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THE HUMAN STERNUM

THE HUMAN STERNUM

THREE LECTURES

DELIVERED AT THE

ROYAL COLLEGE OF SURGEONS, ENGLAND

NOVEMBER, 1903

BY

ANDREW MELVILLE PATERSON, M.D.

DERBY PROFESSOR OF ANATOMY IN THE UNIVERSITY OF LIVERPOOL
HUNTERIAN PROFESSOR AT THE ROYAL COLLEGE OF
SURGEONS OF ENGLAND

PUBLISHED FOR

THE UNIVERSITY PRESS OF LIVERPOOL
WILLIAMS & NORGATE

14 HENRIETTA STREET, COVENT GARDEN, LONDON
1904
U

, a
At the University Press of Liverpool

No. 53. March, 1904. 250

THE MEMORY OF
MORISON WATSON

M.D., EDINBURGH

AT ONE TIME PROFESSOR OF ANATOMY IN THE
OWENS COLLEGE, MANCHESTER

PREFACE

HERE are so many features of interest to the anatomist, morphologist,
and embryologist, associated with the subject of this memoir: so
much has been written about the sternum in detached papers: one’s

conclusions from a prolonged study of the subject agree so little with
accepted notions of its morphology and development: that it seemed
desirable to gather up the facts at one’s disposal, and present one’s ideas in
book form rather than a series of separate papers. At the same time, it is
recognized that the following pages present no complete account, and form
by no means a survey of the entire field.

Having been collecting materials for many years, I have to express
my thanks to many friends, associated with hospitals in Dundee, Edinburgh,
and Liverpool. For the personal observations I have made on the
comparative anatomy of the sternum, I am greatly indebted to Professor
Stewart, of the Royal College of Surgeons of England, and to Dr. H. O.
Fores, of the Liverpool Museum, for the facilities which they have
afforded to me for the examination of the collections under their charge.

I have also to express my grateful thanks for the valuable assistance
rendered in different ways by Mr. Doucias-Crawrorp and Mr. Epwarp
Wuitzey in the preparation and publication of this memoir.

A. M. PATERSON

Tue University oF LIveRPOOoL
March, 1904

CONTENTS

Preface
Introduction

I, Development and ossification
A. General considerations

B. Early development
(a) Fishes ...
(6) Amphibia
(c) Reptiles...
(ad) Birds
(ec) Mammals
C. Ossification
(a2) Before birth
(6) After birth
1. Presternum
Sternal cornua
2. Mesosternum ... :
Number of elements
Fusion of elements...
3. Metasternum ...
(c) Union of mesosternum with presternum and metasternum
1. With presternum
2. With Metasternum
(d) Summary

II. The form of the adult human sternum...

A. Length and Breadth ...
(a2) Age differences...

(6) Sex differences...

PAGE

II. The form of the adult human sternum—continued

B. Variations in form
(2) Suprasternal ossifications
(6) Sternal foramina
(c) Asymmetry
(d) Metasternum

III. Comparative anatomy and morphology of the sternum

(A) Fishes
(B) Amphibia dee
(C) Reptiles... hee ae

Dine a. de ae ae
(E) Mammals
IV. ‘Teratology
Conclusions
Tables oe a ee
Bibliography

Explanation of Figures

PAGE

39
39
42
43
47

49

49
51
51
52
52
61
62

67

THE HUMAN STERNUM

INTRODUCTION

HE subject of this memoir furnished the material for three Hunterian
Lectures delivered at the College of Surgeons of England, in
November, 1903. In preparing the lectures, one was forcibly struck

with the difference between our outlook on anatomical problems and the
view which Hunter took. He was essentially practical, teleological, and
a strict adherent of nature. All his works are strikingly illustrative of the
correlation of structure and function, and his writings are singularly devoid
of transcendentalism. While he invariably takes a broad and comprehensive
view of any anatomical problem, his investigations are never disturbed or
warped by the influence of nebulous hypotheses. He had the insight, the
imagination of the investigator, but it was controlled by sanity and common
sense.

Hunrer’s writings, further, do not contain a single suggestion bearing
upon the subject of these lectures. The difficulties that we are now grappling
with had not then arisen. He never uses the work ‘Morphology,’ and
homologies had not then acquired the importance that they now possess.

VERTEBRAL SEGMENTATION

The sternum is as good an illustration as any part of the animal
economy of the tendency of modern anatomical thought, which, essentially
morphological and architectural, has been influenced by views of vertebral
segmentation to regard the sternum asa part of the skeleton intrinsically
associated with the costal arches.

It is conceivable that we are going too far in our adherence to the idea
of segmentation of the vertebrate body. In the construction of the skull,
and the relation of the limbs to the trunk, evidence may be adduced to

B

2 THE HUMAN STERNUM

show that the notion of segmentation may be stretched too far. And even
in regard to the trunk itself, where segmentation is most obvious, each
particular segment has its specific differences. Segmentation, indeed, may
be looked upon as an architectural device utilized for what it is worth, both
in invertebrates and in vertebrates, and modified and discarded where it
would be hurtful. Modifications of segmentation occur in the formation
of the sacrum, in the cervical vertebrae of cetacea; and there is no real
segmentation in the urostyle of the frog, or in the mammalian basis cranit.

Segmentation, as Herserr Spencer points out, is essentially con-
nected with progressive movement. The elongated worm-like organism
for purposes of progression becomes segmented, on account possibly of
alternate stress and strain produced by the attempt at a wriggling move-
ment. Where stability and fixation are required segmental features are
modified or absent.

In a mammal, the thorax presents the most complete and characteristic
example of segmentation, by the association together of the vertebrae,
ribs, and sternum. In other regions the costal elements are rudimentary
or absent, and it is almost certain that no element comparable to the sternum
exists elsewhere in the mid-ventral line of the trunk.

In such an animal as the ant-eater (Myrmecophaga jubata, Pl. IX,
Fig. 63), the sternum consists of presternum, mesosternum, and meta-
sternum. The mesosternum is composed of eight separate median pieces,
which regularly alternate with costal attachments.

In most other quadrupedal mammals the general structure of the
bone is the same, but (e.g., jackal, Pl. IX, Fig. 64) there is a failure in the
segmental alternation of mesosternal and costal elements ; and the costal
cartilages of the last two sternal ribs unite at the same point with the
junction of mesosternum and metasternum.

Again, in reptiles, birds, and certain mammals (notably bipeds) the
same three elements are present—presternum, mesosternum, meta-
sternum—but the mesosternum here consists only of a single bone, or
fewer pieces, without any proper segmental relation to the ribs, but afford-
ing by its lateral margins attachment for a certain number of costal
cartilages.

THE HUMAN STERNUM 3

Lastly, by a study of comparative anatomy of lower forms, we are
faced with the presence, in certain reptiles and amphibia, of a sternum which
is not connected with ribs at all, but is associated with the shoulder-girdle
only ; and in fishes, with one exception, an entire absence of the sternum,
although there is a complex development of both shoulder-girdle and ribs.

The commonly accepted view of the morphology of the sternum is
that it is made up of ‘sternebrae,’ derived from the costal cartilages ; and
that each ‘ sternebra,’ with a pair of ribs and a vertebra, constitute a typical
and complete body segment.

The more one examines the subject (and even admitting the attrac-
tiveness and plausibility of this view) the more one feels that it is founded
upon an insufficient basis, and presents such unsatisfactory features, that one
is forced to enquire for some other hypotheses for the formation of the bone
which will more adequately account for its special characteristics, and for
the differences which it presents throughout the vertebrate series.

Apart from any hypotheses or theoretical considerations, the sternum
is a bone which occurs in air-breathing animals possessing four limbs; and
its chief modifications are dependent upon differences in the mode of
progression. It is the central bone of the thorax, comparable in that way
to the basis cranii, an adjunct of the shoulder girdle, of prime importance
especially in bipeds, and an essential factor in the construction of the osseous
respiratory chamber. At the same time it forms a platform or a shield for
the heart, and serves important mechanical purposes. Muscles radiate from
it for the neck, fore limbs, thorax, and abdomen.*

The main thesis which I have to present is that there is insufficient
evidence of the origin of the sternum from the rib elements, and indeed
that the association of sternum with costal cartilages is a secondary one, and
that the primary condition of the sternum is one in which it is in association
with the shoulder-girdle. One does not claim originality for this view. It
has been already foreshadowed by T. J. Parker, and Professor CLeranp
apparently does not accept the view held by most anatomists.

* One may draw particular attention to the correlation of the infra-hyoid muscles and the diaphragm,
and the fact of their associated innervation, with the intervention of the sternum : and also to the presence of the
internal mammary artery, and its prolongation downwards to the abdomen.

4 THE HUMAN STERNUM

Professor CLeLanv’s position as one of the most independent and
philosophical anatomists of the present day, entitles any view which he
expresses to weighty consideration. ‘The sternum,’ he says, ‘ there can be
no doubt, is at first laid down in the form of two lateral slips at right angles
to the tips of the costal cartilages. It is impossible, however, to refer these
slips to prolongations from the costal tips. Judging from the appearance
in the third month, the manubrium may be originally continuous with
the first pair of costal cartilages, but the mesosternum takes its origin distinct
from costal cartilages, and the xiphisternum is at first separated from the
mesosternum by the sixth and seventh costal cartilages meeting in the
middle line.

‘The origination of the sternum in lateral parts explains the well-
known case of M. Groux, in whom the two halves remained ununited,
and could be pulled separate by the great pectoral muscles when the hands
were clasped. Less complete division also occurs. The original distinct-
ness of the lower parts of the sternum from the costal arches may be well
kept in mind when comparing it with the chelonian plastron.’ *

This is the main idea that I have to bring forward. The studies into
which one has been led in this investigation have, however, brought to the
front several minor questions of some importance to which one may be
allowed to draw attention in due course.

Fissura STERNI

One aspect of the subject has not been personally studied—teratology
of the sternum; so that it may be contended that thereby one of the
strongest arguments (namely, the existence of a sternal fissure) in favour of
the bilateral and costal origin of the sternum is evaded. Now the occurrence
of fissura sterni has been one’s greatest difficulty in an a priori acceptance of
the orthodox view of the origin of the bone. From a study of the literature
of the subject and from searches in museums, one finds it an extreme rarity.
The museum of the Royal College of Surgeons does not possess a specimen.
If the sternum is costal in origin this ought surely not to be so. All the
factors engaged are tending, during the process of growth and ossification,

* Cleland and Mackay, Text-Book of Anatomy, pp. 130-131.

THE HUMAN STERNUM 5

to expand the chest well and keep the halves separate. The expansion of
the thoracic viscera, and the tendency of the curved ribs to straighten them-
selves, should offer every inducement to the occurrence of a cleft sternum.

On the other hand, fissura sterni may possibly be explained on other
grounds. May it not be caused by an abnormal ratio of growth of the
thoracic viscera and the chest walls at an early period, which may cause a
stretching of the embryonic pre-chondral tissue out of which the sternum is
normally produced? This in consequence does not chondrify or ossify,
but instead gives rise to the membrane described as connecting together the
two hemisterna or the ventral ends of the sternal ribs, in cases of fissured
sternum. In the embryo chick, where the sternum is in two halves at first,
there is a genuine ectopia cordis, and the two halves of the sternum are
united by a thin layer of embryonic mesenchyme.

6 THE HUMAN STERNUM

I. DEVELOPMENT AND OSSIFICATION OF THE
STERNUM

A, GENERAL ConsiDERATIONS. THE MATERIALS AND THE PROcESS

One of the fundamental difficulties in the study of the growth of all
mesoblastic tissues arises from the gradual alteration of the primitive
embryonic cellular tissue into more complex tissues—connective tissue,
cartilage, and bone. At first there is a simple uniform tissue, in which
and out of which, in definite areas, conglomerations of cells occur so as to
produce a clearly demarcated mass in the situations in which, for example,
vertebral column, ribs, sternum, and shoulder-girdle are to arise. Out of
this mass of cells are produced connective tissues, cartilages, and bones.
The usual mode of proceeding is the conversion of the cellular mass into
cellular and afterwards hyaline cartilage, and in that, either by a perichondral
or an endochondral mode of ossification, the cartilaginous model is converted
into bone.

Besides the histological changes, there are also fundamental chemical
differences to be remembered in the conversion of this embryonic tissue
into its more stable and permanent form ; a gelatinogenic change in the
formation of fibrous tissue, a chondrinogenic change in the formation of
cartilage, and an osteogenic change in the formation of bone.

In exceptional cases (e.g., lower jaw, vault of skull, and clavicle in part),
the chondral stage is omitted and a process of periosteal or membrane bone
formation takes place before any cartilaginous an/age can appear.

It is important in connexion with the development of the sternum
particularly to be conversant with the pre-chondral stage in its formation,
as by a recognition of this it can be demonstrated that the shoulder-girdle
is associated with it, that the girdle becomes cartilaginous, while the sternal
anlage is still in the cellular condition, and that the ribs are independent of
the sternum at first, and in mammals become joined to it as cartilaginous
bars, when the sternum itself is a cellular layer. And, further, after
the bone has been modelled out into its definite form in cartilage, the

THE HUMAN STERNUM 7

process of ossification which sets in presents somewhat difficult characteristics.
In man the process is endochondral. In the rat it is perichondral. In
man the ossification of the sternum proceeds in the way characteristic of the
development of epiphyses of long bones, the bodies of the vertebrae
and the basis cranii, The bone is deposited as separate nuclei in the interior
of the cartilage, and replaces it by gradual absorption of the latter, only a
thin layer on the surface being formed as perichondral or periosteal bone.
In the rat, on the other hand, each part (or so-called segment) of the sternum
is produced like a digit, by a cylindrical growth of perichondral bone, which
encloses the primitive cartilage and replaces it.

Much stress—probably too much—has been laid upon the occurrence
of centres of ossification. At the lower end of the humerus there are four ;
at the lower end of the femur only one. One inclines to the belief that in
such cases—and the sternum appears to be an exactly parallel case—the
difference in the two bones named is due to the difference in the forces
applied. In the case of the lower epiphysis of the humerus there are four
forces acting upon it ; the masses of muscles attached to the two epicondyles
and the contact of radius and ulna, In the case of the femur, a single
mechanical force is at work in the pressure of the upper end of the tibia.

So in the case of the sternum. While it has been the subject of con-
siderable research and the most conflicting statements, we cannot regard
the centres of ossification in the sternum as of essential morphological
importance. There are three reasons which may be named in support of
this opinion. (1) They are subject to enormous variation ; (2) the ossifying
process is one which is delayed till a comparatively late period; and (3) it is
not inconceivable that mechanical forces, such as the expansion of the
chest, associated with the growth of the contained viscera, and pressure and
traction due to the attachment of the costal. cartilages, may be the cause of
the excitement which induces ossification.

Another important consideration of a general kind may be mentioned
here. In the attempt to form a clear idea of the individuality of a particular
structure, such as the sternum, what features are to be looked upon as
typical and characteristic? At what stage in its growth are we to compare
it with the corresponding structure in another organism? Is it the adult

8 THE HUMAN STERNUM

bone, or the separate centres of ossification, the cartilaginous model, or
the pre-chondral tissue which is the most important feature in its history ?
Obviously, the only fair picture can be obtained by a composite view. All
these phases in the growth of the bone must be taken into account, and it
is only from a complete knowledge of its development, structure and
variations that we can form a proper notion of its morphological position.

B. Earry DeveLopMENT OF THE STERNUM
(2) Fishes.

Elasmobranchs. In Acanthias vulgaris (Pl. III, Figs. 25, 26), I find that
in young embryos the appearance of the shoulder-girdle and fore limb is
foreshadowed by the occurrence of a conglomeration of cells such as has
been described above, which is ultimately converted into hyaline cartilage.
This stretches across the median ventral line and forms a continuous bar,
without any differentiation into sternum and coracoids.

(6) Amphibia (Anura).

In the frog, the sternum (Marsati”) appears to be developed
subsequently to the formation of the limb girdles. After the approximation
of the precoracoid and coracoid bars, and the formation of the longitudinal
epicoracoid elements on each side, further growths occur anteriorly and
posteriorly. The omosternum is formed in front by the growth of bilateral
processes, as continuations forward of the epicoracoids, which fuse to form
a median cartilage. The sternum and xiphisternum are formed in the same
way by an extension backwards and fusion together of similar bilateral
projections from the hinder end of the epicoracoids.

(c) Repitles.

RatuKke® describes two distinct bilateral strips in Lacerta agilis, which
unite to form the sternum. ‘They are not at first continuous with the ribs ;
and in Anguts fragilis he finds the embryonic sternum further removed from
the ribs in earlier than in later stages, thus indicating a separate origin for
the sternum. The two halves of the sternum fuse together from before
backwards.

THE HUMAN STERNUM 9

Goetre’s observations” differ, to some extent, from Ratuxe’s, but
bear them out in the essential point under consideration. In Cuemidophorus
he found the sternum formed in two triangular halves, each portion being
united at its proximal end with the distal end of the corresponding coracoid,
and united at the lateral border with one rib. Later on he finds three ribs,
and still later four ribs, associated with the sternal border. At the same
time, he expresses his adherence to the view of the origin of the sternum
from the ribs. In Anguis fragilis he finds, as RatuKe did, the sternum in
two lateral halves, and admits that at an early stage the union of the ribs
to the sternum cannot be demonstrated, or else they have already separated
from the sternum. He maintains, however, that the anlage of the sternum
is the distal end of the first rib, which is successively joined by the succeeding
ribs.

(2d) Birds.

In the chick (Pl. III, Figs. 27, 28) the formation of the sternum is
characterized by the presence, at an early stage, of two separate cartilaginous
halves, between which is suspended the heart in the pericardium, with a thin
connecting cellular layer beneath it, joining together the two halves of the
sternum. It is generally described as being derived on each side from the
ventral ends of costal elements (two cervical and five thoracic), of which
the two cervical lose connexion with the half sternum, and the fifth
thoracic ribs become abortive and disappear.

I have made personal observations of the development of the sternum
in the chick, and may summarize my observations here. At the end of
the sixth day one finds no sternum and no ribs. The shoulder-girdle is
present in a cartilaginous form, but the clavicle is not distinct. During
the seventh day the halves of the sternum appear, cellular at first, and
later on becoming cartilaginous. The coracoids are joined to the cellular
sternum at first directly ; afterwards, when the sternum becomes cartila-
ginous, by the intervention of a joint cavity ; the ribs appear, become
cartilaginous, and grow forwards to get connected with the half sternum on
each side. ‘The clavicles are now present and are becoming ossified. The
ninth day shows fusion of the halves of the sternum, and formation of the

keel out of the median border of each half sternum.
Cc

10 THE HUMAN STERNUM

In the chick, during this process of growth, there is a distinct
difference in the development of the sternum and of the ribs. When the
ribs are cartilaginous the sternum is still cellular ; when the latter first appears
the ribs are unconnected with it, and it is only secondarily and after the
sternum becomes cartilaginous that fusion occurs.

