OSTEOLOGY AND ARTHROLOGY IN GENERAL--NOMENCLATURE
--VERTEBRAL COLUMN.
_Osteology_ and _Arthrology_.--Anatomical nomenclature:
median line; lateral parts; the meaning of terms.--Of the
bones in general: long bones (shafts and extremities);
flat bones (surfaces, borders); short bones.--Prominences
(processes, spines); cavities and depressions of bone
(fossæ, grooves).--Bone and cartilage.--The axial skeleton:
the vertebral column.--The vertebræ (bodies, transverse
processes, spinous processes, &c.).--Cervical, Dorsal, Lumbar
vertebræ.--Articulations of the vertebræ.--Movements of the
spine.--Movements of the head (atlas and axis).--The curves of
the vertebral column.--Relation of the vertebral column to the
surface.--Proportions of the parts of the spine.
It is not necessary to emphasise further the importance of a study
of the skeleton. By its means we obtain a knowledge of form and
proportions; by a study of the several articulations we become
acquainted with the complex mechanism by which the whole is knit
together, and by which the movements of the various parts of the body
occur. Further, the relations of the skeleton to the surface forms
of different parts of the body are of fundamental importance. The
science of _Osteology_ is the study of bones (ὀστέον, bone; λόγος,
description); _Arthrology_ is the study of joints (ἅρθρον, a joint):
_Myology_ is the study of muscles (μυς; λογος). The bones are the
levers of movement: the articulations represent the fixed points or
fulcra of these levers; while the powers which produce motion are
represented by the muscles.
[Illustration: FIG. 2.
FRONT VIEW OF THE SKELETON.]
[Illustration: FIG. 3.
SIDE VIEW OF THE SKELETON.]
Before describing in detail the different parts of the skeleton, it
is necessary to consider the method of nomenclature, so that by the
employment of proper terms the subsequent descriptions may be more
intelligible.
NOMENCLATURE.--In the description of the bones, as of other organs, we
have to consider the relation of the portion under consideration to
the rest of the body. The figure is always regarded as occupying the
erect position, with the face, the palms of the hands, and the toes
directed forward. Thus each bone, as well as the other organs or parts,
will be found under one or other of two different conditions: either
it is _median_ in position, and a vertical plane passing through the
longitudinal axis of the body divides it into two similar segments;
or else it is _lateral_ in position, and situated outside this median
plane. As a type of the first class, we will take the _sternum_, or
breast-bone (see Fig. 11, p. 42). This is a central single bone;
it has no fellow, and is composed of two symmetrical portions, one
part on the right and one part on the left. As a type of the second
class, we will take the _humerus_ (Fig. 18, p. 60), which is a bone
situated at the side and one of a pair, inasmuch as there are two,
one on the right and one on the left of the median plane. From these
two examples it is easy to understand that for the description of
each _single and symmetrical_ bone it will be necessary to speak of
anterior parts or surfaces directed towards the front of the body, of
posterior parts (directed backwards), of lateral portions (right and
left), and finally, of parts superior and inferior, looking upwards
and downwards (in the case of the sternum a _superior_ and _inferior
extremity_): on the other hand, in the description of a paired and
_non-symmetrical_ bone, we shall also have to speak as heretofore of
parts superior and inferior, anterior and posterior; but instead of two
similar symmetrical portions, one on each side of an imaginary line, it
has two dissimilar halves, of which the one looking towards the median
plane--towards the axis of the body--is called the _internal_ part,
and the other, looking to the outer side (as away from the axis), is
called the _external_ part. It is necessary, for brevity and accuracy,
to clearly comprehend the meaning of these terms in descriptive anatomy
(anterior and posterior, internal and external, superior and inferior)
which serve to show the relation of the parts to the skeleton as a
whole.
After this first division of bones into single and median, and into
double and lateral, if we glance at the skeleton (Figs. 2, 3), it seems
at first sight that the various bones present an infinite variety of
shape, and defy classification or nomenclature; careful attention,
however, will show us that they may be all included in one of the
following three classes--viz., long bones, flat or broad bones, and
short bones.
[Illustration: FIG. 4.
THE COMPLETE SKELETON (in the attitude of “The Fighting
Gladiator” of Agasias).]
The long bones, which usually act as the axes of the limbs (_e.g._,
the _humerus_, _femur_, _tibia_, &c.), are composed of a central
portion, cylindrical or prismatic in shape, called the _body_, _shaft_,
or _diaphysis_ (διαφύω, to be between), and of two extremities or
_epiphyses_ (ἐπιφύω, to be at the end), usually marked by protuberances
and articular surfaces. The flat bones (_e.g._, the _shoulder-blade_
and the _hip bone_) are formed of osseous plates, on which are various
_surfaces_, _borders_, and _angles_. Finally, the small bones, which
are found in the vertebral column and in the extremities of the limbs,
the hand and foot (_carpus_ and _tarsus_), present a diversity of form
in which cylindrical, cubical, and wedge-like shapes can be made out.
