they grew
out in the sixth month this surface layer of flat epithelium was shed. This
evanescent foetal layer is also known as the periderm.

In the 4th month we find developmental processes in full activity in the
skin ; three strata are recognizable in the epidermis — all derived from
the single germinal layer. These are (1) a basal layer — a single stratum
of cubical or columnar cells, representing the primitive germinal epithelium
(Fig. 481, B) ; (2) an intermediate or mucous stratum, several cells deep ;
(3) a heaped-up superficial or corneous stratum, representing the protecting
but perishing superficial covering of the skin. At the same time the opening
phases in the development of hair follicles, sebaceous and sweat glands
and of skin ridges and papillae are to be detected. In the 5th month
the stratum lucidum becomes differentiated between the mucous and
corneous strata.^

The epidermis rests at first on undifferentiated mesoderm, consisting
of small round cells closely imbedded m a mucoid matrix. This is the

^Comparative anatomy of epidermis: see F. K. Studnicka, Anat. Hefte, 1909,
vol. 39, p. 1.



464



HUMAN EMBEYOLOGY AND MORPHOLOGY



normal structure of undiSerentiated mesoderm. The superficial meso-
dermal cells are condensed beneath the epidermis to form a corium towards
the end of the 3rd month ; an areolar or subcutaneous stratum of tissue
is differentiated at the same time. Connective tissue fibrils begin to de-
velop in connection with the mesodermal cells and by the fifth month the
mucoid substance has almost disappeared ; but even in adult life, when the
thyroid body is diseased or removed, a mucoid substance may reappear,
and a condition resembling the foetal state be thus produced. In the
mucous membranes of the lips, anus and vulva, the superficial layer of
epithelium does not become cornified.

Formation of Dermal Papillae.^ — Up to the end of the 3rd month the
epidermis is easily detached from the corium as a flat membrane, but
early in the 4:th month they become more closely united by ridges
of epidermis becoming folded within corresponding furrows on the




epitrich.
germ. I.
dermis.



_^ -_ corneous. I.

.^^'^s?^?^^- mucous. I




A.



B.



Fig. 481, A.



—Diagrammatic Section of the Skin at the commencement of the second
month.

B. — Diagrammatic Section of the Skin at the commencement of the fifth
month, a. a. a. Infoldings of the epidermis between the primary
ridges.

corium. About the ith month, the dermal papillae, which are grouped
in lines and ridges as is well seen in the palm, are formed in the following
manner :

Long, linear furrows of epidermis grow down into the dermis (corium)
and divide its surface into narrow ridges (Fig. 481, B). These ridges
are subsequently subdivided into papillae. The down-growing nature of
the ectodermal (epidermal) cells which is here exemplified, is of the greatest
clinical importance. The enamel organs, we have seen, arose by a species
of downgrowth of the epidermis ; so do hairs, sweat glands and sebaceous
follicles. Prolonged pressure and friction welds the corneous cells into a
solid plate, such as the callosities seen on the palms of manual labourers.
Normal desquamation is arrested ; the cells produced in the deeper layers,
unable to grow to the surface, grow inwards and produce corns. In
cancer, the epithelial cells of the skin renew their youth and invade the
dermis and deeper tissues.

1 For literature see E. J. Evatt, Journ. Anat. and Physiol. 1907, vol. 41, p. 66.
Also paper by Walter Kidd, same volume, p. 35. M. Heidenhain, Anat. Hefte, 1906,
vol. 30, p. 419. 0. Schlaginhaufen, Ergebnisse der Anat. 1905, vol. 15, p. 628. H. H.
Wilder, Amer. Journ. Anat. 1901, vol. 1, p. 423. Walter Kidd, The Sense of Touch in
Mammals and Birds, London, 1907 ; The Initiative in Evolution, 1920.



SKIN AND ITS APPENDAGES



465



Sweat glands arise as buds from tlie ectodermal troughs (Fig. 481, B).
Their ducts open on the surface of the skin in lines or rows corresponding
to the ]:)rimary e})idermal furrows. In the 5th month the epidermis
round their mouths is raised up into ridges, and it is these ridges which
give rise to the papillary patterns on the balls of the fingers and elevations
of the palm. It will be thus seen that the epidermal ridges correspond






Fio. 482. — The more common patterns formed by the Dermal Papillae on the Tips

of the Fingers.
A, The Loop Pattern. B, The Triangle Pattern. C, The Whorl Pattern.

not to the lines of dermal ]3apillae, but to the furrows of epidermis lying
between the papillae.

The papillary lines on the palms and fingers give security of grasp
(Hepburn). They are arranged in most variable patterns, but the pre-
vailing types in man are those arranged as loops, spirals or whorls (Fig.
482). So much does each pattern vary and so variable is the sequence
of the patterns on the pulps of the digits, that no two people show exactly
the same pattern in the same order counting from thumb to little finger





HAND FOOT

Fig. 483. — The " Pad " Elevations on the Palm and Sole of a Human Foetus at the
end of the 2nd month of development. (After Retzius.^)

in both hands. Hence the impress of the ten finger-tips has been success-
fully used in the identification of criminals — a practical discovery made and
put into use by Sir Edward Henry.