The existence of two lateral halves in the chick’s sternum is a feature
which is characteristic. It is to be associated, probably, with the early and
excessive development of the heart in the embryo bird, which has heavy
work to perform in the period of incubation ; and by its excessive size
delays the fusion of the parts, and causes a condition of embryonic
ectopia cordis.

As in the frog, the sternum in its development succeeds the shoulder-
girdle and the ribs, and is in a cellular condition when both of the other
series have become cartilaginous. GorTrez*t mentions that in the chick (four
to five days) the sternal anlage was separate from the ribs.

He regards the avian sternum as formed in two distinct parts, a
costal sternal plate and a clavicular anlage, the episternalia forming the crista
sterni. Both portions are paired and laid down as widely separated halves.
He considers that each half of the costal sternum arises probably from
rib-ends.

HorrmMann® also maintains that in the formation of the presternum
an interclavicular element is present uniting with the costal sternum, and
forming the keel.

Miss B. Linpsay® in a valuable paper, in which, however, the costal
view of the origin of the sternum is unquestionably accepted, describes in
the Ganner the occurrence (first stage) of eight ribs which end freely with
no trace of a sternum. Later (second stage), ‘ the sternum is indicated by
thick opaque aggregations in a layer of mesoblast.’ In the next stage ‘ the
sternum is fully formed and fused with the ribs.’ ‘There are seven sternal
ribs.’ In the fourth stage the lateral halves of the sternum are closed, and
the keel is formed.

She describes more particularly in the ostrich the metasternum and
its processes as a backward extension from the mesosternum.

THE HUMAN STERNUM II
(e) Mammals.

My own observations* upon the early development of the sternum
were made on rat and human embryos.

(2) In aseries of continuous sagittal sections through a human embryo
in the second month, prepared and placed at my disposal by Professor Paut,
the first anlage of the sternum (PI. I, Fig. 1) is to be seen as a dense con-
glomeration of mesoblastic cells, occupying the anterior part of the thoracic
wall in the middle line, and tapering off posteriorly : disappearing as a
separate structure about the middle in length of the thoracic wall.

There is no indication of any bilateral arrangement of this cellular
tissue. The inner end of the clavicle and the costal cartilages are composed
of cartilage. The first three costal cartilages join the cellular median
sternum ; the fourth and fifth cartilages join those above them ; the sixth
and seventh have free pointed ends.

In rat embryos of 9 mm. in length a very similar condition is found
(Pl. I, Figs. 2, 3, and 4). In the angle between the jaw and the thoracic
wall (prae-cervical sinus) there is a conglomeration in the middle line of
mesoblastic cellular tissue; traceable laterally into association with the
shoulder girdle, and concerned with the formation of the clavicles, thinning
off as it passes backwards in the thoracic wall on the ventral aspect of the
bulging heart.

At this stage this cellular mass is not connected with any of the
costal cartilages, which are extending in a ventral direction, and ending
distally in tapering points (Pl. I, Fig. 4). There is, moreover, no indication
of bilateral subdivision of the mass, except at its cephalic end ; there the
cells are consolidated into two horns, which are concerned with the formation
of the clavicles and sterno-clavicular articulations, and with the anterior
parts of the presternum.

(2) In rat embryos, 10 mm. in length, an advance in development has
occurred (PI. I, Figs. 5-10). The sternum is still cellular. The presternum
is single and median (Fig. 5), and at its cephalic end the component
cells are more distinctly massed together on each side to produce the

* Read at the Manchester meeting of the Anatomical Society of Great Britain and Ireland, June, 1900,
and published in the Fournal of Anatomy and Physiology, October, 1900.

12 THE HUMAN STERNUM

sterno-clavicular joints, and tie inner ends of the clavicles. The cellular
mesosternum (Figs. 8, 9) consists of two definitely separate strands of
mesoblastic cells, which are separated in the middle line by a layer of sparser
and more loosely arranged cells. The cellular sternum is now joined by
the first six costal cartilages (the seventh is still separate on each side), but
there is an obvious distinction in the character of the cells composing
sternum and costal cartilages (Pls. J, I], Figs. 11, 12). The eighth costal
cartilages are not in any way related to the sternum, and there is no indica-~
tion of a metasternum.

In rat embryos, 13 mm. in length, the process of development has
perceptibly advanced. Though the sternum is still cellular, and not yet
converted into cartilage, the median separation between the two halves of
the mesosternum has disappeared, and the mesosternum consists of a
single median band of cellular tissue (Pl. H, Fig. 14). The metasternum
(Fig. 15) has appeared ; cellular in character and tapering off in the angle
between the chest wall and the diaphragm. The clavicle in its outer part
is undergoing ossification, and its inner end is cartilaginous. It is clearly
demarcated from the presternum, although no joint cavity has yet appeared
in the sterno-clavicular articulation (Fig. 13). All the seven costal cartilages
are now connected with the sternum ; but there is still an obvious distinc-
tion between the character of the tissues composing the sternum on the one
hand and the costal cartilages on the other (Fig. 14).

(c) The third stage in the development of the sternum is associated
with the conversion of its cellular tissue into cartilage—at first cellular,
afterwards hyaline. The process begins in the presternum in the upper
lateral angles, along with the formation of the clavicle (Pl. I, Fig. 11).
In the mesosternum, cartilage cells first appear at the periphery of the
band, in the intervals between the attachment of the costal cartilages
(Pl. II, Fig. 12). The parts of the sternum opposite the costal attach-
ments remain longest in a cellular condition. In the metasternum,
cartilage formation begins at the anterior end (Pl. IH, Fig. 15).

Ultimately the sternum is laid down—a model of the future bone—
as a simple median band of hyaline cartilage, separated at this stage from
the clavicles by cellular connective tissue, but in complete fusion with the

THE HUMAN STERNUM 13

costal cartilages on each side, and presenting absolutely no differentiation of
its component parts.

Three human sterna of the third month, measuring respectively,
8, 9, and 10 mm., show clearly this condition (Pl. II, Figs. 17, 18, 19).
The demarcation of sternal elements and of costal cartilages is shown in the
special arrangements of the cartilage cells in lines, along which, later on,
the joint cavities will be formed. In all three sterna the sterno-clavicular
joint is completely formed, with its inter-articular fibro-cartilage. But in
only one case is there a definite costo-sternal joint cavity (between the
mesosternum and the second right costal cartilage) (Fig. 18F).

(7) The further changes in the cartilaginous sternum preceding
ossification occurs in rodents (rat, mouse) without cleavage of the parts of
the sternum or separation of costal cartilages. Opposite to the costal
attachments the cartilage is distinctly hyaline, poorer in cells, and stained
less deeply than in the intervals between the costal cartilages. In these
intervals the cells are massed together, stain deeply, and the process of
ossification is foreshadowed by the formation of ‘primary areolae,’ which
occur in the centre of the presternum, in the metasternum, and in the
first four so-called segments of the mesosternum (PI. LII, Figs. 20, 21, 22).

These areolae increase in size, while at the same time the perichon-
drium becomes enormously thickened, followed by the formation of
periosteal temporary bone in the usual way (Fig. 22).

Thus at birth (in the mouse, Pl. III, Fig. 23), while there is com-
plete fusion of the sternal elements together, and of the costal cartilages
with them, there is a distinct differentiation of elements entering into the
constitution of the sternum. Bone formation has begun, and cartilage only
exists at the extremities of the presternum and metasternum, in the
mesosternum opposite the costal attachments, and in the interval between
the attachments of the sixth and seventh costal cartilages (Fig. 24).

These observations do not harmonize with accepted views, which have
been derived from the observations of distinguished and accurate observers,
notably Rucz,” whose monograph on the subject has long been accepted
as having settled the question of the early development of the sternum.
There are several points in that memoir, however, which, on careful perusal,

14 THE HUMAN STERNUM

compel us to hesitate before accepting the conclusions arrived at by the
author as final.

It is extremely difficult to summarize and classify the views of authors
regarding the development and morphology of the sternum. RaTuke,%?
who first described its development in the chick, gives nowhere any sugges-
tion that the sternum is not originally separate from the ribs. Brucu
later describes it as formed independently, and secondarily connected with
the ribs. Ruce,5? Horrmann,”® and others have maintained the opposite
view, that the sternum is wholly derived from costal elements, while
GoetTe™ inclines to a middle view, which appears to me to contain a certain
fallacy and at the same time to possess a certain basis in fact. He holds
that the presternum is partly derived from the shoulder-girdle (clavicle),
partly from the ribs, that the mesosternum is wholly costal, and that the
metasternum 1s a derivative of the mesosternum.

On account of the fact that Ruce’s memoir has been the mainstay
of the exponents of the theory of the costal origin of the sternum, one
has had to examine it with the greatest care. It is an admirable piece of
investigation. But, writing with all deference, I venture to submit that,
granting the accuracy of all the observations made, exactly the opposite
conclusion may be drawn from the memoir ; and it is further open to
observe that most of the writers who have followed Rucz have been more
dogmatic than he is in their statements of Rucez’s observations.

Ruce’s evidence in this matter is of such importance, and his observa-
tions of such material assistance in the elucidation of this problem, that it
is desirable to review a few of his statements.

The points brought out by a careful examination of his memoir may
be stated in the following propositions :—

1. The sternum in the earliest stages examined by Ruce consists of
two longitudinal strips (sternal Jeiste), which may be traced into the
abdominal wall, and there fade away into indifferent tissue without a distinct
line of demarcation. But no statement is made of the character of the tissue
that occurs in the middle line between the two strips.

2. These sternal strips are joined by the distal ends of the ribs.
But the earliest stage shown is one in which, while the three proximal ribs

THE HUMAN STERNUM 15

have fused with it, the succeeding (fourth and fifth) ribs have as yet no
connexion with the sternal leiste. Yet the sternal strip is continuous, so
that either the fourth and fifth ribs have advanced and afterwards receded,
or their connexion with the sternum is a secondary one.

3. At this admittedly early period there is no indication of segmenta-
tion of the sternum, and no proof is given of any growth of the rib ends to
produce it by their fusion.

4. tis shown that the inner ends of the clavicles and the pre-
sternum are in intimate organic association at an early period: the parts of
the sterno-clavicular joint, the ligaments, menisci and suprasternal ossicles,
as well as the upper end of the presternum itself, being differentiated out of
a homogeneous material.

5. The presternum Ruce derives from the cartilages of the first
and second ribs, but he cannot determine that the suprasternal ossicles have
a costal origin.

6. With regard to the formation of the sternum and ribs, he states
twice that there is a difference in the microscopic structure of the two.
Once he states that the sternal leiste appear purely cellular, while (in
the same specimen) the ribs have a considerable amount of intercellular
tissue ; and in another place, speaking of the appearance of the elemental
structure of the cartilage, he states that the rib cartilage contains cells larger
than those of the sternum,

7. The metasternum Rucz derives from the distal ends of the eighth
and ninth costal cartilages, which he supposes have lost a primitive connexion
with the sternum. Albeit, he describes a pre-existing tissue continuous with
the mesosternum, extending into the abdominal wall, and, when first seen,
unconnected with the ribs in question.

There are other points of detail which, in addition to those named,
support my contention that Rucx’s observations can be used to support a
thesis exactly opposite to the one advanced by him.

From his observations one contends that a picture is presented of a
mesial presternum and metasternum, and a mesosternum continuous with
both, but thicker on either side than in the mesial plane. The presternum is
continuous with the shoulder-girdle (coracoid and clavicle); the meta-

16 THE HUMAN STERNUM

sternum projects into the abdominal wall. The ribs secondarily join the
sternal anlage, and are distinctly cartilaginous when they fuse with a cellular
sternum. After fusion, lines of separation occur at the junctions to produce
the costo-sternal joints.

One would not venture to criticize in this way the results of so
eminent an observer if one had not had opportunities of largely repeating
the observations made by him, or if one were not convinced that Minot’s
criticism# is a suggestive one, and that a stage earlier than that at which Rucz
commenced his observations must be examined before conclusions such as
he advanced can be arrived at.

From an examination of embryos, both chick and mammalian, and
from an examination of the observations of others on the development of
the vertebrate sternum, one concludes that the preponderating weight of
evidence is in favour of an independent sternum, associated in its early
development with the shoulder-girdle and acquiring secondary and variable
connexions with the ribs.

C. OsstFICATION OF THE STERNUM

The process of ossification of the sternum occurs late in foetal life,
and is characterized in man by deposits of endochondral bone. Such
diverse statements and so many observations have, at different times, been
made upon the occurrence of those centres, that it will be best to summarize
one’s personal observations first, and then to compare them with those of
others.

If these centres of ossification possess a morphological value, and
are, in any sense, indicative of a metameric formation of the sternum, it is
of prime importance to demonstrate their segmental character.

It is to be noted that when ossification commences, the sternum is
an exact cartilaginous model of the future bone; the presternum may or
may not be separated from the mesosternum by a transverse, fibrous
lamina; and the cartilaginous metasternum is continuous with the meso-
sternum.

The process is most conveniently studied in two stages—before and
after birth.

THE HUMAN STERNUM 17
(2) OssiricaTion or THE Human SternuM BeEForE BirtH

The following facts have been derived from an examination of two
hundred and thirty-six sterna of human embryos between the foetal ages of
three and nine months.

Dates of Ossification.

The approximate periods of ossification in the component parts of
the sternum are shown in the diagram (Pl. V, Fig. 35), which essentially
corroborates the accepted account of the dates at which the several centres
appear. The first nuclei appear, generally, in the sixth month for the pre-
sternum and the first piece of the mesosternum. In the seventh month
centres appear for the second and third pieces of the mesosternum. No
further addition to the number of centres occurs during the eighth and
ninth months ; and in full-time foetuses (of which one hundred and twenty-
two examples were examined) the fourth piece of the mesosternum and
the metasternum are usually cartilaginous (Table I).

Position and Number of Centres of Ossification.

In Table I], a summary is given of the situation and number of
centres of ossification in the component parts of the sternum. A more
intimate analysis did not appear necessary ; and for the sake of brevity and
clearness, reference has been omitted to the separation or fusion of double
centres, and to obliquity in their position (such oblique double nodules
being grouped as lateral). The results tabulated, it should be noted, are
reached from an examination of specimens, of which the large majority are
full-time foetuses. The cases of younger sterna are not only fewer in
number, but are also less advanced in ossification.

While there is considerable variability, both in number and position,
the centres of ossification are, as a general rule, single and median, and are
five in number; one for the presternum, one each for the first three
elements of the mesosternum, and one for the metasternum. _—_ Exceptions
to this rule are found in all situations, except in the metasternum, which,
in the few cases in which bone formation has begun, is invariably provided

with a single median centre.
D

18 THE HUMAN STERNUM

The presternum is generally formed by a single centre (79 per cent.)
Where there are two centres, they are, generally, vertically placed. Three
or more centres occur quite rarely (2°6 per cent.)

The first centre for the mesosternum is, as a rule, also single
(78°5 per cent.); when there are two, they are usually laterally placed.
The second and third centres of the mesosternum are less frequently
single than the above-named centres (59°3 per cent.: 60°5 per cent.)
When there are two centres, they are in most cases laterally placed. The
fourth centre in the mesosternum is not generally present (26 per cent.)
When it occurs it is nearly as often double as single; and when
two centres are present, they are as often vertical as lateral in position. It
is of course possible, as Markowsk1* infers, that the lower of two vertical
centres represents the metameric centre of ossification for the fifth piece of
the sternum ; but, as it usually occurs in the interval between the attachments
of the fifth and sixth pairs of costal cartilages, and not between the attach-
ment of the sixth and seventh pairs, it has been looked upon as pertaining
to that, and not to a lower segment of the bone.

The part of the sternum between the attachments of the sixth and
seventh costal cartilages is cartilaginous at birth. It appears to be ossified,
as a rule, by extension from the third or fourth piece, though there is reason
for believing that in some rare cases it may possess a separate and indepen-
dent centre (Markowsk]).

Fissures : Grooves : Foramina.

One remarkable negative feature characterizes the series of foetal
sterna examined. In all there is a complete median plate of cartilage,
grooved it may be on one or both sides, perforated in some instances by
one or more holes, but in no single instance is there a fissura sterni. A longi-
tudinal groove (Table III) is characteristic of the foetal mesosternum,
occurring in one hundred and fifty-eight out of two hundred and twenty-
two examples, or 70°1 per cent., a proportion which obtains generally for
all ages. It may be faint or obvious ; it may be found on one or both
aspects of the mesosternum, and implicate it in the whole or part of its
length, but it never appears in the presternum. This grooving of the

THE HUMAN STERNUM 19

mesosternum is probably associated with the method of early development
of the cartilage, but it is at the same time plain that it is not necessarily
coincident with a bilateral ossification of the mesosternum.

Sternal foramina (Table 1V) occur only occasionally (in seventy-two
out of two hundred and thirty-six cases, or 30°5 per cent.) The number
and situation of the perforations are as follows :—

Cases Per cent.
One foramen in the metasternum. ‘ igs avs 2258
‘Two foramina in the metasternum . : 3 rrr a
One foramen in the mesosternum. ; , Sian 33
Two or more foramina (in the mesosternum
and metasternum) . ‘ : : ‘ Gene eer
TD aga. GOs

The great majority thus occur singly in the metasternum, and the pre-
sternum is never perforated. From an examination of a human embryonic
sternum, 9 mm. in length (third month) (Pl. I], Fig. 19), one is led to
believe that the perforation is due to the persistence of vessels, preventing
the conversion of the embryonic connective tissue into cartilage in the
middle line.

Like the grooves, these foramina are not improbably causally associated
with the mode of early development of the sternum, and it is therefore note-
worthy that neither grooves nor foramina occur in the presternum.

Union of Sternal Elements and Costal Cartilages.

An examination of the mode of union of the parts of the sternum
with one another and with the costal cartilages brings out the fact that the
connexions are much the same in the earlier and later months of foetal life
(Table V).

The presternum and mesosternum are usually connected together by
a fibrous lamina (76°4 per cent.) opposite the attachments of the second costal
cartilages. More rarely they are fused together by cartilaginous union
(23°6 percent.). I have only met with one example (in a nine months foetus)
of a case of alteration in position of the junction to the point opposite the
attachments of the third costal cartilages.

20 THE HUMAN STERNUM

The mesosternum and metasternum are usually (77°9 per cent.) united
by cartilage, and only exceptionally (22°1 per cent.) by a fibrous lamina.

The costal cartilages in the centre of the series are generally fused with
the lateral borders of the sternum, and are not united to it by a fibrous
lamina. The first costal cartilage is also, as a rule (81°3 per cent.), con-
tinuous with the presternum. Occasionally on one side or both the union
is fibrous.