Whether the bone be long, flat, or short, it presents prominences and
depressions. The projecting portions of bone are called by various
names--_tuberosities_, _protuberances_, _processes_, _apophyses_,
_crests_, _spines_, _tubercles_. To some of these names is added an
adjective, which shows, more or less exactly, the form of the process
or projection. Thus we speak of a _spinous process_, _mastoid process_
(μαστὸς, a nipple; εἶδος, form), _styloid process_, &c. The depressions
upon the bones are called by various names--_fossa_, _groove_,
_foramen_, _sinus_, _canal_, _notch_, _cavity_, &c. To these also
are added names which indicate their shape, as the _digital fossa_,
from its resemblance to the imprint of a finger; the _glenoid cavity_
(γλήνη, cavity), the _cotyloid cavity_ (χοτύλη, a basin); but more
frequently still, the added adjective bears allusion to a connection of
the cavity with certain organs, as the _bicipital groove_, that which
contains the tendon of the biceps, or the _canine fossa_, in relation
to the root of the canine tooth.
_Structure of Bone._--Bone is characterised by its density, toughness
and elasticity. If a long bone, such as the femur, is sawn in two
lengthwise, its extremities are found to be composed of a delicate
network of _cancellous_, or spongy bone, in the interstices of which
marrow and blood are contained during life; the shaft of the bone
is composed, for the most part, of a cylindrical tube of dense,
ivory-like compact bone, which encloses the hollow medullary canal of
the bone, also filled with marrow during life. The dense bone of the
shaft is continuous with a thin sheet of hard bone, which covers over
the spongy bone of the extremities.
In the case of the flat and short bones, the structure is like that of
the extremities of the long bones. The mass of the bone is composed of
cancellous tissue, with a surrounding thinner envelope of compact bone.
If a bone is burnt, it loses one-third in weight, becomes brittle,
and loses its organic constituents, retaining its inorganic
materials--chiefly calcium phosphate and calcium carbonate. If it
is subjected to prolonged soaking in an acid such as hydrochloric
acid, its inorganic salts are removed, it becomes soft and pliable,
it loses two-thirds of its weight and retains only its organic
materials--connective tissues. These in boiling produce glue.
In certain situations bone is found in conjunction with a substance
which differs from it in its elasticity, its want of rigidity (it is
soft enough to be divided by the scalpel), and in its translucent
colour. This substance is known by the name of cartilage. Thus the
curved bones termed ribs are prolonged at their anterior extremities by
a portion called the _costal cartilage_, which presents the same form
as the ribs properly so called. The bones forming the freely movable
joints (like the shoulder, hip and knee joints) are capped by thin
layers of hyaline _articular cartilage_, which forms a pliant elastic
cushion in relation to the articulation.
Most of the bones, at the commencement of their formation, are
constructed solely of cartilage, which is gradually transformed into
bone as the animal grows by the deposition in it of lime salts; and
this transformation of primitive cartilage into bone may be more or
less complete according to the species or age of the animal. With
advancing age the bones tend to become more and more calcified. Thus
we find that in the skeletons of old people the costal and other
cartilages may be more or less ossified.
_The Subdivisions of the Skeleton._--The human skeleton is
characterised by peculiarities due to the assumption of the erect
position, the high development of the brain, and the possession of
extraordinary manual dexterity. All these factors leave their impress
on the bones of the skeleton, as may be seen by comparing the human
skeleton with that of such a quadruped as the dog.
The skeleton is subdivided into _axial_ and _appendicular_ parts. The
axial skeleton includes the vertebral column, ribs and sternum, and the
bones of the cranium and face. The _appendicular_ skeleton comprises
the bones of the limbs. In the following pages, for convenience of
description, an account will be given of the vertebral column, sternum,
and ribs first; of the limbs second; reserving to the last the account
of the skeleton of the cranium and face.
_The Vertebral Column._--The vertebral column (Figs. 5, 8) is composed
of a number of bones named _vertebræ_, superimposed on one another, and
partially separated from one another by a series of _intervertebral
discs_. The column is subdivided into groups of vertebræ, by reason of
its connections with other parts of the axial skeleton, or with the
skeleton of the limbs.
[Illustration: FIG. 5.