These epidermal patterns ^ are formed on elevations which appear on
the human hand and foot at the end of the 2nd month, and which certainly
correspond to the horny pads found on the feet of quadrupeds. Besides
the elevations on the terminal phalanges there are five situated on the
palm and sole at the base of the digits. Three others are situated on the



Biolog. Untersuch. 1904, vol. 11, p. 33, Jena,

2g



2 See references, p. 464.



466



HUMAN EMBRYOLOGY AND MORPHOLOGY



proximal part of the palm. In the human foot the elevation corresponding
to the hypothenar elevation of the palm undergoes a remarkable enlarge-
ment to cover the heel (Fig. 483).

The Hairs.^^Hairs begin to develop in the 4th month, although in some
regions, such as the eyebrows and lips, their formation begins a month
earlier. Morphologically, a hair may be regarded as a dermal papilla
which has sunk in the subcutaneous tissue, and become capped by a
process of epidermis. Hairs appear to have been primarily touch organs,
and are modifications of the touch bodies found in the skin of reptilia
(Gegenbaur). These touch bodies are composed of epithelial cells, having
the same shape and arrangement as those which form the taste buds in
the circumvallate papillae of the human tongue. The cells which cap the
hair papilla evidently represent the primary sensory cells of the touch



EPITRICHIUM



MUC STRAT.

BASAL STRAT.



SEBAC. GL.




BASAL STRAT.

CENTRAL STRAT.

PAPILLA



CENT BAS.CE.LLb



OUTER SHEATH^
INNER SHEATH

PAPILLA



PERIPHERAL
CELLS
BUlb



(A)



(B)



(C)



Fig. 484. — Three Stages in the development of a Hair Follicle. (After Stohr.)

A, Hair Follicle, commencing to form in a Foetus of 3 months.

B, The downgrowth of the follicle and mesodermal thickening to form papilla.

C, Invagination of Follicular Bulb by Papilla with formation of Matrix Cone.

bodies ; they are situated in line, and continuous with the basal or germinal
layer of the skin. The primary function of the hairs as touch organs is
seen in the vibrissae round the mouths of carnivora. Fnedenthal has
found that certain of the hair-roots in the lips and eyebrows of the human
foetus develop the same large sensory bulbs as are found in the roots of the
vibrissae of lower mammals.

The first stage in the development of a hair is the ingrowth of epidermis
as a solid bud, which pushes in front of it the dermis to form the papilla
on which the hair grows (Fig. 484). Only the two deeper of the primary
layers of the epidermis are carried inwards to form the hair sheath and
hair root. Three stages in the development of a hair follicle are shown
in Fig. 484. As the follicle grows downwards the fundus of its shaft

1 See Friedenthal, Zeitschrift fur Ethnol. 1911, vol. 43, p. 974; K. Backmund,
Anat. Hefte, 1904, vol. 26, p. 315 ; P. Stohr, Aiiat. Hefte, 1904, vol. 23, p. 1 ; L. Stieda,
Anat. Hefte, 1910, vol. 40, p. 285.



SKIN AND ITS APPENDAGES 467

expands to form a bulb. Outside the follicular bulb mesodermal cells
collect to become a papilla. Presently the bullj is invaginated by the
papilla, which thus becomes clothed by the central cells of the basal stratum
while the surrounding wall of the bulb is lined by peripheral basal cells.
On the papilla and within the shaft of the follicle is produced a mass of
cells— the hair cone (Fig. 484, C) — the first rudiment of a hair. The
central cells on the papilla give rise to the cells which form the core or
pith of the hair shaft ; from the peripheral cells arises the inner root-
sheath. The outer root-sheath is formed by the lining cells of the follicular
shaft. As will be seen from Fig. 484, C, a sebaceous gland is produced
from the shaft of the hair follicle, while the erector muscle arises from the
shaft at a deeper level.

The hairs produced at the 4th month are fine in texture (lanugo), and by
the 7th month the whole body is covered by them. The hair roots of tbe
eyebrows, eyelids, and of the lips and scalp are the first to appear. The
production of hair buds goes on until birth, the later buds and hairs being

termin. of corn, layer

.stratum lucidum

rete mucosum nail fold




.corneous layer
-stratum lucidum
nail bed ^'^^^^^ ^"^rete mucosum

Fig. 485. — Diagrammatic Section across a Nail.

thicker and stronger. After birth, new hairs are constantly reproduced
within the sheaths to replace the old. Probably the manner in which
new hairs are produced resembles that of teeth, viz., from processes of the
original bud. Hairs appear first on the head and then on other parts of
the body. The fat in the subcutaneous tissue takes the place of hair as a
heat conserver. Certain sexual hairgrowths appear at puberty on the
face, pubes and axilla. Morphologically, the pubic region represents the
separated axillary regions, and probably th.e explanation of sexual hairs
in the axilla is due to this correspondence, for there is a persistent tendency
towards symmetry of development in the upper and lower extremities.
The primitive mammary ridges, also sexual structures, end at the axilla
and groin.