The condition of the seventh and eighth costal cartilages has been
particularly noted. It is not uncommon for the seventh pair of cartilages to
articulate together in front of the sternum (thirty-four cases—14"4 per cent.)
(In still fewer cases the same occurs with the sixth costal cartilage). In
8 per cent. of the cases the seventh cartilage fails to reach the sternum on
one side or both. On the other hand, the eighth cartilage on one side or
both is connected with the sternum in still fewer cases (4°8 per cent.) In
rare cases, the fifth, sixth, or seventh cartilage reaches on one side to the
middle line of the sternum, its distal end pushing its way forwards through
the body of the sternum.

Shape of the Metasternum.

Out of one hundred and eighty-nine cases, the metasternum is present
in one hundred and eighty-two (96°3 per cent.) ; absent in seven instances
(3°7 per cent.) I have been as careful as possible to discard all examples in
which there was the slightest indication that the metasternum had been
accidentally lost in maceration.

While the shape is extremely variable, it is more often pointed and
median than bifurcated. An undivided metasternum occurs in one hundred
and seven cases (57°6 per cent.) ; a bifid metasternum in seventy-seven cases

(4ar3 per cent.)

Anomalies in Development.

One of the most striking features of the series of embryonic sterna
examined is the absence of anomalous conditions, and the regularity and
symmetry of development. No case of fissura sterni is recorded. One
example occurs of transference of the pre-mesosternal junction to the level

THE HUMAN STERNUM 21

of the third costal cartilages. A few instances are found in which one or
more costal cartilages (usually the fifth, sixth, or seventh) force their way
through the cartilaginous sternum to the middle line. Absence of the meta-
sternum, asymmetry in the attachment of costal cartilages, and fusion of
contiguous costal cartilages together, also occur in a few examples.

Suprasternal Cartilages.

In two cases suprasternal cartilages have been found, situated at the
cephalic border of the presternum, between the clavicular articular surfaces.
In one case the cartilages are separated from the presternum and fused
together ; in the other case, they are separate from one another, but con-
tinuous with the cartilaginous presternum (Pl. VI, Figs. 43a, B).

(6) OssiFicaTION oF THE Human Srernum ArrTer Birtu

1. Growth of the Presternum after Birth.

After the age of sixteen years the presternum is always a single median
bone. Among younger sterna, between the first and sixteenth years inclusive,
out of eighty-four examples, sixty-nine (82°1 per cent.) possess single median
centres ; eleven (13 per cent.) have double vertical centres (eight fused,
three separate) ; two cases present double centres, lateral or oblique; and
in two cases three and five centres, respectively, are present.

These statistics harmonize with the conditions obtaining in the foetal
presternum, in which there is commonly a single median centre (79 per
cent.), and more often a pair of median, vertical centres (13 per cent.) than
lateral or oblique centres (2°3 per cent.) The larger percentage of single
median centres in the presternum after birth is doubtless due to the previous
fusion of two centres in some cases.

Completion of the Presternum.

After the age of sixteen years, there is a still further growth of the
presternum by means of an ectochondral ossification around and along the
first costal cartilage (PI. V, Fig. 36). The ossification is at first independ-
ent of the presternum, but the bone produced soon completely fuses with

22 THE HUMAN STERNUM

it so as to form spurs or sternal cornua, which provide a more extensive
surface for the articulation of the sternal end of the clavicle. These sternal
horns are present in 71 per cent. of the sterna in my possession over the age
of twenty-five years. Among those between twenty-five and thirty years
there are twenty-five examples out of fifty-six sterna. Of sterna under
twenty-five years, only three possess these projections—two at twenty-one,
and one at nineteen years.

These sternal horns increase the width of the adult bone, and must
be taken into account in the transverse measurements of young and adult
bones, respectively.

There is no doubt that in some cases at least the first costal cartilage
has a share in forming the articular surface for the sterno-clavicular joint—
Testu,” Morris, AntHony'—and these sternal horns are produced by
ossification, which serves the purpose of extending the area of the sterno-
costal articular surface.

The presternum is complete at sixteen, and is supplemented between
twenty-five and thirty years by the addition of these cornua.

ALBRECHT* (quoted by AntHowny') contends that theoretically the
human presternum contains sixteen centres of ossification—two ‘hemi-
sternebrae’ in front of the first pair of thoracic ribs, two between the first
and second pairs; two ‘hemi-post-omosternums’ (the omosternum of the
batrachia) ; two ‘pre-para-sternals’ representing the sternal ends of the last
pair of cervical ribs ; two para-sternals representing the sternal ends of the
first thoracic pair ; two subclavicular epiphyseal points ; two caudal hemi-
epiphyses belonging to the second pair of hemi-sternebrae ; and two hemi-
epiphyses corresponding to the Lophosteon of birds.

Ramsaup and Renaur® describe a case of the occurrence of five
centres of ossification in the presternum of a full-time foetus. In Quarn’s
Anatomy® a presternum is figured with six centres. ANnTHony' found at most
three centres in his cases, and more often two ; one superimposed above the
other. He agrees with the facts adduced by my observations, and by others
(Ramsaup and Renaut, Tegstut,” and Markowsk1*) that the presternum
is usually ossified from one centre and more rarely from two.

THE HUMAN STERNUM 23

2. Growth of the Mesosternum after Birth.

After the age of twenty-one, the mesosternum is a single bone, and
after sixteen years the fusion of its component parts makes it impossible to
determine the precise number of the original elements.

Ridges on the Mesosternum.

In the adult bone the only evidence of the number of component
parts of the mesosternum is derived from the presence of ridges across the
bone ; but these ridges are quite unreliable guides. They are variable in
number and strength, and are not normally present except between the
attachments of the third costal cartilages (692 per cent. of all sterna over
twenty years).

Out of five hundred and twenty-four cases (Table VI) the ridges are
absent altogether in one hundred and forty cases (26°7 per cent.) There
are three hundred and sixty-three examples of a ridge opposite the third
costal attachment (69°2 per cent.) ; two hundred and five examples of a
ridge opposite the fourth (39°1 per cent.) ; twenty-one of a ridge opposite
the fifth (4 per cent.) ; and two of a ridge opposite the sixth costal attach-
ment (0°3 per cent.).

The only evidence which these ridges afford supports the facts of
ossification ; there are at least three elements engaged in the formation of
thc mesosternum. They are of no value in deciding upon the presence or
absence of a fourth element; and there is no justification for the statement”
that the mesosternum ‘is marked on its anterior surface by three slight
transverse elevations at the lines of junction of its four component parts.’

Ossification of the Mesosternum
Number of Elements.

An examination of one hundred and forty-one mesosterna between
the ages of birth and sixteen years confirms the view adopted from an
examination of foetal sterna (p. 18), that it is usual to find three centres
of ossification, but only exceptionally four centres (Table VI).

Three centres of ossification are practically constant ; and in two
cases only was the third centre absent (at the ages of two and nine years,

24 THE HUMAN STERNUM

respectively). In thirty-eight cases (26-9 per cent.) there is a fourth centre
for the ossification of the lowest part of the mesosternum, between the
attachments of the fifth and seventh costal cartilages. This element receives
laterally the attachment of the sixth costal cartilages.

The percentage of cases in which a fourth centre of ossification
occurs is lower than in the foetal sterna examined (29 per cent.), probably
on account of fusion in some cases of the two lower centres. ‘There is,
among the cases examined, no evidence of the occurrence of a distinctly
segmental fifth centre for the ossification of the part of the mesosternum
between the sixth and seventh costal cartilages.

Median and Bilateral Ossification of the Mesosternum.

In the case of each so-called segment of the mesosternum, it is much
more common (Table VII) for ossification to occur by means of single,
median centres, than by two or more lateral or vertical centres. When
more than one centre of ossification are present, it is further much more
common to find them lateral (or oblique) than vertical in position. The
third piece of the mesosternum is the one most frequently bilateral, and
the first is most often single and median (Pl. V, Figs. 38 and 39).

In this respect again confirmation is given to the conclusions drawn
from the examination of foetal sterna. The proportion of single centres in
sterna after birth is, doubtless, higher, on account of pre-natal fusion of
certain additional centres.

Relation of Ossification of Mesosternal Elements to Asymmetry of the Sternum.

Many instances have been noted of obliquity in the lines of junction,
irregularity in the form and shape, obliquity in position, of the ossifying
elements of the mesosternum. These irregularities have doubtless some
influence in causing curvature and asymmetry of the sternum, if only by the
approximation or separation of adjacent costal cartilages.

Fusion of the parts of the Mesosternum.

The number of sterna examined gives sufficient data for the deter-
mination of the approximate order and time of the fusion of the component

THE HUMAN STERNUM 25

elements of the mesosternum. The series may be subdivided at the nine-
teenth year. Sterna of nineteen years and over usually poss +ss mesosterna,
in which the elements are fused together. In sterna of eighteen years or
less the component elements of the mesosternum are usually separate.
Atter the ave of forty-one (in two hundred and ninety-nine sterna between
forty-two and cuchty-two years, inclusive) the mesosternum is always a single
undivided mass.

Out of one hundred and sixty-one sterna between twenty-six and forty-one years,
inclusive, the mesosternum is 4 single bone in one hundred and filty-five cases (96°2 per cent.)
The six exceptional mesosterna consist in every case of two pieces, separated in five examples

between the first and second elements, and in one between the third and fourth.

(1) F aet, 30
(2) M4, 34
(3) My 39
4) M4, 39
(3) M4 41
(6) M , 32 147 +3 4

To 2 ob Up

—————

Among forty-four sterna between the ages of twenty and twenty-five years, the meso-
sternum is a single piece in thirty-six cases (81°8 per cent.) "There are nine exceptional cases,

in eight of which the mesosternum is in two pieces ; in one it is in three pieces.

(1) Ms aet. 21 |

2) ME: 3s 23 |

(sy My th FR Beg ee
(4) F ” 25 |

(5) M , 25 J

(6) F 4, 20 |
(7) M , petra
(83) Ms, 20 "Lap 2 3b &
(9) M_,, 20 To2 3+ 4

In five cases the cleavage is between the first and second clements ; in two cases between the
third and fourth ; in one between the second and third ; and in one instance the second
piece is separate from both first and third.

Among seventeen sterna at nineteen years, the mesosternum isa single bone in ten
cases (58'S per cent.) Of the remaining seven cases, in five the first piece is separate ; in
one the first and second pieces are separate ; and in one case all the elements are scparate.

Below the age of nineteen, as already stated, the mesosternal elements are more frequently

separate than fused.
E

26 THE HUMAN STERNUM

Of eleven sterna at the age of eighteen years, four possess mesosterna (36°3 per cent.)
in the form of a single bone ; in one case all the elements are separate ; in six cases the first
piece only is separate, and in one of the six it is fused with the presternum.

Among seven sterna at seventeen years, there are five in which the mesosternal pieces
are all separate ; and two in which it consists of two pieces (1°2 + 3 + 4, and 1 + 2° 3).

OF sixteen sterna at sixteen years, six have mesosterna, the elements of which are
all separate ;_ five in which they are all fused (31°2 per cent.) ; and five in which the meso-
sternum is in two parts ; of the last-named series, four show a cleavage between the first and
second, and one a cleavage between the second and third pieces of the bone.

Out of seven sterna at fifteen years, the mesosternal elements are all separate in three
cases, all fused in two cases (28°5 per cent.) In two cases the mesosternum is composed of
three separate pieces.

Of seven sterna at fourteen years, the mesosternal elements are all separate in three
cases ; all fused in one case (14'2 per cent.) In two cases the first piece is separate ; and in
one case the first and fourth pieces are separate, the second and third being fused together.

In a sternum of thirteen years of age, the parts of the mesosternum are all separate.

Of five sterna at twelve years, the elements of the mesosternum are all fused in one
case (20 per cent.) ; and all separate in another. In two other cases, the first, and the first
and second, respectively, are separate from the rest.

Among three mesosterna at eleven years, there are two pieces in two cases (lt24 33
1° 2 + 3 + 4); and in one case three pieces (1 + 2° 3 * 4).

Three sterna at ten years, and six at nine years, show all the mesosternal elements
separate.

Of three cases at eight years, the mesosternal elements are all separate in one case ; in
two cases the first two elements are separate (1° 2° 3 + 4).

Among five mesosterna at six years, different conditions are present in each case :—

(1) | ane ae feu
(2) 1r'2°3+4
(3) 1°2 +43

Gj) 1p es

(5) 1+2+3+4

Among five sterna at five years, all the mesosternal elements are separate in four cases ;
in one case the second and third pieces are fused together.

Of three sterna at four years, one has all the mesosternal elements separate ; in two
instances the third and fourth pieces are fused (1° 2° 3 + 4).

In eighteen sterna, aged o—z years, the mesosternal elements are separate in all.

This statement has been made at length to show that although there
is wide variability, the examination of an extended series shows a considerable

THE HUMAN STERNUM 27

regularity in the mode of fusion of the parts of the mesosternum (see
Table VIII). There is a tendency for the parts to unite together from
below upwards, and only exceptionally in the opposite direction. Precise
data are not possible about the fourth element, as it is so often absent ;
when present, it is usually the first to unite with the preceding portion.

Between the time of birth and the end of the tenth year (Table VIII)
three or four pieces of the mesosternum remain separate. Between the
eleventh and fifteenth years inclusive the third (or the fused third and fourth)
segment begins to unite with the second. Between sixteen and twenty
years inclusive the second, third, and fourth pieces have usually united, and
the second is beginning to join the first. Between twenty-one and twenty-
five years all the elements become fused, so as to form a single bone.

Eight anomalous cases have been noted, in which union occurred in
different ways (Table VIII). These anomalies are all traceable to the
behaviour of the second piece of the mesosternum, which fuses with the
first or third piece or with both, leaving the other element or elements
separate.

These observations confirm and amplify the statement of Dwicut™"
and others regarding the date of completion of ossification of the meso-
sternum. They indicate, in my opinion, a greater degree of precision than
Dwicut’s cases admitted, in the process of union of the several elements
(Pl. ¥ Fig. 34).

In examining the ossification of the mesosternum it is difficult in some
cases to decide on the relation to particular ribs of a given centre of ossifica-
tion, and one has to admit the general conclusion that the osseous deposit
occurs with characteristic irregularity. It appears to be stretching the facts
observed too far to assume that a given centre is actually associated with
any given pair of ribs. At the same time, an important point is demon-
strable, that even in the most regular and symmetrical cases there are only
three, or at the most four, sets of centres for the mesosternum. No doubt
five or more centres may occur, but I have searched in vain among the examples
examined for the presence of a distinctly segmental fifth mesosternal element,
such as has been figured and described by OrtTo.* One does not doubt its
existence, but one fails to give it the morphological importance ascribed to

28 THE HUMAN STERNUM

it, and one explains its occurrence on the same grounds as that of other
centres—mechanical causes. It is useless to quote the text-book statements
on this point. They are copicd from one te another, without verification,
and all are morc or less erroncous. The statement that 1s most approximate
to the truth, as L have observed it, is Dwicar’s: ‘Mew examples occur

over twenty years in which the mesosternum is not one prece.,’
3. Growth of the Nletasternum

The date of ossification of the metasternum is extremely variable. A
centre is usually absent before five years, and present after cishteen years,
and may appear at any date between five and cighteen (Table IX). It
appears never to become completely oss lied, being found tipped with
cartilase even in extreme old ave.

It is always ossificd by a median centre, which is practically always
single. Only three cases occur in my collection of additional ossificatons,
Two are examples of additional endochondral ossifications at the extremity
of the main centre. The third case is peculiar : in this two small bilateral

nodules are interpolated between the mesosternum and the metasternum.

(c) Union oF viz Mesosrernom wiri THe PresrerNuM AND

Merasrernum (Pare X)
1. Union of the Mesosternum with the Presternum.

Fusion of the mesosternum with the presternum must be regarded
as an abnormality. It occurs in 8°8 per cent. of the six hundred and
forty-two sterna cxamined. Its frequency increases with age. Itusion
occurs in 3°r per cent. of sterna under thirty, in rovs per cent. between
thirty and forty-nine, and in 15°6 per cent. between fifty and eighty-two.
At the same tine it is not altowether a senile condition. Among tortv-six
sterna between sixty-one and cighty-two years, union of the presternum and
mesosternum only occurs four times (8°7 per cent.) 3 and among twenty-
two sterna between sixty-six and cighty-two it only occurs once.

On the other hand, there are grounds for believing that in some cases
this condition is associated with the occurrence of an arthrodial joint

between the presternum and first costal cartilage (Pl. VI, lig. 40). A close

THE HUMAN STERNUM 29

scrutiny of fifty-three cascs of fusion of presternum and mesosternum
shows in twenty-one the existence of what appears to be an articular tacct
for the first costal cartilage, instead of the usual excavation. One hesitates
to speak more confidently in regard to this point without further investiga-
tion upon sterna ina fresh condition. All my specimens were dry and
macerated ; but the conditions were. striking enough to warrant the si --
gestion that just as separation of the presternum and = mesosternum is
associated with fusion of the first costal cartilage with the presternum, so in
cases of fusion of these two elements of the sternum, an arthre ial articu‘a-
tion may coincidentally occur between the sternum aml the first costal
cartilage so as to permit free movement of the ribs and expansion of the
chest.

Although Muscrove,“ among cichtcen examples of costo-sternal
joints, never found a single case of a diarthrostial joint between the first
costal cartilage and the presternum, Anriony,’ out of sixty-six cases, found
such a joint in seven examples, of which five were bilateral. He quotes

TCcHAussow as recording nineteen examples out of cighty cases.

Fusion of the First Piece of the Mesosternum only with the Presternun.

This rare condition illustrates the fact that fusion of the mesosternum

with the presternum is not dependent upon age alone. As already stated
(p. 20), in one foetal sternum the fibrous septum of separation of the
cartilaginous presternum and mesosternum was situated opposite ihe
attachment of the third costal cartilages. There is, therefore, a possibility
at least that all cases of this sort may be congenital in origin. © Amonzr the
sterna in my possession there are five in which the first piece of the meso-
sternum is separate from the other pieces, and fused with the presternum.
The ages of the sterna were eighteen, nineteen, twenty-five, forty-four, and
fifty-nine years* (Pl. VI, Fig. 41). Dwicir™ has described an example of
this condition, and Turner” has given a case occurring in the skeleton of
an Andaman islander.

In the gibbon it is the normal condition, as described by pe Brarn-

vitvy,® Kerry, and others.

* Another example, not included above, occurs along with other skeletal peculiarities in the skeleton
of an African negro, aged twenty years, in the anatomical museum of this University.®®

30 THE HUMAN STERNUM

2. Union of the Mesosternum and Metasternum.

This, though not normal, is a condition essentially associated with
advancing age (Table X). It may occur at any time after the growth of the
mesosternum is completed. After the age of twenty years, out of five
hundred and twenty-one cases, 65°7 per cent. show the metasternum separate ;
34 per cent. show fusion. Between fifty and eighty-two years separation
occurs in 53°9 per cent; fusion in 45°3 per cent. After sixty years, union
of the metasternum is more frequent than separation (thirty to twenty-five) ;
and of thirteen sterna between the ages of seventy and eighty-two, nine
show fusion and four show separation of the mesosternum and metasternum.
This statement confirms the view that fusion of the metasternum is a senile
change—occurring, as it does, after the age of sixty.