THE VERTEBRAL COLUMN (antero-lateral aspect).--1, the first
cervical vertebra (atlas);--9, 9, its occipital articulating
surfaces;--2, the second cervical vertebra, or axis;--13, its
body;--4, seventh cervical;--5, 5, transverse processes of the
ten first thoracic vertebræ;--8, 8, transverse processes of the
lumbar vertebræ;--10, 11, 12, articular processes;--19, 19,
bodies of the lumbar vertebræ;--20, the sacrum;--21, the coccyx.]
The _head_ is poised on the upper end of the column, and causes the
peculiarities, to be described later, in the first two vertebræ
(_atlas_ and _axis_). The attachment of the ribs to the sides of
the vertebral column causes the separation of three regions: (1)
_cervical_, belonging to the neck, and comprising seven vertebræ;
(2) _thoracic_ (or dorsal), belonging to the _thorax_, or chest, and
comprising twelve vertebræ; and (3) _lumbar_, belonging to the loin,
and comprising five vertebræ. The attachment of the hip bones to the
sides of the succeeding vertebræ leads to the fusion of the next five
vertebræ together, under the name of the _os sacrum_, which will be
described along with the _hip bone_ and _pelvis_. Finally, below the
sacrum are four small, rudimentary vertebræ, known as the _coccyx_,
forming the attenuated remains of a caudal appendage.
There are thus, altogether, normally thirty-three vertebræ: seven
cervical, twelve thoracic, five lumbar (constituting together
twenty-four movable vertebræ); five sacral, and four coccygeal vertebræ
(constituting nine fixed vertebræ, which help to form the pelvic basin).
[Illustration: FIG. 6.
OUTLINE OF A VERTEBRA (upper surface).--_0_, vertebral
foramen;--1, spinous process;--2, body of vertebra;--3, 3,
transverse process with articulating facets (4, 4) for the
tuberosity of the rib (see p. 30);--5, 5, superior articular
processes;--6, 6, the parts which connect the body with the base
of the transverse and articular processes;--7, 7, vertebral
laminæ.]
The vertebral column is intended not only to form an axis for the
rest of the skeleton, but also to serve as an attachment, direct or
indirect, for all the other bony structures; it also forms a bony
canal, within which the spinal marrow is contained. It is for this
reason that each of the pieces which compose it, called a vertebra,
is a sort of bony ring (Fig. 6). The anterior portion of the ring is
very thick, representing the segment of a cylinder, and is called the
body of the vertebra (2, Fig. 6); and the vertebral column, considered
as the median column of support, is essentially constituted by the
superposition of these vertebral bodies upon one another, separated
by the intervertebral discs. Behind each vertebral body is an arch,
the _neural arch_, which encloses the neural ring. The spinal or
_neural canal_ is formed by the combination and connection together of
the neural rings. Each neural arch is comparatively slender, but it
gives origin to certain projections or processes, three in number,
on each side, of which one directed transversely outwards is called
the _transverse process_ (3, Fig. 6). In the thoracic region these
give partial attachment to the ribs. The other two--directed more or
less vertically, one above, the other below--are called the _articular
processes_, superior and inferior. These serve for uniting together the
arches of adjoining vertebræ (5, 5, Fig. 6). Finally, the posterior
portion of the neural arch is prolonged backwards as a protuberance,
more or less pointed, called the _spinous process_ (1, Fig. 6).
Such are the most important parts which we find in each vertebra, but
they present particular characters according to the region to which
each vertebra belongs. The description of the sacrum and the coccyx,
which are formed of vertebræ welded together, and articulating with the
hip bones, will be given with that of the pelvis.
The more important features of the movable vertebræ which contribute
to give to the whole column its general form are: (1) the size,
particularly of the bodies, of the vertebræ; and (2) the characters of
the transverse processes. The bodies of the vertebræ are smallest in
the upper thoracic region, and increase in size upwards and downwards
from the fourth thoracic vertebra. The bodies are largest and most
prominent in the loin; in the neck the vertebræ are broad in the
transverse diameter, but their antero-posterior diameters are less.
The vertebral column is weakest in the upper thoracic and upper lumbar
regions, and most mobile in the neck and thorax. Rotary power in the
loin is practically prevented by the shape of the lumbar articular
processes, which interlock the vertebral arches in this region.
[Illustration: FIG. 7.
THE MOVABLE VERTEBRÆ.--A, atlas (upper surface); B C, axis; D E,
cervical vertebra; F G, thoracic vertebra; H I, lumbar vertebra.]
The _spinous processes_ of the vertebræ, which project more or less
obviously in the middle line beneath the skin in different regions, in
the cervical region are short and bifid; in the thoracic region they
are long, sloped downwards, and “bayonet-shaped”; in the lumbar region
they are directed straight backwards, and are “hatchet-shaped.”
Besides these general characters in each region there are certain
vertebræ which demand special mention owing to the peculiarities of
their shape. These are the first two and the last cervical.
