is carried off along with the placenta, and
constitutes its external membrane, the rest
remains attached to the inner surface of the
uterus. This layer serves as a medium by
which the uterine arteries (fig. 484. a a) and
veins pass from the uterus into the placenta.
Numerous valve-like apertures are observed
upon all parts of the surface. They are the
orifices of the veins which have been torn off
from the uterus. A probe passed into any of
these, after taking an oblique direction, enters
at once into the placental substance. Small
arteries, about half an inch in length, are also
everywhere observed embedded in this layer.
After making several sharp spiral turns, they
likewise suddenly open into the placenta.
These are the uterine vessels, which convey
the maternal blood to and from the interior of
the placenta.
Circumference. The margin of the pla-
centa is bordered all round by the united
membranes which enter into its composition.
Here the amnion and chorion, after lining the
fetal or concave surface, come into contact
with the decidua which covers its uterine face,
and the three membranes then pass off to-
gether to enclose the liquor amnii and foetus.
At this part the decidua is always most dense.
PLACENTA (NORMAL ANATOMY).
Partly within its substance is formed an in-
complete sinus, the circular vein or sinus.
This constitutes an interrupted channel, which
more or less encircles the placenta. Several
orifices are observed in its walls. Some com-
municate directly with the interior of the pla-
centa, and others with the uterine sinuses.
717
Substance. When a clean section has been
made through the placenta (fig. 484.), the two
surfaces already described are observed to
enclose between them a soft spongy sub-
stance, which is made up principally of count-
less ramifications of the foetal villi. These
are attached at their base to the chorion, from
Fig. 484.
Vertical section of the walls of the uterus with the placenta attached. From a woman in the thirtieth week of
gestation. (After Wagner.}
The length of the lines uu, serves to distinguish the uterus; p, the placenta, and dd, the decidua. To
the right of the figure the decidua is separated into ud, uterine decidua, and dp, decidual prolongations
which form the dissepiments dividing the placenta into lobes ; f, funis ; am, amnion ; ch, chorion ; vf,
foetal blood-vessels (divided) upon the surface of the placenta; vv, villi ; us, uterine sinuses; a a, curling
arteries in the substance of the uterus.
which they spring, while their opposite extre-
mities are united to the decidual layer form-
ing the uterine boundary of the placenta.
The interspaces left between the villi and
their ramifications form what have been
termed the cells of the placenta. They are
widest between the roots of the villi, and
much smaller between their extremities. In
these spaces the maternal blood circulates.
When injections are thrown into the placenta
from the uterine arteries or veins, these spaces
become filled, and the mass, when broken,
exhibits a peculiar granular appearance. Dip-
ping down among the villi, and reaching in
some cases as far as the foetal surface of the
placenta, are numerous sheet-like prolonga-
tions of decidua (fig. 484. dp). These con-
stitute the dissepiments which separate the
entire mass into its several lobes or cotyle-
dons. At the placental margin, the decidual
layer generally dips under the villi, forming a
return end or border, which is directed in-
wards, and is attached at a distance of 3 4"'
from the margin to the outer surface of the
chorion. The exact relation of the decidua
to the villi, in various parts of the placenta,
will be better understood after a more minute
description has been given of each of these
structures.
The tufts and villi. A placental tuft
has been often compared to a tree. It con-
sists of a trunk giving off numerous branches,
which ultimately end in finer subdivisions
or villi (fig. 484. vv and fig. 485. a). The
trunks may be said to take root in the cho-
rion, from which they spring, while the
branches and finer subdivisions spread la-
terally and upwards, until they come into
contact, at their sides, with the adjacent tufts
and villi, and above with the decidua which
bounds the placenta towards the uterus.
Many of the villi, instead of branching like
trees, proceed thread-like from the floor to
the roof of the placenta, only sending off
short knotty side branches. The tufts are
so closely set, that their forms cannot be
readily discerned until they are floated out
in water. The stems are tough and fibrous,
or coriaceous, while the branches and finer
villi, though strong, are of a more brittle
texture. When one of these is broken offj
and examined by the microscope, it presents
the following characteristics the subdivi-
sions are abrupt, contorted, and singularly
718
UTERUS AND ITS APPENDAGES.
devoid of symmetry ; from all parts of their
surface spring numerous short pullulations,
which render them knotty and uneven.