(2) Summary.

Reviewing the facts detailed above relating to the ossification of the
sternum : the bone is laid down, primarily in association with the shoulder-
girdle, as a median cartilaginous lamina, modelled in the form of the future
bone, and connected in a definite and symmetrical way with the cartilages
of a certain number of ribs. The conversion of the cartilaginous model into
the osseous sternum is by endochondral ossification, as in the case of the
vertebral bodies, basis cranii, and the epiphyses of the long bones. It is a
slow process, and allows of expansion of the bone in all directions in relation
to the growth of associated parts.

We need not discuss the question of the significance of the mode of
ossification, or the value to be attached to it ina morphological sense. One
does not attach any special morphological significance to the occurrence of
‘special centres’ in the cartilaginous sternum—or, indeed, in any cartilaginous
mass in which this method of ossification occurs. In cases of endochondral
ossification the deposition of bony ‘centres’ appears to be determined by
mechanical causes, more or less obscure. One inclines to the belief that
bone production may be excited by stress or strain affecting particular points
in the cartilaginous mass, causing vascularization and ossification. Reference
has already been made to the difference in ossification of the lower end of
the femur and the lower end of the humerus. In the former only one
epiphysial endochondral ‘ centre’ appears ; in the latter there are four. It

THE HUMAN STERNUM 31

is not impossible that the single centre in the femur is produced by the
exercise of a single fundamental force—the direct relation of the femur to the
tibia; while on the other hand, at the lower end of the humerus, causes of
ossification may be found in the pressure of the radius and ulna, and the
traction of the muscular masses attached to the epicondyles of the humerus.

So in the case of the sternum. Its cartilaginous condition may
justifiably be taken as the basis of comparison in different groups of
animals ; and the endochondral centres of ossification may be regarded as
altogether secondary, and devoid of any special morphological significance.
As in the case of epiphyses, the centres of ossification in the sternum may
be explained by the exercise of traction or pressure on the part of the ribs
and costal cartilages.

This view appears, on the whole, more satisfactory than a nebulous,
transcendental notion, of a hypothetical representation of sternebrae or sternal
segments.

It affords an explanation of the intercostal and median deposit of bone
in the cartilaginous sternum, which produces ultimately an apparent, and it
may be permanent, segmentation of the sternum in such animals as the
Edentata, possessing a mobile sternum. Per contra, it explains the condition
found in man, cheiroptera, birds, etc., in which, along with well-developed
clavicles and shoulder-girdles, the mesosternum remains a simple bone, and
even becomes more consolidated than in the embryonic condition. Lastly,
it explains the irregularity in the occurrence of the centres of ossification of
the sternum. Even leaving out of account the ossification of the meta-
sternum, the ‘segments,’ of which the mesosternum is composed, are
not (except in certain mammals, to which reference will be made later)
equal to or alternate with the number of costal cartilages associated
with it.

In the human mesosternum there are, in my experience, never five
segmental centres of ossification ; at the most four, or still more commonly
only three are present. There is very rarely a centre in the interval between
the articulations of the sixth and seventh costal cartilages ; and there is
more often than not no centre in the space between the fifth and sixth. Their
absence needs no explanation, but may be suggested as due to the close,

32 THE HUMAN STERNUM

but variable, approximation of the lower costal cartilages at their junction
with the sternum ; this approximation causes the traction of these costal
cart laves to be exerted simultaneously in the growing sternum, and so
prevents the formation of an osseous centre, which might make its appearance
it the sternal attachments of these cartilages were further removed from one
another.

If the terms ‘sternebrae’ and ‘sternal segments’ possess any real
morphological significance, some more adequate explanation is required of
the .bsence of these sternebrae between the sixth and seventh, and between
the fitth and sixth costal attachments, than the bare and meaningless
statement that * they have been lost in the process of evolution.’

From this process of ossification one is led to a belief that morpho-
Jovically the sternum is a simple cartilaginous structure (possibly bilateral
in origin), median in position, associated with shoulder-girdle and ribs, and
subserving the purposes (in different degrees in different animals) of shoulder-
girdle movements and of resp ration. — Its ossification is conducted by a slow
and jJastic method, by means of which its form can be adapted to the necds
of the animal, and which leads to the ultimate production of different types
of sternum -on the one hand, the clavicular or human type; and on the
other hand, the purely thoracic or respiratory type, as in the carnivora or
ungulata.

This view appears further to explain those cases in which on one or
both sides the distal end of a rib overlaps the sternum, and reaches the middle
line; the sternum itself persisting or being absent at the same time ; it
accounts for the non-occurrence of scementation in the cartilaginous meso-
sternum 3; and for the variability in the matter of fusion or separation oi the
cartilaginous mesosternum in relation to the presternum and metasternum.

In dealing with the observations of previous observers, it would he a
matter of great difficulty to summarize the extraordinarily discrepant
accounts that are given of the ossification of the sternum. My task is
lightened by reason of the appearance of an important memoir by
Maxiowsk1,4 in which these statements are given with some fulness.
‘This author also deals exhaustively with some parts of the question under

consideration. As, however, his statistics are compiled from an examination

THE HUMAN STERNUM 33

of the sterna of embryos and children after birth—raken together—it is not
quite easy to harmonize them with those presented in my own personal
observations. This is a matter for regret, as it appears that we have been
simultaneously working along similar lines. Our results do not agree
statistically, but as will be shown later there are certain broad generalisations
which can be stated from a comparison of the two series of observations.

Dates oF OssIFICATION OF PARTS OF THE STERNUM

Presternum.

Whereas my observations show that a centre (or centres) for the
ossification of the presternum usually occurs in the sixth month, Markowsk1
states that it begins to appear at the fourth or fifth month. In regard to
the number of centres, a general agreement exists between us. A single
centre is most common (M., 68-4 per cent. ; P., 79 per cent.) ; double
centres are less common (M., 29 per cent. (vertical); P., 17°2 per cent.
vertical, 1 per cent. horizontal) ; three centres (or more), 2°6 per cent. in
both cases. He does not make any observations on the union of the pre-
sternum and mesosternum ; and regards the presternum as morphologically
associated with the first and second ribs.

Mesosternum.

There is general agreement in the two series of observations, in regard
to the main facts observed, although statistically wide differences are apparent.
The comparison between the two series of observations of the ossification of
the mesosternum may be readily shown in a tabular form :—

MesosTERNUM Dare or APPEARANCE
Dare or Union M. P.
ist | 5th month 6th month (66°6%)

P. M.
16—20 year} 3h year...
2nd| 5th month ath month (77°4%)
2I—25 years

complete

7—II years sot)
11—15 years 3rd | 7th & 8th month 7th month (54°8%)

See. 59% before birth

4th | 15% after death 8th month (37°54)
26% absent
5th} oth month (once) never

34 THE HUMAN STERNUM

Metasternum.

MarkowskI mentions one case in which the metasternum is absent
altogether. He states that it begins to ossify just after birth, or in the first
three years ; and among his cases the latest date was in the sixth year.
He states further that ossification occurs by two lateral or by one median
centre. My observations agree more with those of previous observers,
who lay stress upon the variability in the time but the stability in the mode
of ossification of this part of the bone—from one median centre.

The general conclusions at which Marxkowsk1 arrives, and upon
which I would lay most stress, are (1) the agreement in our observations
that the sternum is ossified from single median centres more often than
from bilateral double centres, although he asserts that this is a higher and
later mode of development ; and (2) the obvious and striking irregularity
in the mode of ossification of the mesosternum.

Although great stress is laid upon the metameric arrangement of the
parts of the sternum, Markowsk1 admits that this is secondary ; and he fails
to indicate—just as my specimens fail to indicate—a metameric relation-
ship between the lower true ribs and parts of the mesosternum. And
although he pins his faith on the association of the presternum with the
first and second costal cartilages, he admits that a single median centre is
much the commonest for this part of the bone (68°4 per cent.)

In my opinion, the great value of Markowsk1’s researches is to show
again how very variable is the deposition of the centres of ossification. He
rightly lays stress upon the asymmetry of centres ; and, more particularly,
upon the occurrence of additional accessory centres round the main centres.

These points, to my mind, are supporting evidence in favour of the
view that the centres of ossification are not of morphological value; but
that the sternum is essentially a shield, in whose substance these osseous
deposits occur as bosses to strengthen it, not through hereditary influence,
but through causes which are mechanical, functional, and adventitious.

Markowsk’s memoir is a valuable and thoughtful contribution to
the subject, and has been of great interest in the final preparation of the
present work.

THE HUMAN STERNUM 35

His main morphological thesis is diametrically opposed to the view
at present advanced. He considers the sternum as made up of a series of
intercostal segments corresponding to vertebral bodies ; and the absence of
centres in the lower segments as indications of reductions (from below
up). He does not, however, depend for his proof upon segmental ossification.
This, he admits, cannot be maintained, but he relies on transient cleavages
in the cartilaginous sternum (Rippen linien), which, he says, occur opposite
the costal attachments. But one would submit that prior to these cleavages
(which cannot be said to be, by any means, constant or obvious) the cartila~
ginous model is a longitudinal strip without, at an earlier period, any trace
of a metameric structure, either in cartilage or before the formation of
cartilage, that has yet been satisfactorily demonstrated. The segmental
character appears to be introduced by the association of the ribs with this
sternal anlage. The only obvious cleavage is at the manubrio-sternal
junction ; and that in the foetal sternum is by no means constant in
occurrence or definite in time.

36 THE HUMAN STERNUM

Il. THE FORM OF THE ADULT HUMAN STERNUM
A. Lenotu ano BreaDTH

The measurements of five hundred and four sterna are given in
Table XI—three hundred and ten male, one hundred and twenty-six female,
and sixty-eight unclassed. Among those whose ages where known, one
hundred and eighty-three were under forty, and two hundred and fifty-
three were over forty years of age. An analysis of the measurement shows
distinct differences dependent upon age and sex.

(a) Age Differences.

In Table XI, the male and female sterna over twenty-one years of
age have been grouped together in two series—under and over forty years
of age. From a comparison of the measurements of the two groups, it
appears that a certain increase, both in breadth and length, takes place after
the age of forty. This increase is apparent, as to breadth, in all three parts
of the sternum. On the other hand, while the total length of the bone is
greater over forty years, the increase (in both sexes) is seen to be due to
the greater length of the metasternum, and (in the male only) to the
greater length of the mesosternum, The male presternum and the female
mesosternum are actually shorter in the series over forty years of age than
in the younger bones.

(o) Sex Differences.

While it is obvious (Table XI) that the male sternum is absolutely
larger, in both length and breadth, and in all three parts, than the female
sternum, there are several important differences in the relative size of the
parts of the bone, in the two sexes, as shown already by Hyrrtv,” Srraucu,®
Dwicut," and others.

1. Presternum.

Among the specimens in my collection, the average male presternum
measures in length 52°0 mm., and in breadth 65°3 mm. (Table XII). The
female presternum measures in length 47°3 mm., and in breadth 58:3 mm.

THE HUMAN STERNUM 37

The breadth index — of the male presternum is 125°5; of the

female presternum is 123°2. The /ength index aS) of the male pre-

sternum is 79°6; that of the female presternum, 81:1. The male pre-
sternum, is, therefore, wider and shorter; the female, longer and
narrower.

2. Mesosternum.

The average measurements (Table XII) of the mesosternum are :—

Length Breath
Male 103°7 mm. . 40°5 mm.
Female gt'omm. . 36:8 mm.

The indices for the two sexes are :—

Length index Breadth index
Male 256 é 39

Female 247 ‘ 40°4
The male mesosternum is, therefore, on the average, longer and narrower ;
the female shorter and broader.

The statistical method does not very clearly indicate a point that was
often obvious in the examination of the series. In the female, the meso-
sternum is often extremely broad in the lower part (PI. VI, Fig. 42a), so as
to give rise toa ‘type’ of sternum which can be almost designated the
‘female’ type; although it must be admitted that this type is not unfrequently
found in male sterna as well (Pl. V1, Fig. 428).

3. Metasternum

The measurements of the metasternum are vitiated because of the
dry and shrivelled condition of this part of the bone. However, as all
measurements have been taken in the same way, they are included for what
they may be worth. The measurements are :—

Length Breadth
Male 40°O : 20°0
Female 33°5 : 16°0

The indices are :—
Length index Breadth index
Male 290° ‘ [<exe)

Female 209°3 : 47°7

38 THE HUMAN STERNUM

The male metasternum, therefore, appears to be shorter and broader ; the
female longer and narrower.
The male sternum thus differs from the female in having a longer

and narrower mesosternum, and a presternum (and possibly a metasternum)
which are shorter and broader.

Relative length of Presternum and Mesosternum in the Male and Female

The law propounded by Hyrri” is not obeyed by the series under
examination (Table XIII), that ‘she manubrium of the female sternum exceeds
half the length of the body; while the body in the male sternum ts at least twice as
long as the manubrium.’ ‘This law was supported by Srraucu,™ quoted by
Dwicut ; but has been somewhat adversely criticized by the last-named
author. An analysis of the lengths of the presternum and mesosternum
in the adult (22-82) shows that on the average the relative length of the two
parts is the same in both sexes (1: 1°9), and the presternum is thus usually a
little more than half the length of the mesosternum.

When, however, the series of adult sterna is broken into two groups
—those under and those over forty years of age—a difference in relative
length comes to light, due probably to an increase in length in the male
mesosternum in later years. Between the ages of twenty-two and thirty-nine
(Table XIII), the relative length of the presternum and mesosternum is as
stated above (1: 1°9); but between forty and eighty-two years, there is a
difference. The mesosternum of the male is twice the length of the pre-
sternum (1: 2); that of the female is less than twice the length (1: 1°8);
so that the older group coincides with Hyrrt’s statement.

The measurements of one hundred and six young sterna (sixty-five
male, forty-one female), between two and twenty-one years, have beenadded in
Table XIII. The shrinkage of cartilage, and the consequent loss of length,
more particularly of the mesosternum, probably renders these measurements
fallacious ; but the series agrees with Hyrru’s statement, the relative length
of presternum and mesosternum being in the male 1: 2, and in the female

sae og F

THE HUMAN STERNUM 39
B. Variations In Form

(a) Suprasternal Ossifications

From an examination of two hundred and thirty-six foetal sterna
between three and nine months (p. 20), only two examples were obtained
of additional suprasternal cartilages (0°8 per cent.) Ina female sternum of
six months, two ovoid cartilages occur upon its upper border, fused together,
but separate from the presternum. In another female sternum of nine
months, two cartilages occur separate from each other, and fused with the
upper border of the presternum (PI. VI, Fig, 434, 8).

Since the above investigation was published the statement has been
made‘? that such conditions are much more common than appeared
from my observations. To verify or reverse my previous statement,
Dr. W. H. Broad, Robert Gee Fellow of Anatomy in the University of
Liverpool, has examined for me fifty-four foetuses, with, however, a
completely negative result. None of the sterna presented suprasternal
cartilages. The particulars of age and sex of the foetuses examined were :—

Sex

Age Number
M. F.
Months 3 I I °
ae | I I °
» 5 3 2 I
» 6 6 6 °
» 7 12 5 7
» 8 10 9 I
a) 21 se) 11

54

With this controlling observation one appears to be justified in
regarding the appearance of these additional cartilages as a rare and
-anomalous condition.

40 THE HUMAN STERNUM

Out of five hundred and sixty-three sterna, young and adult, between
the age of two and eighty-two years, four hundred and sixty-seven, or
83 per cent., present what may be styled a normal presternal notch. The
notch in these cases varies considerably in width and depth. In one
hundred and ninety-three cases it is a well-marked wide concavity (Pl. VII,
Figs. 45, 46, 47); in one hundred and eighty-nine cases it is shallow
(Figs. 45, 46); and in eighty-five cases it is narrow (Fig. 47).

In ninety-six cases (17 per cent.) there are deviations from the normal,
accompanied by the absence of a presternal notch. The variations that occur
are devisable into two main groups: (1) fifty-one cases (gO per cent.) in
which the upper border of the presternum is flat, convex, or raised into a
median projection ; and (2) forty-five cases (7-9 per cent.) in which two
lateral projections or separate ossicles occur.

In the first series sixteen examples occur in which the upper border
of the presternum is a horizontal, straight, or irregular line. In twenty-
three cases it is convex ; in twelve cases it is raised up into a distinct median
projection (Pl. VII, Figs. 48, 49).

In the second series the projections from the presternum are bilaterally
placed. In one case only (0°17 per cent.) are there two separate supra-
sternal ossicles (Fig. 50). In another case there is a prominent tubercle on
the left side fused with the presternum, and on the right side a sessile
tubercle surmounted by an articular facet* (Pl. VII, Fig. 51).

The remaining cases, forty-three in number, are examples of the
presence of lateral tubercles on the upper border of the presternum, which
fall into four groups not clearly separated from each other. (1) There are
eleven examples of prominent tubercles bilaterally placed upon the upper
border of the presternum (Pl. VII, Figs. 52, 53) ; (2) there are ten cases in
which the projections form ridges separated by grooves or notches from the
clavicular facets (Fig. 52). Again, there are twenty-two cases in which
there are distinct articular projections (Fig. 54) (3) separate from or (4)
continuous with the clavicular articular surfaces.

Of the last-named series, probably not all are undoubted suprasternal
ossifications. Some (in group 3) appear to be facets for the articulation of

* More recently I have obtained a third example (Pl. VII, Fig. 44) not included in the series, with an
articulating separate ossicle on the left side, and an articular facet upon the right side.

THE HUMAN STERNUM Al

ossicles which have been lost in maceration ; and others (in groups 2 and 4)
may be associated with variations in the mode of articulation of the clavicle
or attachments of the sterno-clavicular ligaments.

Significance of Suprasternal Ossifications.

It is thus evident that these suprasternal ossifications are both rare
and variable in their occurrence. The nodules may be separate or fused
together ; separate from the presternum or attached to it; bilateral or median
in position.

They are vestigial structures, and several views have been formulated
regarding their homologies. They have been regarded as representative of
costal, shoulder-girdle, or sternal elements. Brescuet,* who first described
them, inclined to the belief that they represent the persistent distal ends of
cervical ribs, the more proximal portions of which had not been developed.
This notion does not bear investigation ; in cases in which cervical ribs are
fully developed, and reach to the sternum, their attachment to the pre-
sternum is by means of costal cartilages to the lateral borders of the bone
below the articulation of the clavicles.”°

There is more difficulty in deciding if the ossicles are homologous
with shoulder-girdle or sternal elements. GzcrnBauR’™* and ParKER®
both seem to agree that they represent structures which are not, strictly
speaking, appertaining to the shoulder-girdle, but are median in position
and placed in front of the sternum (interclavicle and omosternum).