Nails. — The nails are made up of three strata representing the basal
layer of cylindrical cells, the stratum mucosum and the stratum lucidum
of the skin, the corneous layer being lost after the 4th month of foetal life.
They appear first in the 3rd month as fields of thickened epidermis on the
tips of the digits (Fig. 473), but are afterwards shifted dorsally, carrying
their palmar nerves with them, so that the terminal phalanx is wholly
supplied from the palmar digital branches. At the end of the 3rd month
the germinal layer of epithelium at the proximal margin of the nail field
forms a lamina which grows into the dermis to form the root and is thus
overhung by a reflection of skin — the nail fold (Fig. 486). The nail of the



468



HUMAN EMBRYOLOGY AND MORPHOLOaY



little toe, a digit in a retrograde phase of development, is frequently
shaped like a claw, probably a reversion to a primitive form. The nail
is produced on the scattered papillae (the matrix) at its root. The area of
production is marked by the lunule. On the nail bed, in front of the
lunule, the papillae are arranged in longitudinal rows. If the nail be
pressed, as by the boot, the lateral papillae, under the nail fold (see Fig.
485) are directed downwards, and their epithelial outgrowths follow the
same direction, thus causing ingrowing nail.

About the end of the 7tli month the matrix of the nail root becomes
differentiated, active growth sets in and the terminal margin of the nail
becomes free ; it grows forwards over the corneous layer which covers the
terminal row of papillae of the nail bed. The ridge of corneous epithelium



SUBUNGUAL WELT
(Frog-)




FIBROUS CAP



TERM PHAL:



Fig. 486. — Diagrammatic Section of the Terminal Joint of tiie Digit of a Human
Foetus to show the Cap of the terminal Phalanx and the Subungual Welt.

under the nail-tip represents the central part of the hoof (" frog ") of
ungulates (Fig. 486).

The nail is carried by the terminal phalanges. Professor Leboucq
observed that the tip of the terminal phalanges of the foetus is covered
by a special fibrous cap ^ (Fig. 486), which undergoes ossification directly
from membrane, while the rest of the phalanx is laid down and ossified
in cartilage. The terminal phalanges have thus a special element added
to them for the support of the nail and for the fixation of the terminal bulb
of the digits.

Sweat Glands.^ — In the 4th month solid processes of epidermis grow
into the dermis from the ectodermal troughs and also from the necks of
hair follicles and produce sweat glands (Fig. 481, B). They arise at the
same time and in the same manner as, and often in common with, the buds
of hair roots and sebaceous glands. They are produced within the epi-
dermal ridges, and hence the ducts of sweat glands, as may be seen on the

^ F. A. Dikey, Journ. Anat. and Physiol. 1906, vol. 40, January.
2 P. Diem, Anat. Hefte, 1907, vol. 34, p. 187 ; C. Schoeppler, Anat. Hefle, 1907,
vol. 34, p. 429.



SKIN AND ITS APPENDAGES 469

palms and fingers, open along the summits of these. The sweat glands in
the axilla arc peculiar. In section they resemble the acini of the mammary
gland, also believed to be highly modified sweat glands. The axillary
glands contain much epithelial debris. They appear to be sexual in
nature. The wax glands of the external auditory meatus are also modified
sweat glands.

Sebaceous Glands. — The sebaceous glands are outgrowths from the
more superficial part of hair buds (Fig. 484). Their epithelial lining is
derived from the germinal layer. In hair sheaths which have become
occluded after their hairs have been shed or lost, or when the mouth of a
gland is blocked, the secretion is retained, and a sebaceous cyst or wen,
so frequently seen in the scalp, is produced. Round the mouth, on the
lips and nose, the sebaceous glands, especially in disorders of the sexual
organs, are apt to retain their secretions and become inflamed, smaU
pustules being thus produced. The Meibomian glands in the eyelids are
modified sebaceous glands. At birth the child is covered by the vernix
caseosa, which is composed of desquamated corneous epithelium and the
secretion of sebaceous glands.



MAMMARY GLANDS.

Evolutionary History. — It is a remarkable fact that although the milk
glands do not come into use until adult life and although they must be
regarded as among the later evolved structures of vertebrate animals,
yet they are the first of all the glands arising from the epidermis to appear
during development of the embryo. In the human embryo of the 6th
week or in the corresponding stage of a pig (Fig. 487), or of any other
mammal, the primary mammary ridge or milk line — a mere surface thicken-
ing of the ectoderm — is seen extending along the body wall on either side
from axilla to groin. Breslau ^ regards these primary ridges as representa-
tives of the brooding organs of the ancestors of mammals, from which
structures he supposes that the mammary glands were evolved. In a



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