Every villus is composed of two distinct
parts, viz. an outer leathery sheath, and an
inner softer and vascular structure, which is
contained within the former like a finger en-
cased in a glove. The distinction between
these two structures is not easily observed,
except in parts where the outer sheath has
been accidentally broken off, leaving the more
pulpy internal substance exposed. Or in cases
where the placenta has become stale by keep-
ing for a few days, when the inner portion by
shrinking has retired from the end of the vil-
lus, so that a small interspace has been here
left (fig. 485. b).
When a terminal tuft so prepared is viewed
by transmitted light, under slight compression,
the outer case is seen to consist of a trans-
parent non-vascular structureless membrane,
embedded in the substance, or attached to the
inner surface of which are numerous flattened
spheroidal cells, forming generally a single
layer. In the apex of a growing tuft, or
forming a distinct bud projecting from its
extremity, may be often observed a group of
similar cells which appear to be passing off
from a spot in the centre of the mass.* These
cells perform important parts in the growth
and offices of the villi, which will be presently
noticed.
The internal portion (fig. 485. 6) consists
of a soft and pulpy structure which envelopes
the blood-vessels of the villi. In its substance
also are embedded numerous cells of a similar
nature to those observed in the structure-
less sheath.
Termination of the foetal vessels. The ar-
rangement and terminal divisions of the blood-
vessels within the villi varies considerably
according to the age of the placenta. The
following distribution is observed from the
third to the sixth month (fig. 485. a). Each
villus contains one or more arteries and veins,
together with numerous capillaries. The ar-
teries pass up the centre of the stem, and
divide into branches according to the number
of the terminal subdivisions. Within these
the branches split up into numerous capilla-
ries, which present various forms of arrange-
ment, in some parts resembling Malpighian
bodies, and in others the arrangement of pul-
monic capillaries. From these capillaries the
blood is collected by veins which pass back
through the tufts accompanying the corre-
sponding arteries. All these vessels, with
their subdivisions, are enveloped and sup-
ported by the pulpy granular substance that
forms the interior of every villus (fig. 585. b).
Towards the end of pregnancy, the true
capillaries of the villi gradually disappear, so
that in a placenta at term the blood-vessels
present the condition accurately described by
C. H. Weber and Goodsir. A single vessel
generally enters each terminal tuft, and after
* J. Goodsir. Anatomical and Pathological Ob-
servations, 1845.
Fig. 485.
a, terminal villus of a foetal tuft, from placenta
of six months. The arteries, veins, and capillaries
are minutely injected. The latter, which disappear
towards the end of gestation, are here very abun-
dant. The arteries and veins occupy the centre, and
the capillaries the surface, of the tuft, immediately
beneath the non-vascular sheath. The nucleated
non-vascular sheath is shown at b, separated from
the internal softer structure in which the vessels
ramify. (Ad Nat.")
forming an open loop, it returns again, either
dividing within the villus, or leaving it as it
entered. Or a single vessel may enter, and
retire from two or more villi, before it termi-
nates in a principal vein. Many modifications
occur in the forms of the loops, which may be
simple, compound, wavy, or much contorted,
and in parts varicose.*
Such, then, are the structures belonging to
the fcetus which are brought into contact with
the maternal blood in the interior of the pla-
centa, viz. the portion of chorion that forms
the floor of the placenta, and the tufts or villi
which spring from its surface. The office of
the former is simply mechanical in confining
the maternal blood to its proper course, and
preventing rupture of the organ ; the latter
constitutes the potential portion of the pla-
centa.
On the other hand, the sole parts belonging
to the mother, the existence of which can be
anatomically demonstrated in the substance of
the placenta, are formed out of the decidua.