Parker lays great stress on the morphological distinction between
‘membrane’ bones and ‘cartilage’ bones in the lower vertebrates, and
points out that the shoulder-girdle (scapula, coracoid, and prae-coracoid)
and sternum (including omosternum) are cartilaginous in formation ; whereas
the clavicle and interclavicle are membrane bones.

There seems to be a general consensus of opinion regarding the fate
and homologies ot the elements of the shoulder-girdle. The reptilian
coracoid appears to be represented in man by the coracoid process, and
possibly also by thc costo-coracoid ligament (along with the epicoracoid
bone, present in certain mammals (Pl. X, Fig. 66), such as Mus mus-

culus, crocidura (GEGENBAUR). The prae-coracoid, extending in reptiles
G

42 THE HUMAN STERNUM

from scapula to sternum, is rudimentary in man, being overlaid and
replaced by the clavicle. Its extremities, however, are possibly persistent
in the form of the sterno-clavicular meniscus, and the cartilaginous inner
end of the clavicle on the one hand, and in the acromio-clavicular meniscus
when present on the other hand.

The other elements present in the neighbourhood of the presternum
are the interclavicular ligament and the supra-sternal ossicles, the former
developing in membrane, representing probably the interclavicle, and the
latter, developing in cartilage representing the occasional recurrence of the
omosternum (Pl. VIII, Fig. 55).

It is not impossible, however, that the suprasternal ossicles may
represent persistent and detached portions of the prae-coracoids,® which are
responsible for the production of the menisci and inner portions of the
clavicles.

In June of last year, in the Anatomische Gesellschaft at Heidelberg,
Dr. H. Ecce.ine® read a paper on the subject of the suprasternal ossicles,
fully confirming the foregoing observations. The chief differences in the two
series of observations lie in the fewer number of cases and the higher
proportions of ossicles or tubercles in Dr. EccEtino’s experience.

(6) Sternal Foramina cannot be said to be uncommon. One hundred
and thirty cases (20°2 per cent.) have been found among six hundred and
thirty-one sterna. The proportion among foetal sterna was 30°5 per cent.
They are always median and usually single. Occasionally two foramina occur
in the metasternum ; and in three instances foramina are found coincidently
in both mesosternum and metasternum. They are much more frequent
(Table XIV) in the metasternum (16-4 per cent.) than in the mesosternum
(3°8 per cent.) When in the mesosternum the foramen is always in the
lower half of the bone ; most commonly opposite the attachment of the fifth
costal cartilages: and more frequently above than below that position
(PL VII, Fig. 6),

It is noteworthy that nothing in the least approaching the condition
of sternal fissure has been met with in this series. The foramina are small
in all cases ; the largest admitting nothing larger than an ordinary pencil.

The explanation one has to offer for the occurrence of sternal foramina

THE HUMAN STERNUM 43

is that they are deficiencies in the primitive cartilaginous sternum, owing to
the failure of the praechondral tissue to form cartilage on account of the
presence or formation of a bunch of vessels with its surrounding envelope
of connective tissue in some part of the tissue in which the cartilage is being
laid down. This view is supported by the occurrence of such a condition
in an early foetal sternum (p. 19). One cannot ascribe the formation of
the foramina to the failure of the centres of ossification to unite together ;
nor can any special morphological significance be attached to their presence.

One rejects the idea of sternal foramina being associated with the
occurrence of fissura sterni; the causes of the two conditions seem to be
distinctly different.

(c) Asymmetry of the Sternum (Plate IX, Fig. 61).

This condition is not uncommon. It is shown in curvature of the
long axis of the bone, obliquity in the pre-mesosternal junction, inequality
in position and number of costal attachments, and in differences in the
position and form of the clavicular facets.

The foetal sternum is itself usually straight, and is asymmetrical
only by reason of occasional irregularities in the position and number
of costal attachments. The causes of asymmetry appear to operate for
the most part after birth; and the chief causes appear to be obliquity
in the union of the ossifying centres, right-handedness (or the reverse) ; and
the asymmetrical position of the thoracic and abdominal viscera.

1. ILrregularity and Obliquity in the Union of the Component Elements of
the sternum is one of the commonest causes of curvature, and of asymmetry
in the costal attachments. The most notable illustration of this condition is
found at the junction of the presternum with the mesosternum (Table XVI).
This occurs in 30°3 per cent. of all sterna examined. The obliquity of
junction is most often downwards and to the left (left, 17:0 per cent. ;
right, 13°2 per cent.), so as to produce a convexity to the right, and a down-
ward displacement of the second left costal cartilage. It is more frequent
with advancing age (Table XVI), showing that it is caused not merely
by the shape of the sternal elements, but by influences such as right-
handedness, operating after completion of the individual elements.

44 THE HUMAN STERNUM

This statement does not coincide with that of BrrnmincHaM,’ who states
that among thirty sterna examined by him, twelve were symmetrical in this
particular ; while in the remaining eighteen, ten showed an obliquity down-
wards and to the right ; eight showed an obliquity downwards and to the left.

2. Lateral Curvature of the Sternum (Table XV). In 35-6 per cent. of
the sterna examined, the long axis of the sternum presents a convexity
slight or well marked to the right or left. The convexity is more often to
the right (23°2 per cent.) than to the left (12°3 per cent.), and the curvature
occurs more often in adult (38°9 per cent.) than in young sterna (32°3 per
cent.)

This agrees with Birmincuam’s conclusion. He found out of forty-
six cases examined, a curvature to the right in fifteen examples (32°6 per
cent.), a curvature to the left in twelve cases (26:0 per cent.)

3. Asymmetry in Attachment of the Costal Cartilages (Tables XVII-XIX).
The young sternum is naturally straight and symmetrical. The alteration
in costal attachments appear to be caused by (i) alterations in the form
of the thorax brought about by movements, curvature of the spine, and
conditions affecting the viscera, (ii) obliquity in the osseous union of
the sternal elements, (i11) differences in the depth of the first pair of costal
cartilages, and (iv) alterations in the relative position of the sixth, seventh,
and eighth costal cartilages.

Some degree of costal asymmetry is common, and is found in the
majority of adult sterna.

It is more frequent in the adult (69°9 per cent.) than in the young
sternum (41°2 per cent.) It is rather more common to find downward
displacement of the costal attachments on the left (57:4 per cent.) than on
the right side (42°5 per cent.); and in some cases there is asymmetry of
the costal cartilages in two directions in the same sternum. The cartilages
in or near the middle of the series are most frequently asymmetrical
in position ; and most rarely the seventh and first. The following is the
order of frequency :—

Adult: 3rd, 2nd, 4th, gth, 6th, yth, rst.
Young: 4th, sth, 3rd, 6th, 2nd, 7th, rst.

THE HUMAN STERNUM 45

The first costal cartilage is occasionally asymmetrical by reason of
increased depth and more extensive junction than usual with the presternum.
This occurs much more frequently on the right than on the left side
(28:5) (Table XVIID.

The sixth costal cartilage and its attachment to the sternum are interest-
ing on account of occasional asymmetry, and also because of the light
thrown upon the possible existence of a fifth element in the mesosternum.
It is normally attached to the third or fourth piece of the mesosternum
(72°6 per cent.), and it is never displaced to a higher point. It is displaced
downwards in 27°3 per cent. of cases; and more frequently articulates
between the mesosternum and the metasternum than with the metasternum
only. Asymmetry in attachment occurs forty-seven times out of one
hundred and forty-eight cases (31°7 per cent.) It is much oftener displaced
downwards on the left (thirty-nine cases = 82°9 per cent.) than on the right
side (nine cases = 17°0 per cent.)

The seventh costal cartilage has its normal attachment between the
mesosternum and metasternum in 68-1 per cent. of cases (Table XIX). It
is not commonly raised to articulate with the mesosternum alone (9°3 per
cent.), and when this does occur it is more frequent on the right than on the
left side (8:3). It is attached to the metasternum only in 15°7 per cent.
of cases ; and when asymmetry occurs, the left cartilage is more often dis-
placed downwards than the right (44:12). It may fail altogether to articulate
with the sternum on one or both sides (6°6 per cent.) ; and the left is more
frequently absent than the right (4:30). This corroborates the idea
derived from an examination of higher cartilages : that a shifting downwards
of the costal cartilages in relation to the sternum is more common on the
left side than on the right, and, similarly, that there is a firmer and closer
attachment of the right costal cartilages than of the left.

The eighth costal cartilage has an occasional attachment to the sternum,
on one or both sides ; and more often on the right than the left side. The
results of observations made upon young sterna are given in Table XXI.

As the adult sterna in my collection were mostly dry, and in many
cases lacked costal cartilages, one was unable, from their examination, to
arrive at conclusions which would be reliable. I have, therefore, specially

46 THE HUMAN STERNUM

examined another series of fifty-six sterna with the costal attachments
intact, with the result that forty-five sterna (80°3 per cent.) were found to
be normal ; eleven sterna (19°5 per cent.) were abnormal (Table XXII).
Of the eleven abnormal cases, two (3°5 per cent.) are examples of failure
of the seventh costal cartilage to reach the sternum, in one case on both
sides, in the other case on the left side. The nine remaining cases (16 per
cent.) are examples of a junction of the eighth costal cartilage with the
sternum on one side or both. In two cases there is a junction with the
sternum on both sides ; in two cases the junction 1s on the left side only ;
and in five cases the eighth costal cartilage is attached to the sternum on
the right side only. Coupled with the larger series of younger sterna these
examples confirm the conclusions of CunnincHam" and others, and showing
that the eighth costal cartilage has a greater tendency to become attached
to the sternum on the right side than on the left.

Examination of the attachment of costal cartilages to the sternum
indicates (1) an individual variability in the connexion of the ribs with the
sternum unrelated to any essential change in the structure of the latter,
(2) a greater tendency to detachment and downward displacement on the left
side, and (3) a firmer and more extensive attachment on the right side,
particularly at the ends of the series.

4. Asymmetry of Clavicular Facets.

There are slight differences in the size and concavity of the pre-
sternal facets for articulation of the clavicles (Table XX). A more
important difference, however, which is to be associated with asymmetry of
the sternum, occurs on the /eve/ of the two facets. It was found unequal
in one hundred and ninety-nine cases (38°9 per cent.), and the left facet
is much oftener in a higher position than the right one (left, 145 = 28°5
per cent. ; right, 54 = 10°3 per cent.)

This agrees with Birmrncuam’s’ conclusion on this point. He
found the right facet more often lower in position than the left, and he
associates rightly the vertical depth of the interval between the first two
costal cartilages with this condition. On the other hand, it is possible to
regard the occurrence of a higher clavicular facet as compensatory for
curvature caused, it may be, by the costal attachments.

THE HUMAN STERNUM 47

These somewhat minute differences in the form of the bone, and its
relations to the costal cartilages and the clavicles—differences that appear for
the most part to be caused by influences operating after birth—by their very
minuteness serve to emphasize the essential character of the sternum as a bone
of a fixed and determined type not subject to large variations or much
affected by changes in correlated structures. It particularly impresses
one with the idea of the secondary relation of the ribs to the sternum, the
former being subject to very considerable variations without producing any
appreciable alteration in the sternum,

(2) Shape of the Metasternum.

The shape of the metasternum is extremely variable. Without
describing in detail these variations, it is enough to point out that it is
normally median in position, and is usually not bifurcated at the extremity
(four hundred and twenty-nine cases—= 57°8 per cent.) It is bifid in a
minority of ¢ases (three hundred and thirty-four cases = 42 per cent.)
The constancy of its median position, and the constant occurrence of a
single median centre of ossification, lend support to the idea of the formation
of the sternum in the middle line, rather than by fusion of lateral structures

derived from costal cartilages.

SUMMARY

Surveying the foregoing facts, one has a clear picture of a series of
conditions caused by various influences (of which right-handedness is
probably the most important), and giving rise, in many cases, to a slightly
asymmetrical form of sternum. The right costal catilarges have a firmer
adhesion to the sternum than the left. At the ends of the series this is
most obvious: the first right costal cartilage has oftener a larger and
deeper connexion with the presternum than that on the left side; the
left seventh costal cartilage is more often detached from the sternum than
the right ; and the right eighth costal cartilage is more often adherent than
the left. Coincidently there is a greater tendency on the left than on the
right side for a downward displacement of the costal attachments. These
conditions are, probably, the cause of lateral curvature of the sternum, with

48 THE HUMAN STERNUM

which two other asymmetrical features are associated (probably compen-
satory in character)—obliquity of junction of the presternum and meso-
sternum, and a higher level of the left sterno-clavicular articulation.

The development, ossification, and form of the human sternum
taken together present a picture of which the essential feature is median
position. If the human sternum were bilateral in origin, more frequent
examples might be expected of cleavage of its lateral parts. Sternal
foramina are not indications of fissure ; the presternum is always median
and imperforate ; the mesosternum in its cartilaginous form is median,
and its ossification is more often median than bilateral. The metasternum
is always median and ossified by a single median centre; and is more
often pointed or rounded than bifid at its extremity.

The connexion of the sternum with the costal cartilages is variable,
but without producing variations in its own intrinsic form ; the develop-
ment and ossification of the bone give no indication of any essential
metameric dependence upon the association with rib elements, except what
may be due to mechanical influences.

Neither the cartilaginous sternum nor the centres of ossification agree
or vary with the number of associated ribs.

THE HUMAN STERNUM 49

IN. THE COMPARATIVE ANATOMY AND MORPHOLOGY
OF THE STERNUM

A study of the characters of the sternum and associated parts
in vertebrate animals lends support to the notion that the bone is axial in
position, and primarily connected with the shoulder-girdle, and secondarily
with the ribs ; that the human sternum is more primitive in type than the
quadrupedal sternum usually regarded as typical of the mammalia; and
that the so-called segmentation of the sternum is a specialization associated
with the adoption of the quadrupedal position and the needs of locomotion.
Too much stress has in my opinion been laid by many anatomists upon the
similarities in structure of the fore and hind limbs. Even among quadrupeds
there are as many essential differences between them as likenesses, both in
structure and functions. And the sternum is in my view fundamentally
associated with the fore limbs.

In the following pages a summary only is necessary of the comparative
anatomy of the sternum. The elaborate memoir of Parker may be con-
sulted for the requisite detail.

A. Frsuegs

The sternum is absent among fishes as a rule. Instead, the elements
of the shoulder-girdle meet on the ventral surface of the body and form a
shield for the protection of the heart and branchial chamber. Fishes are
characterized by the great variety in the arrangements of the parts of the
shoulder-girdle. The simple type is found among cartilaginous fishes, such
as Acanthias vulgaris, in which the lateral halves of the shoulder-girdle are
confluent, and a continuous cartilaginous bar extends across the middle
line. Among chondrostean fishes, such as Acipenser sturio, the coracoid
elements fail to meet, and clavicles and interclavicle are interposed between
them. In Protopterus annectens there is present a median element, separate
from the coracoids and receiving the ends of the clavicles. In bony fishes
clavicles are present, and there is a variable development of the coracoid

element.
H

50 THE HUMAN STERNUM

The only fish which is known to possess a sternum is Notidanus
indicus, the Perlon shark. This is described by Professor T. J. Parker® in
Nature. It had previously been described by Haswext.* It is present in
the middle ventral line as a distinct, four-sided, lozenge-shaped cartilage,
‘let into the arch as it were in front.’ Two elements are distinguishable
in it—anterior and posterior. Its remarkable feature from our present
point of view is its intimate association with the shoulder-girdle, and the
entire absence of connexion with the ribs. This sternum is an addition to
the shoulder-girdle, and is added without the intervention of the ribs at all.

Attention may be directed in this place to an interesting and
remarkable paper by Professor T. J. Parker® on the origin of the sternum.
Enquiring if there is any genetic connexion between the ‘omosternum’ of
Notidanus and of Amphibia and the ‘costal’ sternum of Aminiota, he
inclines to the belief that the latter is derived from the former. He thinks
it possible that in early Amniota a ‘ post-omosternum”’ existed, joined late
in development by the first pair of ribs. Later on the succeeding ribs
would join it. In Apteryx he found that in the earliest stage in which a
sternum was present, it extends back to the level of the third rib ; the first
and second pair are attached to it by articulation, the third by fibrous tissue.
Tn the next stage, the first three pairs are attached by joints, the fourth pair
by fibrous tissue, from which he concludes that the sternum undergoes a
backward growth. Retardation of chondrification of the sternum would
lead to the development of a sternum of indifferent tissue, subsequently
chondrified from the ribs.

He states certain hypothetical stages in the phylogeny of the sternum,
tracing it by gradual differentiation from an association with the shoulder-
girdle (probably bilateral), a secondary union with the ribs, a delay in
chondrification, and a subsequent extension of cartilage from the ribs.

These views are valuable. The facts I have adduced regarding the
early development of the sternum give support to the hypothesis of Parxgr,
and indicate that a view different from that of Rue and his followers is at
least tenable.

The essential feature of the parts under consideration in fishes is the
union of the elements of the shoulder-girdle in the mid ventral line, for
the construction of the girdle itself and for the protection of the heart.

THE HUMAN STERNUM 51

B. AMPHIBIA

Among Amphibia wide variations exist in the shoulder-girdle and
sternum. In some examples, as in Proteus anguinus, a fish-like character
persists. Praecoracoids and coracoids are present, but no sternum. In
others, as in Bufo, a leaf-like sternum is present between the ventral ends of
the shoulder-girdles. In other cases again, as in Rana (Pl. VIII, Fig. 57),
an additional element, the omosternum, makes its appearance, separated
from the sternum proper by the praecoracoids, epicoracoids, and coracoids.
Neither omosternum nor sternum have any connexion with the ribs: but
as shown already are derived from the shoulder-girdle, and are in intimate
association with it.

C. Repriies

Among reptiles there are many differences in the condition of
shoulder-girdle, sternum, and ribs, and in their relations to one another.
There is, however, a distinct gradation as we proceed from lower to higher
forms.

Among Opbidia, in which the limbs and their girdles are absent, the
sternum is absent too ; and at the same time the ribs are highly developed
and of great importance.

In Chirotes canaliculatus, ‘the only annulate saurian with anterior
limbs,’ there is together with a shoulder-girdle, a sternum which is differen-
tiated into presternum, mesosternum, and metasternum (Pl. VIII, Fig. 58).
‘The ribs do not appear to be connected with the relatively extremely small
bone’ (Parker).

In Anguis fragilis, clavicles, interclavicle, and articulating coracoids are
present, together with a presternum, connected by fibrous tissue with one
pair of ribs (PI. X, Fig. 65).

In such reptiles as Iguana tuberculata and Lemanctus longipes, clavicles,
interclavicle, precoracoid, and coracoid, are closely associated with a broad
shield-like sternum. In Lemanctus longipes (Pl. VIII, Fig. 59), there are
eight cervical vertebrae, of which the last four carry ribs. Beyond these
there are four sternal ribs, i.e., vertebral ribs, articulating by synovial joints
with the lateral borders of the sternum. ‘There is no differentiation of

52 THE HUMAN STERNUM

presternum and mesosternum, but the hinder end of the sternum is bifurcated,
and each bifurcation is again subdivided into long, narrow cartilaginous
processes. This condition is directly comparable with the arrangement of
the parts in the Ornithodelphia (Ornithorhynchus and Echidna), which cannot
but be said to have developed upon parallel lines.

In the Crocodiles (Pl. VIII, Fig. 60) there is a further modification of
the parts by a simplification of the connexion of the sternum and shoulder-
girdle, and its closer association with the ribs. The elements forming the
shoulder-girdle are more rudimentary. There are no clavicles. There is
a rudimentary interclavicle : and a simple coracoid, articulating with the
sternum. The sternum itself is more highly developed. There is an
indication of differentiation of presternum, mesosternum, and meta-
sternum, but no separation of the several parts has occurred, and the
mesosternum is unsegmented. The metasternum is bifid, and may
articulate with one or two ribs. The costal connexions of the sternum are
much closer than in other reptiles. There are two pairs of cervical ribs,
and six, seven, or eight sternal ribs, articulating in pairs with the borders
of the sternum.

The above examples indicate the primary and essential association of
the sternum with the shoulder-girdle, and its secondary connexion with the

ribs.

D. Birps

In birds the sternum is broad and simple in character, its chief
variability being in the development of the keel. It is characterized by its
close and intimate association with the parts of the shoulder-girdle—in which
coracoid, precoracoid, clavicles, and interclavicles, are usually present—
and by the variability in the number of sternal ribs. Cervical ribs also are
commonly present.

In Rhea Americana there are only three pairs of sternal ribs, crowded
together in their articulation with the anterior part of the lateral border of

the sternum.

THE HUMAN STERNUM 53

E. MammMatia

The mammalian sternum is usually divisible into the three parts,
presternum, mesosternum, and metasternum ; but there are considerable
variations in the form and disposition of the three elements, to a large
extent dependent upon and correlated with the arrangement of the shoulder-
girdle and ribs.

Ornithodelphia. At the outset it is to be noted that Ornithorhynchus
and Echidna present a sharp contrast to the rest of mammals in the possession
of a shoulder-girdle, of a complex, reptilian type, associated with a mammalian
form of sternum. For several reasons I propose first to describe and contrast
these two forms separately (Echidna (Pl. X, Fig. 68), from a specimen in the
Zoological Museum of the University of Liverpool; Ovnithorbynchus
(Pl. X, Fig. 69), also from the University Museum). From a comparison
of Parxer’s figure with the Liverpool specimens, I conclude that his
example of Echidna was immature and imperfect.

The shoulder-girdles are very similar in Ornithorbynchus and Echidna.
The coracoids laterally and the interclavicular portion of the clavicles
in the median line articulate with the anterior margin of the presternum.
The presternum is also much alike in the two cases; in all cases it is
separate from the mesosternum, The mesosternum in two examples of
Echidna consists of two pieces. In Ornithorhynchus in one instance it is
composed of two pieces, in another of three. The metasternum is an
oval bone imbedded in cartilage in Echidna. It is absent in both examples
of Ornithorbynchus.

One would direct particular attention to the mode of attachment of the
ribs. In Echidna the presternum carries the first two ribs. In Ornithorbyn-
chus the first rib articulates with the presternum ; the second with the
interval between the presternum and mesosternum. In both Echidna
and Ornithorbynchus the first piece of the mesosternum carries the third
pair of ribs. The fourth and fifth pairs of ribs articulate with the second
piece of the mesosternum (or, in one example of Ornithorhynchus, with the
second and third pieces respectively). The sixth pair in Echidna articulates
between mesosternum and metasternum ; in Ornithorhynchus, with the
lower end of the mesosternum. One noticeable feature of these sterna is

54 THE HUMAN STERNUM

that, although the mesosternum is segmented, there is no metameric co-
ordination between it and the ribs.

The mammalian presternum.

The presternum is the one element of the mammalian sternum which
is constantly present. It is sometimes the sole representative of the bone,
as in some Cetacea (Balaenoptera, Balaena), and in the Manatee. In
Balaena (Turner) only one pair of ribs reaches the sternum. In the
Manatee (PI. X, Fig. 67), two ribs articulate with it on each side; and it
possesses a bi-lobed projection in front of the articulation and a pointed
process behind it.

Usually the presternum articulates with the first and half the second
pair of ribs. Besides the Manatee, exceptions to this rule are Tamandua,
Beaver, Coypu, and Hylobates (two and a half ribs), and Echidna and Tamandua
bivittata (two ribs).

The presternum is usually composed of a single bony element. A
remarkable exception is found in Hy/odates, in which it is normally made up
of two elements, one in front of the other, and receiving two and a half
pairs of ribs (corresponding to a condition which occurs occasionally in
man, as observed already, by inclusion in the presternum of the first piece
of the mesosternum).

The form of the presternum varies considerably. That characteristic
of the human sternum occurs among Primates, but only exceptionally in
other orders, as among some Rodents (Pteromys volucella, Arvicola amphibia,
and in Censetes among Insectivora.

In those orders where the shoulder-girdle is deficient and the clavicle
is absent (as in Ungulata), the presternum becomes considerably reduced in
size : in contrast to the mesosternum, which is greatly increased in width,
particularly in its posterior portion, On the other hand, in exceptional cases,
the presternum is keeled on its ventral surface in a more or less marked
degree. This character has been noted in the lesser Rat Kangaroo
(Hypsiprymnus minor), Tree Kangaroo (Dendrolagus inustus), Vulpine Phalanger
(Trichosurus vulpinus), Hystrix cristata, Cheroptera, Mole, and Aye aye.

Special attention has been given to the form of the anterior end of

THE HUMAN STERNUM 55

the presternum. It appears to be much commoner for it to project
forwards in front of the first rib than not: and this forward projection
may be a single median spine, or may be bi-lobed. There is no forward
projection in Ungulata as a rule (except in the Peccary, Gazelle, Chamois
(PI. IX, Fig. 62), and Ox, in which there is a slight median projection) ;
nor in sterna of the human type (Primates, Tamandua, Myrmecophaga jubata
(Pl. IX, Fig. 63), Centeres, and flying Squirrel).

The process in most mammals is single and median, and gives attach-
ment to the clavicles (when present). It occurred in a characteristic form
in all the Carnivora examined (Pl. IX, Fig. 64), and, usually, also in
Rodents. It was found in most Marsupials (Dasyurus mangoei, Wombat,
Kangaroo, Hypsiprymnus minor, tree Kangaroo, vulpine Phalanger, and Koala) ;
among Edentata in the climbing Ant-eater, three-toed Sloth, and giant
Armadillo; among Ungulates in the Gazelle, Peccary, Chamois, and Ox; in
the Mole and Elephant ; and in Hyrax, in which case it is tri-lobed.

A bilateral projection in front of the presternum is less common.
It was present in Peraurus, Bandicoot, Porpoise, Manatee (Pl. X, Fig. 67),
Tatusia, Armadillo, two-toed Sloth, Water Rat, and Java Loris.

The mammalian mesosternum.

The mesosternum may be absent or indistinguishable from the pre-
sternum (e.g., Manatee (P|. X. Fig. 67), Balaena). In Mammaliait isthe portion
of the sternum in which the most striking differences occur. In the embryo it is
at first a simple bar of cartilage in which ossification (endo-chondral or
periosteal) occurs, so as to produce in the adult of different orders two
extreme types—the segmented and the unsegmented. The simple
unsegmented type of sternum is associated with a powerful development
of pectoral muscles, and with a specialization of structure of the shoulder-
girdle. It occurs in Man, Orang-outan, and Gibbon (I have not met with
an example of an unsegmented mesosternum in the gorilla or chimpanzee) ;
in Cetacea (in the Porpoise the mesosternum is fused with the presternum and
metasternum), Cheroptera, and in the Mole.

The segmented type of mesosternum is characteristic of animals in
which the shoulder-girdle is reduced to its simplest form, the clavicle is absent

56 THE HUMAN STERNUM

or deficient, and the fore limb is adapted to quadrupedal locomotion. This
type of sternum, therefore, is associated with the need of pliancy and elasticity
in lateral movement of the trunk.

The mesosternum is comprised of a number of ‘ segments,’ usually
median and intercostal, and separated from one another by plates of cartilage
or by fibrous laminae. The extreme example of this form of mesosternum
occurs in the polymorphous Edentata, in which, as a rule, the mesosternal
segments regularly alternate with costal attachments (Pl. IX, Fig. 63).
Even here, however, exceptions may occur (Manis longicauda) in which there
are five mesosternal segments giving attachment to seven and a half sternal
ribs (PARKER).

This regular alternation of mesosternal segmentation and costal
articulation I have found only exceptionally in other orders, e.g., in Cerco-
leptes caudivolvulus, Nasua narica, and Mustela vulgaris, among Carnivora ;
in Paca, Dasyprocta, Atherura africana, among Rodents; in an exceptional
example of Bos indicus among Ungulata ; in an example of Ateles Goe ffroyi ;
and in a Chimpanzee with only six sternal ribs.

In most orders the number of mesosternal segments is less than the
number of associated ribs, although at the same time there is undoubtedly
a general correlation between the two, and an increase in the number of
sternal ribs is co-incident with an increase in the number of mesosternal
segments. In the rat, for example, there are four mesosternal segments
and seven sternal ribs: in the rabbit, five mesosternal segments and eight
sternal ribs. I have notes of three examples in the Orang-outan, pos-
sessing three mesosternal segments and six pairs of sternal ribs ; and of
five examples in which there are four mesosternal segments and seven pairs
of sternal ribs.

While Mammals exhibit considerable variability in the relative
number of mesosternal segments and ribs, there is one important point of
general agreement. ‘There is usually one mesosternal rib in excess of the
number of mesosternal segments (Pl. IX, Fig. 64). The penultimate
sternal rib is attached to the side of the last mesosternal segment, or along
with the last sternal rib to the junction of mesosternum and metasternum.
The last sternal rib articulates either with the junction of mesosternum and

THE HUMAN STERNUM 57

metasternum, or more rarely with the metasternum alone. The so-called
metameric arrangement fails through the mode of attachment of the penul-
timate rib and the presence of a rib in excess of the number of mesosternal
‘segments.’

It has been shown that only in exceptional cases (e.g., Edentata) does
a true alternation of mesosternal segments and costal articulation occur.
There are, on the other hand, orders in which specialization of the sternum
occurs in the opposite direction, leading to equally exceptional conditions.
Such examples occur among Ornithodelphia, Sirenia, and Cetacea. The
arrangement of the ribs and mesosternum in Ornithorhynchus and Echidna
has already been discussed. In Sirenia, Parker’ describes the sternum of
the Manatee as representing the presternum alone, and receiving the attach-
ments of only the second pair of ribs. In two examples in the Liverpool
Museum, a sternum of a simple type occurs, with a pair of bilateral forward
projections and a pointed posterior extremity. ‘The first ribs are cervical,
and the sternum receives the second and third pairs (Pl. X, Fig. 67).
From Parker’s description of the Dugong’s sternum, which I have not had
an opportunity of examining personally, it is evident that there is a com-
plete sternum, comprising presternum, mesosternum (of three segments),
and metasternum. Yet there appear to be only two or three pairs of
sternal ribs.

Among Cetacea, in the Do/phin there is no metasternum. The
presternum receives one-and-a-half pairs of ribs, and the mesosternum,
composed of two elements, articulates with two-and-a-half pairs of ribs.
There are, in two examples, thirteen and fifteen pairs of ribs respectively
altogether, and in both cases four pairs of sternal ribs. In Balaena mysticetus
a simple sternum occurs (ParKER) with a pointed posterior extremity, and
only one pair of ribs articulating with it.

Examples such as these are as much at variance with the notion of a
metameric sternum with a costal origin as the Edentate sternum is in
harmony with it. Reviewing the whole series, and bearing in mind the
general mammalian arrangement and the exceptional conditions of an
opposite character which occur, one is forced to the conclusion that, while

the differences in costal attachments are probably responsible for differences
I

58 THE HUMAN STERNUM

in the form of the sternum, by affecting its mode of ossification and
influencing the conversion of the embryonic structure into a homogenous
or a segmented bone, these differences do not affect its essential structure,
as a longitudinal, ventral, thoracic axis.

The mammalian metasternum.

The metasternum is not constantly present in mammalia. It is absent
in Ornithorhynchus, present in Echidna. It is generally present in Edentata
but is absent in Unaw (Parker), and in the three-toed and two-wed Siloths.
In Sirenia it is present in the Dugong (Parker), but it is not differentiated
—if present—in the Manatee. It is generally present in Ungulata, but is
absent in the Tapir. Among Cetacea, it is not present in the Do/phin, and
is absent or undifferentiated in Balaena and Balaenoptera.

The mammalian metasternum is always median in position, elongated
as a rule, and either broadened out and capped with cartilage at its free end
(Rodentia, Chiroptera, Carnivora, Marsupials) or pointed and angular
(Echidna, Ungulata). The occasional bifurcation of the metasternum met
with in man does not appear to occur in other mammals.

The position, form, ossification, and occasional absence or want of
differentiation of the metasternum, make it difficult to accept the view of its
metameric origin from the ventral ends of the ribs. It is essentially median,
and a backward prolongation of the ventral thoracic axis.

SUMMARY

A survey of the differences in the character of the sternum in the
different classes of vertebrates enables one to form a clearer opinion of its
morphological position. Its peculiarities appear to be associated, to a certain
extent, with differences in the mechanism of respiration, but to a much
greater degree with differences in the functions of the fore limb.

In simple types of Fishes (such as Elasmobranchs) the place of the
sternum is occupied by the ventral elements of the shoulder-girdle, which
may, as in the dog-fish or skate, form a continuous bar extending across
the middle line beneath the heart. With one exception (Notidanus), how-
ever, no fish possesses a sternum proper, and the heart is protected and
supported by the shoulder-girdle.

THE HUMAN STERNUM 59

In air-breathing vertebrates the sternum is not always present ; nor
is its most primitive form to be looked for in animals like Ophidia, without
limbs, and with a high development of the ribs. .

Among Reptiles the sternum is found in several different forms. In
its most primitive form (Batrachia, Lacertilia) it is a median structure, not
connected with the ribs, but associated with the two halves of the shoulder-
girdle.

Chirotes canaliculatus presents a remarkably complete sternum, without
costal connexions, and associated with a rudimentary shoulder-girdle.

Among the higher Reptiles (Crocodilia) the sternum acquires costal
connexions in addition to its attachments to the shoulder-girdle, and
assumes a character directly comparable with that characteristic of the most
primitive Mammals (Ornithorbynchus, Echnida) but at the same time it is
a simpler structure, as shewn in the want of differentiation of the several
elements.

The study of the sternum in Reptiles encourages the idea that the
association of sternum and shoulder-girdle is a primary and fundamental
relation ; and that the connexions with the ribs is a secondary event.

In still higher vertebrates—Birds and Mammals—the modifications
of the sternum which occur are directly associated with alterations in the
construction and uses of the shoulder-girdle. Costal connexions are always
present, but with wide variations in different groups. The differences in
the type of shoulder-girdle appear to determine the constitution of the
sternum.

In bipedal animals, or animals whose fore limbs are used for flight,
burrowing, etc., in which the coracoid and clavicle (Aves, Ornithodelphia),
or clavicle (cat, mole, man) are powerfully developed, the sternum is of a
simple type, forming a broad, flat bone, keeled, it may be, for the attach-
ment of powerful pectoral muscles. In such cases the presternum is broad
and strong, and the mesosternum is in the form of a single bone.

Among quadrupeds, on the other hand, and Mammals in which the
fore limb is more assimilated in type to the hind limb than in birds and
bipedal mammals, and is used for supporting weight and for locomotion,
mainly or solely, the shoulder-girdle is more rudimentary, the clavicle is

60 THE HUMAN STERNUM

absent or defective, and the sternum, at the same time, is modified in a
different direction. The presternum is more rudimentary, and the meso-
sternum may be throughout life in a quasi-segmented condition.*

In the case of the biped, fusion of the sternal elements is a necessity,
in order to obtain the proper use of the fore limb. In the quadruped,
segmentation and pliancy of the sternum are equally a necessity to ensure
ease and readiness in lateral movement of the trunk. In both the respiratory
necessities are the same, and are equally provided for by the bony and
muscular mechanism.

The mammalian sternum is essentially characterized by the possession
of presternum, mesosternum (simple or compound), and metasternum,
The only element constantly present, however, is the presternum, the other
elements being in different cases either absent or undifferentiated. Lastly,
the most variable features of the sternum are produced by the variations in
the connexions of the ribs (Tables XXIII, XXIV).

* In the ox the mesosternal ‘segments’ are united by cartilaginous laminae ; and one finds, in old examples, a
gradual fusion of the segments by osseous union, beginning in the hinder part of the mesosternum. In one case in
my collection, the mesosternum is in one piece, and has the metasternum fused with it.

THE HUMAN STERNUM 61

IV. TERATOLOGY OF THE STERNUM

I have had no opportunities of making personal observations on this
aspect of the subject. Awruony devotes a chapter to the subject in his
valuable memoir," from which the following summary is taken :—

Absence of the Sternum.

The metasternum is occasionally wanting in the human subject, and
in rare cases the mesosternum and metasternum are both absent together,
the presternum alone being present. It is doubtful if the presternum alone
is ever absent : such conditions as have been described being possibly cases
of fissure. One case is recorded of absence of the presternum and mesosternum
together, the metasternum alone being present. In this case the thorax was
enclosed by a fibrous membrane.

Defective or Excessive Attachment of the Sternal Ribs.

The first or the seventh pair of ribs may fail to reach the sternum.
The former is the rarer condition, and is found most commonly in
anencephalic monsters. It may or may not be associated with fissura sterni.

On the other hand, the last cervical vertebra may carry sternal ribs
(Turner™”), This isa very rare occurrence. It is much more common to
find the eighth pair of ribs associated with the sternum (TREpDGoLp,”
CunnincHaM"™). It iscommoner in the male than in the female, and on the
right side than on the left.

Perforations in the metasternum are regarded by AnrHony as indications
of the primitive lateral separation of the parts. Perforations never occur in
the presternum.

Fissures may be partial or complete; the former occurring in the pre-
sternum or metasternum, separately or together ; the latter occurring so as to
produce two lateral plates, each associated with its corresponding ribs. Many
cases occur in anencephalic foetuses. The two hemi-sterna are in such cases
(Turner’) united across the middle line by a fibrous membrane.

62 THE HUMAN STERNUM

GENERAL CONCLUSIONS

The conclusions at which one has arrived from the foregoing considera-
tions may be summarized here.

Surveying the whole field, the sternum is to be regarded as evolved,
developed, and constructed rather in relation to the shoulder-girdle than to
the thorax. It primarily subserves the functions of the fore-limb, and is
secondarily utilized for purposes of respiration. It therefore receives the
attachment of a variable number of ribs.