The decidua. A general description of this
membrane, as it forms the roof of the placenta,
and sends off dissepiments into its substance,
has been already given. It only remains to
* These are the only terminations of the fcetal
vessels of the placenta which have been hitherto
described. The true capillary system disappears
towards the end of gestation, and apparently, on this
account, has escaped the attention of observers, as
far as I am aware, except Schrceder van der Kolk,
who, in his recent work, has described and figured
them in a placenta of three months. Scanzoni also
(Lehrbuch der Geburtshilfe,^. 99.) reproduces the
figure of Meckel and Gierse, in which the capilla-
ries have evidently been injected ; but this is given
as an example characteristic of a dropsical placenta,
and not as representing a normal state.
PLACENTA (DEVELOPMENT).
719
explain the exact relations of this structure to
the villi, within the placenta. All the extre-
mities of the villi which are sufficiently long
to reach across the placenta from the chorion
to the opposite surface formed by the decidua,
become firmly attached to the inner side of
the latter. This attachment takes place not
by any actual perforation of the decidua, but
by the ends of the villi being simply inserted,
in an early stage of the formation of the pla-
centa, into little shallow pits or cup-like de-
pressions in the decidual substance, into which
they are received, and from which they may
be withdrawn.* In other cases, the ends of
the villi become blended with the decidua, to
which they are apparently fixed, by a growth
of decidual cells. These attachments are for
the purpose of giving strength to the placenta,
and of mechanically supporting the villi. They
take place not only between the ends of the
villi and the decidua forming the roof of the
placenta, but also wherever decidua and villi
come into contact. Hence similar attachments
are also formed between the villi and the septa
or dissepiments (fg. 484. d /?), which divide
its substance into separate lobes. Upon the floor
also of the placenta all round the margin, where
the decidua turns downwards and inwards to
become united with the chorion, and to form
the placental margin, the decidua is found for
a short distance attached to the bases of the
villi. And this arrangement gives to the parts
an appearance as if the decidua had been here
penetrated by the villi, but one which is actu-
ally occasioned by the former having, in the
course of growth, become extended around
the roots of the latter long after these were
first formed. Occasionally also decidual cells
may be found upon the surface of villi, con-
necting together their extremities, or forming
here and there rough irregular belts upon their
stems.
Termination of the maternal vessels. No
extension of the maternal blood-vessels into
the substance of the placenta among or be-
tween the villi, can be demonstrated to take
place. So far as anatomical evidence goes,
the maternal vessels all terminate at once and
abruptly upon the inner surface of the decidua.
The curling arteries, after passing from the
muscular coat of the uterus, obliquely for the
most part, through the layer of decidua which
forms the roof of the placenta, open directly
into the interior of the latter; while the veins
commence by equally abrupt openings which
* The difficulty of understanding the early steps
in the construction of the placenta has arisen from
the belief commonly prevalent, that the ovum on
first reaching the uterus remains upon the outside
of the decidua, and that the villi of the chorion
penetrate its substance or enter the uterine glands
in order to form the placenta. But there is no
actual penetration of the decidua at any period,
except that which consists in the entire ovum
gaining a situation in the interior of this membrane
shortly after its arrival in the uterus. The tips of
the villi at a certain stage, as above described, be-
come superficially imbedded in the walls of the
foetal chamber, which is formed of decidua ; but
this is not a penetration of the decidua, as commonly
understood, but only a means of fixing the ovum. "
conduct through the decidual layer to the
venous sinuses in the uterine walls. These
venous orifices occupy three situations. The
first and most numerous are scattered over
the inner side of the general layer of decidua
which constitutes the upper boundary of the
placenta ; the second form openings upon the
sides of the decidual prolongations or dissepi-
ments, which separate the lobes from each
other ; while the third lead directly into the
interrupted channel in the margin, termed the
circular sinus.
Development of the placenta. The early
steps in the formation of the placenta have
been described in the account which has been
already given of the development of the deci-
dua during gestation (p. 653.). These first
steps consist in the formation out of the deci-
dua of a perfectly spherical chamber, in the
centre of which lies the impregnated ovum.
The surface of the ovum is at this time
covered everywhere by short club-like villi of
equal size. The extremities of these villi are
simply in contact with, but are not as yet
attached to the walls of the containing cham-
ber. Subsequently both the villi and the de-
cidua forming the foetal chamber undergo con-
siderable metamorphoses. Certain portions of
these become intimately united, in order to
form the placenta ; while other portions suffer
retrogression, and take no part in its construc-
tion. The following are the principal features
in these metamorphoses.
Foetal portion. The surface of the ovum
does not long retain the peculiarity just men-
tioned, of being equally covered by villi. Dur-
ing the second month at least, if not earlier,
those villi on the side furthest from the uterus
cease to grow, and in consequence of the
increasing expansion of the ovum become
more widely scattered over this part of its
surface, while those nearest to the uterus
rapidly increase in size and extent, so that
this portion of the ovum soon exhibits a pro-
fuse growth of villous processes, which send
out their ramifications in all directions.
According to Professor Goodsir, the deve-
lopment and growth of the villi proceed from
the groups of cells already described as occu-
pying their bulbous extremities. These swell-
ings on the sides and ends of the villi are
their germinal spots, and are the active agents
in the formation of these parts. The villus
elongates by the addition of cells to its extre-
mity, the cells passing off from the germinal
spot, and the spot receding on the extremity
of the villus, as the latter elongates bj the
additions which it receives from it.
As the villi increase in size, their strength
is gradually augmented by the conversion of
the membrane and cells forming their stems
and larger branches into a tough white fibrous
texture ; while frequently, towards the end of
gestation, calcification is observed to begin
within the finer villi, and to proceed sometimes
to so great an extent that a considerable num-
ber of them become filled up and obliterated
by solid matter. While these changes are
going on in the outer portion of the villi. or
720
UTERUS AND ITS APPENDAGES.
that which is derived from the chorion, im-
portant modifications occur in the interior
structures. Up to a certain period of gesta-
tion, the chorion and its villi contain no
blood-vessels. According to the author last
quoted, blood-vessels first appear in these parts
when the allantois reaches and applies itself to
a certain portion of the interior surface of the
chorion. The umbilical vessels then commu-
nicate with the substance of the villi, and be-
come continuous with loops in their interior.
Those villi in which the blood-vessels do not
undergo any further development, as the ovum
increases in size, become more widely sepa-
rated, and lose their importance in the oeco-
nomy. The villi, again, in which vessels form,
in connection with the umbilical vessels, in-
crease in number, and undergo certain changes
in the arrangement of their constituent ele-
ments. As the blood-vessels increase in size,
the cells diminish in number, but are always
found surrounding the terminal loop of ves-
sels in the situation of the germinal spot.
The injections of Schrceder van der Kolk *
show a profusion of capillaries within the villi
as early as the third month. And at later
periods of gestation, up to the sixth month, I
have succeeded without difficulty in display-
ing, by the aid of fine injections, such an
abundant development of these vessels, as is
exhibited in fig. 485, Before the end of ges-
tation, however, the greater part or all of
these fine capillaries have disappeared, and
the vessels within the villi then show only
the long tortuous varicose loops which Good-
sir has so well described.
Such are the principal modifications which
normally take place during the development
and growth of the foetal portion of the pla-
centa. The changes occurring in the maternal
portion, or that which is supplied by the de-
cidua, are not less remarkable.
Maternal portion. Four principal stages
may be observed in the formation of this por-
tion of the placenta.
The first stage is that in which the decidua
constitutes a perfectly spherical chamber *j*
surrounding the ovum, but having as yet no
structural connection with it (fig. 486.). This
is the condition of the ovum in the early part
of the first month of gestation.
The second stage is marked by the com-
mencing attachment of the villi all round to
the inner surface of the containing chamber,
so that now the ovum becomes fixed, and can
no longer be turned out, except by breaking
off the villi, or drawing out their ends from
the little pits, or anfractuosities, already de-
scribed in the walls of the decidua, in which
they have become embedded. At this period
(latter half of the first month), the decidua
forming the walls of this chamber is suffi-
ciently firm to admit of dissection, and already
there may be traced, upon its inner surface,
* Loc. cit. pi. i.fig. 1.
f For an account of the formation of the foetal
chamber, and of the early steps in the construction
of that portion of the placenta which belongs to the
decidua, see p. 653.
Fig. 486.