In those animals in which the fore-limbs are limited in functions and
serve solely for quadrupedal movement, the limitation is associated with
(1) a failure in development of the shoulder-girdle, and (2) a coincident
modification of the sternum, which in such cases is connected only with the
ribs, and assumes a quasi-segmental character.

If the quadrupedal mammal is to be regarded as the type, then it
might be fairly assumed that its sternum is also typical. But can this be
asserted ? The mass of evidence seems to point to the conclusion that the
fore-limb is primarily developed as a prehensile organ ; that its more
primitive type is to be looked for in animals with a strongly developed
shoulder-girdle : and that the fore-limb of quadrupedal mammals is a
modification from this primitive type, associated with defects in the forma-
tion of the shoulder-girdle and modifications in the form of the sternum.

In other words, segmentation of the sternum is not a primitive con-
dition. It is a secondary event, in two senses—both ontogenetically and
phylogenetically. It is an architectural device adopted to suit the particular
needs of a modified type, utilized for a particular end in one case and
abandoned in another; just as in the formation of the dorsal axial
skeleton, segmentation is made use of in the formation of the vertebral
column, but is discarded in the construction of the basis cranii, and is
modified in the formation of the sacrum.

In advancing a view which is opposed to orthodox belief, one has to
face the essential foundations of current opinion. In this light, I venture
to express the view that there is no solid evidence of the ontogenetic or
phylogenetic origin of the sternum from the ribs. In my opinion and by

THE HUMAN STERNUM 63

positive observations the weight of evidence is all on the side of the primary
association of the sternum with the shoulder-girdle, and its secondary
connexion with the ribs. And I venture, further, to submit that my view
is borne out and supported by the evidence of comparative anatomy.

In favour of the view put forward are the following arguments :—

1. The early development of the sternum shows it closely related
to the shoulder-girdle and independent of the ribs.

2. The ossification of the several parts, whether median or lateral,
is not truly segmental, and is not dependent upon costal connexions,
except in the mesosternum.

3. The so-called ‘segments’ of the mesosternum are not truly
metameric. Asa rule they are not the same in number as the ribs which
articulate with the mesosternum, but fewer. They are not costal but
inter-costal. And the formation of segments is due to the deposition of
bony masses, whose separation from one another—like their origin—is
associated with the mechanical effects produced by the costal attachments
to the borders of the sternum.

4. Individual variations are common in the arrangement of the ribs
in relation to the sternum. But they occur without producing any funda-
mental differences in the morphology of the bone itself.

5. A study of the comparative anatomy of the sternum demon-
strates that the bone may exist without any connexion with the ribs; and
that the ribs may be highly developed and of great functional importance
in cases of absence of the sternum (and shoulder-girdle). Moreover,
modifications in the form of the sternum are more dependent upon modifi-
cations in the shoulder-girdle than upon differences in its rib-connexions.

In opposition to the views submitted, the following are the chief
arguments :—

1. The accounts given by previous observers—notably Ruce—of
the early development of the sternum. In previous pages the memoir of
this distinguished observer has been referred to, and it need only be re-stated
here that, admirable as Rucx’s observations are, and valuableasa demonstration
of fact, there is really nothing in the observations made that prevents one

64 THE HUMAN STERNUM

from coming to a conclusion diametrically opposed to the view which he takes,
and which has become crystallized into dogmatic assertion by succeeding
writers. As Minor suggests, an earlier stage in development must be
observed in order to ascertain whether a sternal anlage exists or not before
the elementary growth of the ribs is completed. This I believe has been
demonstrated as a cellular strand projecting from the shoulder-girdle, and
secondarily connected with the rib elements.

2. The ‘segmental’ ossification of the sternum and the bilateral
ossification of the mesosternum (in certain of its segments and in certain
animals) have been cited as arguments in favour of its costal origin. It has
already been conclusively demonstrated (1) that the centres of ossification
are not costal but inter-costal, (2) that they do not agree as a rule with the
number of associated ribs, and (3) that they are more usually, both in
individuals and in species, median than bilateral.

3. The type of sternum characteristic of mammals (quadrupeds) is
segmented (e.g., Edentata). It has been already shewn that this form of
sternum may be regarded rather as a modification than as the primitive
elemental type.

4. Teratological conditions lend absolutely no support to the idea
of the costal origin of the bone. From abnormal conditions only tentative
conclusions can be arrived at as to what the procedure should be in normal
conditions. But in most cases either excess or arrest of development is
the cause of an abnormality. The most remarkable form of arrest of
development of the sternum is fissura sterni, in which an absence of the
bone altogether or its longitudinal fissure into two lateral halves may occur.
There is no doubt that the normal mode of development of the bone in
many lower forms is in two lateral strips separated by the heart. But there
is no instance of the occurrence of ‘segments’ of the sternum isolated from
one another, and connected with the ventral ends of the ribs, as one would
expect if the various parts of which the bone consists were originally derived
from the ribs. In other words, obvious proof of the existence of a sternum
without rib connexions has been adduced, but no evidence exists of the
persistent association of elements of costal origin arrested in the process of
development into a composite sternum.

THE HUMAN STERNUM 65

In conclusion, one may point out that there still remains a great deal
that is uncertain and problematic in regard to this question. One has only
succeeded in partially covering the ground in the foregoing pages; and the
teratological aspect of the subject has not been personally investigated at
all. The observations recorded and the general interest arising out of the
study of the various problems associated with them appear to me to be
enough to warrant the attention they have received, and to justify my
taking advantage of this opportunity to formulate my views on a subject
which has occupied my attention for several years.

TABLES

THE HUMAN STERNUM

TasLe I.—Ossification of the Sternum (percentages)

Centre Presenr

Centre ABSENT

ae. he g BE | ¢ e
Sl5-.| 24 3 Mesosternum a 2 Mesosternum I

Sieeise| 3 2] 8 3

: z |*"1 £ | st [and 3rd] 4th g & | 1st |2nd 3rd | 4th g

mm.

IX} 122 | 64:4]| 96°7199°2 | 99°2] 83°3} 29 | 8°x]] 3°73} 8) «= -8) 16°6) 71 | g1°8
VII} 21} 52°73] 81 [81 | 76-2) 47°6] 975] 14°31 19 | 19 | 2378] 52°3) 90°5| 85°7
VIE} 16} 49°31] 93°3/87°5 | 87°5| 62°5) 37°5| 12°5]| 6-6) 12-5] 125) 37°5| 62°5| 87's
VI| 31] 40°7]] 80°6183°8 | 77°4| 54°8) 6:4] © |] 19°3| 16°31] 22°5) 45°1) 93°5| 100

Vij 24| 36°5|] 62°5/66°6 | 41°6] 1275} 4°r} © |) 37°5] 33°3] §8°3| 8775] 95°8| 100

IV| 13| 29:6] 15-3123 | 15-3] 7°7| 7°7| © |} 846177 | 84°6| 92°3| 92°3] 100
III} 9} 23°9]) 2°2) 2-2] 2:2] 1} o © || 77°7| 77°7| 77°7| 88°8] 100 | 100

69

THE HUMAN STERNUM

2)
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9.2 + § ° fo) (o (0 I jo ¢ + ‘slo Jo sary, =“
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62 8 Lt1 z z ZI tz 6 F1 tg : ‘ * a[suig aruad
SAOVINGOUA WOY, | WII AI A IA IIA IIA XI ; ; : SHINOTA]

uO1IDIYISSC) [vABIVT puo uvipay—|]

aTav 7

THE HUMAN STERNUM

Taste Il].—Grooves

Per cent.
Months | ~ Cases Present Absent

Present Absent
IX 113 75 38 66°3 33°6
VIII 17 6 11 35°2 64°7
Vil 16 13 3 812 18°7
VI 30 24 6 800 20°0
Vv 24 19 5 7971 20°8

IV 13 13 ° 100
III 9 8 I 88°8 Il
Total 222 158 64. 701 29°0

THE HUMAN STERNUM

72

FI.z 4 gb.€ 8 Fiz S Lg.zz| +S | +.69 | +91] 9gtz
‘yuao sad §.of = zZ = quasaad x10], ie ° see ° ap ° “s z LLL Z 6 lll
c= uinusa}s : z et I as ro) we ¢ 6.£5 £ £1 AI
-vjaul pur UINUIE}s
-osaul UI a10UI 10 Z ay I ees ° oes ° “ee Zz £.68 Sz gz A
g WINUIa}SOsaUI Ul I
c= UINUIaIsEJOW UT Zz mee ° : fe) ae 1 : 6 9-99 ot of IA
+o== WINUJa}se}IU UI I
—qwosag | °° fc) 3 ° oe ° ee 9 $.z9 ol gI IIA
2 I oe I ae fo) oe + of +1 oz IIA
‘yuso sod +.69 = +91 : : > quasqy
wae I is 9 Su + ar gz $.L9 1g Oxi XI
*yuao Xi *quao - *yu20 A *yuad . *yua. 2
1g °N lag oN Jag oN dag oN 13d oN
anutereese IA Pue WNU19} 808 9JAT WINUI3}seJI IAT UINUII}SE}9 sasec)
IWLOY, ra ausousese IN ul 2uQ, Ul OM, ul 2uQ, i Jo squoW
ul alow 10 OMY, : 7 ; LNasEy ‘ON
LNASAY g

DUIMDADT [DUAN S— A] ATAVT,

THE HUMAN STERNUM 73
TasLe V.—Union of Sternal Elements and Costal Cartilages

Months F : F : | IX |VIIT} VIL) VI} V | IV | IIL | Per cent.
Union of Fibrous “r| 78: -9| 8 8°3| 69°2| 66°6| =76°
Presternum si ; + 75°T) 79°9) 99°9) 87 | 50°3) Og'2 =70°4

and Cartilaginous 2 21 o | 12° 1'6| 30° -3| =22°6
Mesosternum 8 ; 4:7 9| 4 39°7| 33°3| =23
Union of Fibrous I I° 1°z| 40 o | Irn] ==22°1
Mesosternum é : | 3 315} 3 4 9 =
d ely :
ice od Cartilaginous : 68°9| 68:4] 68-7] 60 | go:g|100 | 88:8} =77°9

First Costal

Fibrous : j 14 | 25 66] 0 O | 15°3) 111
Cosflave [cin : | 76 | 68°7] 80 |100 | 78:2] 76:9] 88°38) =81°3
Dissimilar . : | 9°99] 18:7] 13°3] Oo | 216] 76) 0
Seventh Separate from sternum .| 6°6] 5:8) 18°7| 15°3| 0 9 co) = 8
Costal
Cartilage United : : | 93°4] 94:2] 81°3] 84°7| 100 | gt {100 | =g2
Eighth Costal Separate from sternum .| 90 |100 |100 | 96°7/100 | go-g| 88°8) =95'2
Rartilage United ‘ i | 99) of oO 3°5| © g | irr) = 48
TasLe VI.—Ridges on the Mesosternum
Present opposite
No. of cases Years Absent
3rd C.C. | gthe.C. |} sth C.c. ) 6thC.c.
44 20-25 21 20 4 I )
34 26-29 9 9 16 fe) fc)
127 30-39 26 97 51 7 2
110 40-49 30 72 41 4 re)
141 50-82 4l 97 51 7 °
68 Unclassed 13 53 37 2 °
524 140 363 206 21 2
Per cent. 26°7 69°2 39°71 4'0 o3

74

THE HUMAN STERNUM

Tasre VII.—Ossification of the Mesosternum; 141 cases, O—16 years inclusive

Centres Double centres
Segments No. of cases} Per cent.
Single Double | Three Vertical | Lateral
First IqI 100 136 4 1 fo) 5
——
5
96°4 per cent. 3°5 per cent.
Second Iq] 100 115 23 3 3 23
——— a”
26
81°5 per cent. 18°4 per cent.
Third 139 98°5 gI 42 6 4 44
——— a”
48
65°4 per cent. 34°5 per cent.
Fourth 38 26°9 29 9 fo) fe) 9
———
76°3 per cent. 23°6 per cent.
Taste VIII.—Fusion of Mesosternal Elements together. Percentages
Normal process Abnormal process
Age No. of P24+3 [1 +243 1+2°34+ Usual condition
cases arg or or 4 I+2°3+) 14-24
or 2344) 24+ 3+11 424+ 3/024 3°4 or 4 34
1°2°3"4 4 +4 14253
1-5 29 82°7 6-9 3°4 6'9 All separate
(3 or 4 pieces :
6-10 17 64°7 | 17°6 5°8 5°8 5°8 first three sep-
fag arate.
I-4 pieces :
Ii-ry | 23 34°7 | 130 | 26:01 17°3 43 43 pen separ-
47°7 43°3 a
16-20] 59 22°0 | 50 | 25°74 | 40°6 1°6 33 1.6 |{ 1-4 pieces :
ae ec Ufirst separate.
27°0 66'0
21-2 6 13°8 | 82: . I-2 pieces:
5] 3 138 | 833 7 in eed
971

THE HUMAN STERNUM

TasBLe 1X.—Ossification of the Metasternum

Age No. of cases Centre absent Centre present
O24 24 1g 5
5-9 18 5 13
10-14 17 764 9 729 8 735
15-18 29 15 14
88 48 :¢54°5 per cent. ae 45°4 per cent.
19-25 55 6: 9'0 per cent. 4g : 890 per cent.
26-82 428 6: 14 per cent. 422 : 98°6 per cent.

ie:

THE HUMAN STERNUM

Taste X.—Union of the Mesosternum with Presternum and Metasternum

A. Wirn Presrernum

Age No. of cases Per cent. separate Per cent. fused
O-I4 62, 982 1°7
15-19 58 stal 1°7
198 96°8 371
20-25 44 veal 98
26-29 34. 100°0 oxo)
30-49 234 89°3 10°5
375 {364 li
50-82 141 83°6 15°6)
Unclassed 68 89°7 10°3
642 go'9 per cent. 8°8 per cent.
B. Wir Merasrernum
Age No. of cases Per cent. separate Per cent. fused
O-14 62 100 ro)
15-19 58 96's 3°4
20-29 72 80°6, 1973
30-49 240 60°9 391
65°7 34
50-82 141 $3°9 45°3
Unclassed 68 67°6 32°3

Tasie XI.—Length and Breadth of the Adult Sternum

THE HUMAN STERNUM

77

Breadth Length
Sex Age Total length
Pre- Meso- | Meta- Pre- Meso- | Meta-
sternum | sternum | sternum | sternum | sternum | sternum
Male (123) 22-39] 62°5 | 39°4 | 19°70 | 52:2 | 102°8 | 363 191°3
» (187) 40-82] 68:1 | 41°6 | 2170 | 51°8 | 1047 | 43°7 202°5
Total Male (310) 22-82 | 65°3 | 40°5 | 20°0 | §2°0 | 103°7 | 4o°o 196°9
Female (60) 22-39 | 57°3 | 35°38 | 150] 47°71 | g270 | 31°8 1739
»» (66) 40-82) 594] 37°8 | 17°70] 47°5 | goo | 35°2 172°8
Total Female (126) 22-82] 583) 368 | 16:0 | 4773] 91 33°5 173°3
Unclassed 2 63°3.| 39°77 | 192 | 49°1 | 102°8 | 3674 189°
Average Male and
Female (183) .-.| 22-39 | 57°8 | 384] 17°99 | 504 | 98°8 | 35°2 185'4
Average Male and
Female (253) ..-| 40-82 | 63°7 | 39°77} 190) 49°6 | 97°3] 39°4 187°6
Total average (504) .| 22-82 | 61°6 | 392 | 187 | 49°7 | 99°6 | 37°70 187°5
and un-
classed

78 THE HUMAN STERNUM
Tasie XII.—Sternal Indices (Adult, over twenty-one)
A. Breaptu
Male Female Unclassed
Total
average
Age :— 22-39 40-82 Average 22-39 40-82 Average Average
Presternum ...| 119°7 1314 125°5 121°6 125°0 123°2 1289 | 125°8
Mesosternum . 38°3 39°7 39°0 384 42°0 40°4 38°6 39°3
Metasternum . 523 48°0 50°0 471 48°2 47°7 52°7 501
B. Lencru
Male Female Unclassed
Total
average
Age :— 22-39 40-82 Average 22-39 40-82 Average Average
Presternum ... 83°5 76:0 79°6 82:1 799 811 77°3 79°4
Mesosternum .| 260°9 251°6 256°0 256°9 238°0 247° 258°9 | 254°0
Metasternum | 191'0 208°0 200°O 212°0 207°0 209°3 1892 | 19975

THE HUMAN STERNUM

Taste XIII

Relative Length of Presternum and Mesosternum in the Male and Female

No. of cases Age Presternum Mesosternum Proportion
65 M. M. 37°9 769 I: 2
106 2-21
41 F, Ke 237°3 73°0 1: 1°9
123 M. M. 52°2 108°8 I: 19
183} 22-39
60 F. FL 471 920 1: 19
187 M. M. 51°8 104°7 Ti 2
253} 40-82
66 F. By. Ap goro 1: 1°8
310 M. | M. 52°0 103'7 I: 1°9
Total 436 22-82
126 F, F. 473 gI‘o I: 19

80

THE HUMAN STERNUM

Taste XIV.—Svernal Foramina

Agei—

0-24

25-82

Total per cent.

No. of cases
Absent

Present :—

Metasternum

(1)...
(2)...

Mesosternum ...

Situation :—
Ci@ eg vss
C.C, 4-5
On Ore errr
C.C. 5-6
Junction

In both Meso- and
Metasternum

152

-| 80°2 per cent.

“| 1571 ”
12°5 aii
2°60 5
46»

I

2

3

Oo

479

79°1 per cent.

77
158s,
P85
371 oy

I

5

Z

2

°

631

79°6 per cent.

16°4.
14°]
avn (20°? Per cent.
:

13°O per cent.

30°44

434 59
87 oy
4°3 ”

3 per cent.

Taste XV.—Lateral Curvature of the Sternum

No, of cases

Straight

Curved

Total per cent.

Right

curved
Left

Young 139
Adult 490
Total 629

g4—67°6 per cent.
299—61'0 sy,
393—64°3 ow»

133—27

27—19Q'4 per cent.

160—23°2

” 58—11°8 ”

” 76—-12°3 ”

18—12°9 per cent.

32°3 per cent.
389
356

THE HUMAN STERNUM

Taste XVI.—Odiiquity of Junction of Presternum and Mesosternum

81

Oblique (per cent.)
Straight
Age No. of cases per cent.
Down to right Down to left Total
0-19 115 89°5 6:0 4°3 103
20-29 70 67°1 18°5 14°2 42°7
30-39 130 6671 16°1 17°6 33°7
40-49 110 609 15°4. 23°6 39°70
50-82 140 5355 18°5 27°8 46°3
Unclassed 68 70'S 11°8 17°6 29°4
Total 633 69°5 13°2 17°0 30°3
Taste XVII.—Asymmetry of the Costal Cartilages
Adult Young
Number Per cent. Number Per cent.
Symmetrical 156 32°0 94 58°7
Asymmetrical 355 67°9 68 41'2
Down to Right... 151 42° 36 54°5
Down to Left 204 $74 32 45°4
Both Directions 24 67 2 2°9

82 THE HUMAN STERNUM

Taste XVIII.—Asymmetry of the First Costal Cartilages

Number Per cent.
{ R. 20 90°9
Adult 22 git
l le 2 g'0
R. 8 9227. 5
Young \ II 71
L 3 27°2
R. 28 81°9
Total 33 61
5 18°1

TasBLe XIX.—Attachments of Seventh Costal Cartilages

Attachment Numbers Per cent.
To mesosternum—both ioe 83°3\
Right only ... $) Les 727 9°3
II | {366 |
Left only 3 ‘ie 27°2)
To junction of mesosternum
and metasternum—both ... 407 | a)
Right only ... 64 479 88°8 68°1
i pel
Left only 8 II‘l
To metasternum—both 55 | 49°5 \
Right only ... 12) III 214 15°7
56| tse 4
Left only... 44) 78°5)
Absent—both 137 27°63
Right only ... 4 i 11°7 | 6°6
{3¢| tres|
Left only... 30 88-2

THE HUMAN STERNUM 83

Taste XX.—Asymmetry of Clavicular Facets

1. Size

Number of Cases

Equal

Unequal

486

410 : 87°2 per cent.

R. larger L. larger
35:5°4 per cent. | 20 : 3°9 per cent

~-

54 : 9°4 per cent.

2. Concaviry

Number of Cases

Equal

Unequal

486

464 : 95°6 per cent.

R. greater L. greater
12:1°9 per cent.| 10:1°3 percent.

C

22 : 3°2 per cent.

3. Lever

Number of Cases

Equal

Unequal

506

307 : 61 per cent.

R. higher L. higher
54:10°3 percent. | 145 :28°5 perct.

~~

199 : 38°g per cent.

84 THE HUMAN STERNUM

TasLe XXI.—Astachmenis of the Eighth Costal Cartilages in Young Sterna

(under 2.1)

Condition Number Per cent.
Absent... ia ihe ea 123 git
Present—Both ... a8 on 6 50

Right only “ses dl ap 12 66°6 8°8
\ 6 50
Left only ian Se 2 333
Attachment—Junction ... se \ 20°
10
Metasternum se 8 80°0

Taste XXII.—E/even Cases out of fifty-six Sterna of Variation in Attachment
of Seventh and Eighth Costal Cartilages (adult)

Failure of 7th C.C.|/8th C.C. reaches Sternum
Sex Age
R. | L. | Both R. L. | Both
De 40 ora pees 1
M. 44. ites I
F. 32.36 doer lL csseets «| che roe tigi oe,
M. P22 GO. aon! ses. ] ote 3
M. 39-44. vee fees | vee | 2 (almosit)...
? pot perea Wee ran) (ere — 2
I I 5 2| 2

THE HUMAN STERNUM

85

Taste XXI11.—Connexions of Mesosternum with the Ribs in some Mammalia

(ComPiLeD FROM PErsonaL OxsERVATIONS AND OTHER AUTHORS)

use Tota ribs | Sternal ribs | ton | segments of | Attachment of hinder
mesosternum | mesosternum

Ornithorhynchus 16 6 45 2 Peculiar
Echidna 16 6 44 2 Peculiar
Marsupiaqs 13 7 ie 4 6 meso., 7 junct.
Wompat 14 6 45 3 5, 6 junct.
EDENTATA 10-21 5-11 34-94 3-9 Last junct.
Tatusia septemcinctus ... 10 5 34 3 35 i
Tatusia peba II 6 45 4 ” ”
Armadillo 11 7 5a 5 ” ”
Giant Armadillo 12 8 64 6 ” ”
Three-toed sloth 14 L. 8, R.g 64-75 6 55 3
Myrmecophaga jubata ... 18 Il 95 7 ” ”
Cyclothurus didactylus(1) 16 9 63 7 J 95

a9 3 (2) 16 10 7% 8 5 -
Tamandua 15 10 8 8 ” ”
Tamandua bivittata 17 10 8 8 iy ”
Two-toed sloth 21 L. 10, R. 11/R. 94, L. 84 9 ” ”
Carnivora :—Jackal 13 9 73 6 8, 9 junct.
Risen. rufa 14 9 7% 6 8 meso., 9 junct.
om 13 9 74 6 8, 9 junct.
Lioness... 13 9 7% 6 » 9
Dog 13 9 74 6 ”» 9
Puma 13 9 qs 6 ”» 9
Otter 14 10 8h 7 g, 10 junct.
Cercoleptes ree 15 10 8h 8 Last junct. alternate

86

THE HUMAN STERNUM

Tasie XXIJI.— Continued

Name Total ribs | Sternal ribs ae on pou a pet eto
mesosternum | mesosternum
Nasua narica 13 9 73 8 Last junct. alternate
Mustela vulgaris 14 10 84 9 55 *
Ropentia :—Beaver 14 8 54 3 6,7 meso., 8 junct.
Water rat 13 63 4 7 junct., 8 meta.
Coypu ... 13 8 54 4 5 5
Jumping hare ... 2 7 5s 4 6 meso., 7 meta.
Flying squirrel ... 12 7 s4 4 6, 7 junct.
Hystrix cristata 15 L.,14R 8 64 5 7, 8 junct.
Spalax typhlas ... 13 8 64 5 6 meso., 7 junct.
Paca 13 4 53 5 7 junct. alternate
Dasyprocta 13 a 53 5 i 5
Atherura afr. 14 7 53 5 5 55
CHEIROPTERA II 6 44 I 6 junct.
Uneurata :—Peccary ... 14 7 5a 4 6 meso., 7 junct.
Ox 13 7 53 4 7 junct.
Bos indicus (1)... 13 7 $s 4 5s
ie (2)..+ 13 6 44 4 6 junct. alternate
Chamois 13 8 64 4 7 meso., 8 junct.
Bison If R79) Les 8 53-64 5 7, 8 L. junct.
Gazelle... 13 8 64 5 7 meso., 8 junct.
Antelope 13 8 64 5 95 s
Reindeer ? 8 64 5 7 i
InsecTivora :—Mole ... 13 4 54 3 & 4, 5, 63, 7 3rd
piece meso., 7 junct.
Centetes 15 7 54 4 6 meso., 7 junct.

THE HUMAN STERNUM
Taste X XIII.—Continued

87

Ribs attached | Number of :
Name Total ribs Sternal ribs to segments of aeachitent ef Tanase
ribs to mesosternum
mesosternum | mesosternum
Hyracoiwga we 18 6 44 4 5, 6 junct.
Srrenta :—Manatee (2) . 17 2 2 1 Sternum simple
Czracza :—Porpoise 12 4 2 I Fused with pre-
and metasternum
Primates :—Tarsier 13 7 54 3 {#2 grd picce meso.
6, 7 junct.
10 junct., II in
Lemur ... 12 II 94 4 jon of meta. and
with each other
Aye aye 12 9 74 6 8 meso., 9 meta.
Nycticebus tardigradus... 15 a 94 8 10, II junct.
Ateles geoffroyi... 14 8 64 6 8 junct. alternate
Colobus vellerosus 12 R. 7, L. 8 53-05 5 7 junct., 8 L. meta.
Cynocephalus anubis ...| R. 12, L. 13 8 64 7 junct., 8 meta.
Macacus inuus... 12 9 7 6 8 junct., 9 meta.

88 THE HUMAN STERNUM

Taste XXIV
ANTHROPOIDS
weet ates] ments of Matera | Atichment of hinder
sternum
I. Hytopares—
R.C.S. 58 seq] 3 7 4 |? oth — nen iets
61 (young).) 13 7 4]? are ‘ie aaeld
63 seal) SBS 7 4 I si ses ...| 6 meso., 7 junct.
65 -f 13 [R.8,L.7/R.42.34] 2 (median) et ...| 6,7 meso., R. 8 junct.
66 see) a 7 ss i se nee ...| 7 lower end meso.
67 wel 14 8 55 | 2 (median) oe -..| 7 meso., 8 junct.
L’pool 1 cep D4 7 44 | 1 one ies ...| 6 meso., 7 junct.
2 ee ee 7 43 | 3 (median) eas ...| 6 meso., 7 junct.
II. CuimpanzEs—
R.C.S. 1 se) 3 7 sh] 2 6 meso., 7 junct.
2 sie] 13 6 44 | 3 6 meso.
3 ite) 2 ? ? 4 i
4 aaelt 12 7 53 1 3 6 meso., 7 junct.
5 see] 2 ? ? 4 ?
13 (young).) 13 7 54d 4 aa 6 meso., 7 junct.
Iq 5 13 vi) 44 3 6 meso., 7 junct.
15 5 13 7 sto 4 6 meso., 7 junct.
1654 14 8 sho 4 7, 8 junct.
L‘pool (2) vel 13 6 45 | 4 6 junct.—alternate.

THE HUMAN STERNUM
Taste XXIV.—Continued

ANTHROPOIDS.—Continued

89

Number pf tena stacked Segments of Mecosteraum | Attachment of hinder
sternum
III. Gortra—

R.C.S. 20 13 7 5s | 4 (median) -| 6 meso., 7 junct.
21 (young) 13 ? ? 3 (1 and 2 bilateral) |?
21a 4, 13 ? ? 3 (bilateral) ?

23 13 7 54 | 2 (median) 5, 6 meso., 7 junct.
24 13 ? ? 2 (median) ?

29 (young).| 14 7 54 | 4 (bilateral) .| 6 meso., 7 junct.
30 13 7 54 | 3 (bilateral) .«-{ 7 junct.

L’pool (1) 13 7 54 | 3 (median) .| 6 meso., 7 junct.
(2) 13 6 44 3 (median) .| 5 meso., 6 junct.

IV. Oranc—

R.C.S. 37 12 |R.7,L.6) 54, 44] 3 (median) .| R. 7, L. 6, meso.
38 12 6 53 | 3 (median) 5 meso., 6 junct.
38a 12 6 54 | 3 (median fused)... 5 meso., 6 junct.
40 12 7 45 | 3 (median) .| 6 meso., 7 junct.
40a 12 7 53 | 3 (median fused)... .| 6 meso., 7 junct.
40b(young)) 12 |R.7,L.6) 54, 44] 3 (1 and 2 bilateral .| R. 7, L. 6, junct.
4oc gy, 12 7 53 | 3 (bilateral) .| 6 meso., 7 junct.
47 3 12 7 54 | 4 (median, exc. 2 bilateral)) 7 junct.

48 93 12 7 54 | 4 (1, 2, 3 bilateral) .| 6 meso., 7 junct.
50 7 12 |R.7,L.6) 54, 43] 4 (2, 3, 4 bilateral) .| 6 meso., 7 junct.
Si 3 12 7 53 | 4 (bilateral probably) .| 6 meso., 7 junct.
52 e 12 ? ? 4 (bilateral possibly) ?

56a ? 6 4% | 3 (2 bilateral) .| 5, 6 junct.

56b_ C=» ? 7 54 | 4 (bilateral probably .| 6 meso., 7 junct.

L’pool (1), 13 ig 54 | 4 (3 bilateral) .| 6 meso., 7 junct.
(2) 5, 12 8 64 | 4 (median) .| R. 7, 8 junct.; L. 7

meso. ; L. 8 junct.

BIBLIOGRAPHY

ON

BIBLIOGRAPHY

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BIBLIOGRAPHY

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EXPLANATION OF FIGURES

Fig.
Fig.
Fig.
Fig.

Fig.
Fig.
Fig.
Fig.
Fig.
Fig.
Fig.

Fig.

Fig.

Fig.
Fig.

Fig.

Fig.
Fig.
Fig.

Fig.

Fig.

Fig.

Fig.

20.

21.

22.

23.

EXPLANATION OF FIGURES

PLATE I

Human embryo (sagittal section).
Rat g mm. Section across bottom of praecervical sinus.
35 3 Sagittal section through chest wall.

a a Transverse section through middle of chest wall, showing ossifying
ribs and absence of sternum.

Rat 10 mm. \

” ” Magnified seven times, showing clavicles, presternum, mesosternum,
” » and costal cartilages.

Magnified twenty-eight times, to show characteristic features of

Rat 10 —
53 presternum and mesosternum.

”

Rat 10mm. Higher magnification, to show differentiation of tissue in sternum
and costal cartilages.

PLATE II

Rat 1omm. Higher magnification, to show differentiation of tissue in sternum
and costal cartilages.
Rat 13 mm.

Magnified seven times, to show structure and connexions of sternal

e elements.

”? ”
Rat 13 mm \ Magnified twenty-eight times, to show general structure of sternum
; and connexions of costal cartilages.
Human sternum, third month. 8 mm. long, magnified twenty-eight times.
$9 - 10 3 i twenty-one __,,

os 7 9 35 possessing sternal foramen, and eight
costal cartilages—magnified eighteen times.

PLATE III

Rat embryo. Coronal section of cartilaginous sternum, showing primary areolae—
magnified seven times.

Coronal section of older rat’s sternum, showing primary areolae and absence of
ossification in last piece of mesosternum—magnified seven times.

Coronal section of older rat’s sternum, showing primary areolae, thickened peri-
chondrium, and absence of bone formation between sixth and seventh costal
cartilages.

Coronal section of sternum of mouse a few days after birth, showing bone formation
and cartilages.

ii EXPLANATION OF FIGURES

PLATE [I—Continued

Fig. 24. Sternum (lower end) of rat nine days old, showing ossification.

Fig. 25. Section through shoulder-girdle, and sternum of an embryo of Acanthias vulgaris
(advanced), showing the continuous cartilaginous structure.

Fig. 26. Section through early embryo of Acanthias vulgaris, showing the undifferentiated
tissue of shoulder-girdle and sternum.

Fig. 27. Section of chick embryo (six days eighteen hours), showing the formation of the
sternum on one side (St.) and the ribs (RR).

Fig. 28. The half-sternum (St.) of the same embryo more highly magnified, showing the
difference in the tissue of the sternum and the rib cartilages (CC).

PLATE IV

Figs. 29-34. Sections of Rat embryos, showing the differentiation of sternum and clavicles.
P.st., praesternum; V, vein; I.H.mm., infrahyoid muscles; S.S., synovial
sac.

PLATE V

Fig. 35. Diagram of human sternum at birth, showing the average dates of appearance of
centres of ossification, and the percentage of centres present and absent at
the time of birth.

Fig. 36. Sternal cornua. Ossification of the first costal cartilages, in the sternum of a male
aged fifty-four years. (Reduced two-thirds).

Fig. 37. Diagram to illustrate the process of fusion of the several parts of the sternum, with
the average dates of coalescence.

Fig. 32, (&=—0) Varieties in the number and arrangement of the centres of ossification of
8 : the sternum. A, B, male sterna, get. four years. C, female, four
Fig. 39. (D, E) years. D, male, E, female sternum, aes. five years.

PLATE VI

Fig. 40. Sternum of a female, ger. fifty-four, showing fusion of presternum and mesosternum
with an arthrodial surface, for the first costal cartilage (half natural size).

Fig. 41. Sternum of a male, aet. fifty-nine, showing fusion of first piece of mesosternum
with presternum, pre-mesosternal articulation opposite third pair of costal
cartilages, and two-and-a-half pairs of costal cartilages articulating with pre-
sternum (half natural size).

Fig. 42a. Sternum of a male, aev. forty-six. Male type (half natural size).

Fig. 428. 9 » female, ger. thirty-eight. Female type (half natural size).

Fig. 43a. 5 », female foetus in the seventh month, showing supra-sternal cartilages,
fused together, but separate from the presteruum (natural size).

Fig. 438. Sternum of a female foetus at full time, showing two supra-sternal cartilages,
separated from one another, but each fused with the presternum. The
metasternum is absent, and there are only six sternal ribs on the right side
(natural size).

Fig. 44. Presternum of an adult, viewed from behind, showing a supra-sternal ossicle
articulating with the supra-sternal notch on the left side, and an articular
surface on the right side (natural size).

Fig.
Fig.
Fig.
Fig.

Fig.

Fig.
Fig.

Figs.
Fig.

Fig.
Fig.
Fig.
Fig.
Fig.
Fig.

Fig.

Fig.
Fig.
Fig.

Fig.
Fig,
Fig.
Fig.
Fig.

45.
46.
47-
48.

49.

50.
SI.

EXPLANATION OF FIGURES ili

PLATE VII

Supra-sternal notch, normal, broad, and shallow.
a ” ” ” ”
* * 9» deep, and narrow.

Male sternum (ae¢. nine years). Supra-sternal border raised into a median pro-
jection.

Male sternum (aez. forty-six years). Supra-sternal border raised into a median
projection.

Example of supra-sternal ossicles.

Supra-sternal ossicles (from behind). A fused ossicle on the left side ; a raised
articular facet on the right side.

52, 53. Examples of two fused tubercles on the supra-sternal border.

54.

55-
56.

57:
58.

59-
60.

61.

62.
63.
64.

65.
66.
67.
68.
69.

Example of tubercles on the supra-sternal border, with articular facets.

PLATE VIII

Scheme of the homologies of the shoulder-girdle and sternum.

Sternum of a male (ae¢. thirty-five), with sternal foramina (half natural size).
The shoulder-girdle and sternum of the frog (Rana temporaria), from Parker.
The sternum of Chirotes canaliculatus, from Parker (x 9).

Sternum and shoulder-girdle of Lemanctus longipes, from Parker.

% 5 Pe the Crocodile (young), from Parker.

PLATE IX

Scheme of prevalent variations of the sternum, affecting (i) clavicular facets ;
(ii) pre-mesosternal junction ; (ili) thickness of first costal cartilage ; (iv) dis-
placement downwards of left cartilages (2-7) ; (v) detachment of left seventh
costal cartilage ; (vi) attachment of right eighth costal cartilage ; (vii) sternal
foramen ; and (viii) bifurcation of metasternum.

Presternum of chamois (natural size).
Sternum of ant-eater (Myrmecophaga jubata) (two-thirds natural size).
o jackal (two-thirds natural size).

PLATE X

Shoulder-girdle, etc., of Anguis fragilis (x 6), from Parker.
Presternum and supra-sternal ossicles in Mus musculus, from Parker.
Sternum of Manatee (one-quarter natural size).
Sternum, etc., of Echidna.

“9 +5 Ornithorhynchus.

PRINTED BY DONALD FRASER, 37 HANOVER STREET, LIVERPOOL

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PLATE 4.

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PLATE 5.

CONDITION AT BIRTH. DATE OF APPEARANCE.
: ee ant el eel

Centres Present. Centres absent.

"967% 3-3% Sm20nths.

(18876
between
20 &64

years.

99-2 0:8
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Figure 69.