Decidua at the beginning of gestation, exhibiting the
foetal chamber in the first stage of its formation.
T/ie ovum, being at this time unattached, has dropped
out of it. (After W. Hunter.}
orifices communicating with canals in the de-
cidua that lead into the uterine sinuses. The
maternal blood already flows freely into the
foetal chamber, and, after passing everywhere
among the villi, is returned into the uterine
veins. Thus a temporary placenta is formed,
which, as in Pachydermata, Cetacea, &c., en-
tirely surrounds the ovum (fig. 487.).
The third stage is the most important. It
marks the transition from the temporary to
the permanent form of the placenta. Coinci-
dently with the increased development of the
villi on one side of the chorion, and their cor-
responding arrest of growth on the opposite
surface, there occurs an increase of the space
between the decidua and the ovum on one
side, and a corresponding decrease of it upon
the opposite side. The increase is always on
the side next the uterus, where the villi are
most abundant, and the decrease upon the
opposite surface, where they are fewest. And
this change continues progressively, until,
upon the bald side of the ovum, the decidua
reflexa and the chorion come into so close
contact that the interspace is obliterated, and
the blood, which formerly flowed freely among
the villi, is now no longer admitted to this
part of the circumference of the ovum ; while,
upon the side which is directed towards the
uterus, a large space is left which now takes
the form of a meniscus. In order more effec-
tually to confine the blood within this limited
space, an increased development of decidual
cells now takes place, which pass off from the
uterine walls, and attach themselves to the
chorion all round the circumference of this
space, and thus is formed the margin of the
permanent placenta. During all this time, the
ovum, by its growth, has been gradually rais-
ing the decidua above and around it, just as
the common integument becomes raised dur-
PLACENTA (FUNCTIONS).
Fig. 487.
721
The formation of the foetal chamber more advanced than in Jig. 486. The ovum is still evenly covered by villi,
the ends of which, about this time, begin to be attached to the walls of the chamber in which the ovum is
contained.
o, ovum ; d r, decidua reflexa, forming the foetal chamber; dv, decidua vera; , uterine walls.
ing the formation of a subcutaneous abscess j
while in proportion as the base of the chamber
becomes extended by the gradual retiring, from
the centre, of the line of reflexion of the de-
cidua, like waves receding from a central point,
so, at the same time, an increasing surface is
produced by the expansion of the uterus itself;
and the layer of decidua here formed, com-
monly termed the decidua serotina, is simply
the mucous membrane reproduced to supply
the place of that which had been consumed
or pushed off in forming the decidua reflexa.
The fourth or final stage consists in the
partitioning of the permanent placenta into
smaller portions or lobes by the extension of
the layer of decidua (serotina) which lies op-
posite to the developed villi inwards at vari-
ous points towards the chorion. In this way
are constructed the dissepiments already de-
Supp.
scribed as bounding the several lobes or co*
tyledons. This partitioning of the placenta
commences about the fourth month.
Thus, during these several stages in the
formation of the placenta, two processes may
be said to be concurrently carried on which
tend in opposite directions a process of
positive enlargement and growth combined
with one of relative retrogression or limita-
tion. For while the bulk of the placenta is
progressively increasing up to the completion
of pregnancy, the relative amount of surface
of the ovum appropriated to it is, on the other
hand, diminished. The entire surface of the
chorion being, in the first stages of develop-
ment, employed as a placenta ; while in the
latter half of gestation, one third of it suffices
for that purpose.
Function* of the placenta. By means of
3 A
722
UTERUS AND ITS APPENDAGES.
the placenta, the blood of the mother is
brought into mediate relation with that of
the foetus. Two currents, the one foetal, and
the other maternal, are continually flowing
into and from this organ, yet in channels so
perfectly distinct that no direct commingling
of the streams can ever take place. Never-
theless, though no passage of the form-ele-
ments of the blood can occur, yet through
the partition- walls which separate these two
currents, all the materials necessary to the
growth of the foetus are conveyed by endos-
motic processes, and all the changes necessary
to the respiration of the foetus, and to the
elimination of effete materials, are effected.
The mechanism by which these ends are