The _first cervical_ vertebra (Fig. 7, A), called the _Atlas_, because
as directly supporting the head, it has been compared to the giant
Atlas, supposed by the ancients to support the heavens, is a bony ring
with only transverse processes, and on the upper and lower aspects of
its lateral portion, two pairs of articular surfaces; the superior
articular surfaces are hollow oval surfaces which articulate with the
convex condyles of the occipital bone; and by the _occipito-atlantoid_
joints provide for flexion and extension of the head on the spinal
column. The inferior articular surfaces are flat and directed downwards
to articulate with the axis and form the _atlanto-axial_ joint, which
is responsible for the movement from side to side of the head upon the
trunk. The _axis_, or second vertebra (Fig. 7, B C), is so called from
the presence on the upper surface of its body of a tooth-like process,
the odontoid process (ὀδοὺς, tooth; ἔιδος, form), which projects
upwards in an osseofibrous ring formed by a transverse ligament in
the anterior part of the ring of the atlas. Ligaments extend from this
process to the occipital bone, and it forms a pivot round which the
head _and_ the atlas move in the lateral movements of the head upon the
spinal column.
In nodding the head the movement occurs primarily at the
occipito-atlantoid joint; in shaking the head, the chief movement
is between the atlas and axis. These functions, of no moment in the
production of surface forms, are of too great an importance in respect
of the articulations of the head and trunk to be omitted here.
[Illustration: FIG. 8.
VERTEBRAL COLUMN (lateral view).--1 to 7, bodies of cervical
vertebræ;--8 to 19, bodies of thoracic vertebræ;--20 to 24,
bodies of lumbar vertebræ;--A, A, spinous processes;--B, B,
articular surfaces of transverse processes for the tuberosities
of the ribs;--C, auricular surface of sacrum.]
The _seventh_ cervical vertebra, or _vertebra prominens_, is so
called because of the extraordinary length of its spinous process,
which, except in very stout people, forms a projection easily visible
beneath the skin; and this projection is also more conspicuous as
it corresponds to that part of the neck where the trapezius muscle,
represented only by a fibrous layer--not fleshy--forms a flat surface
at the back of the neck. In the centre of this surface the projection
of the seventh cervical spine appears on the level of a transverse line
passing through the superior border of the shoulder (see Fig. 3). It
may be observed that when the model bends the head forward the spinous
process of the seventh cervical becomes very prominent. It should
also be noted that in the majority of cases the spinous processes of
the _sixth cervical_ and _first thoracic_ vertebræ also give rise to
superficial projections above and below that produced by the _vertebra
prominens_.
We have been disconnecting the vertebræ in order to account for
the construction of the vertebral column; we must next see how the
different vertebræ are placed one upon the other--how they articulate
in such a manner as to form a column, not rigid, but elastic and
curved. The vertebræ are placed one on each other so that the inferior
articular processes of one fit exactly on to the superior articular
processes of the next beneath, and thus throughout the series we see
(Fig. 8) that the bodies of the vertebræ are not in contact one with
the other, the space which separates them being filled in the living
subject by elastic fibrous discs. These _intervertebral discs_ are
very thick in the lumbar region, and become thinner in proportion as we
ascend to the superior dorsal and cervical regions. They are thicker
in the cervical and lumbar regions than in the thorax; and taken
together they form one-seventh of the length of the spinal column.
Being compressible and elastic, these fibrous discs give to the column,
formed by the placing one on another of the bodies of the vertebræ, a
certain degree of flexibility, whereas a column formed of bone alone
would have been quite rigid.
In addition to the intervertebral discs, a series of ligaments which
join together the posterior portions of the neural arches (_laminæ_)
is of great importance. Composed of yellow elastic tissue to a large
extent, they are known as the _ligamenta subflava_. They consist of two
short bands placed on each side of the root of the spinous process,
uniting the inferior border of the lamina of one vertebra with the
superior border of the lamina situated next below it.
The _yellow_ or _elastic_ tissue which composes these ligaments is
similar to a piece of india-rubber; it is elastic--that is to say, it
is able to stretch, and to return again by its own reaction to its
original size when the cause which extended it has ceased to act: so
that each movement of flexion of the column in front results in moving
the vertebræ on one another, at the same time stretching these elastic
ligaments. When the anterior muscles of the trunk which accomplish
this flexion cease to contract, it is not necessary, in order to
straighten the column, that the posterior muscles of the back should
come into play; the elasticity of the ligamenta subflava suffices for
this, as they return to their original dimensions and draw together the
vertebral laminæ. We may say, then, that there is at the posterior
portion of the column within each vertebra a pair of small springs
which keeps the column erect, so that the erect attitude of the trunk
is maintained simply by the presence of the elastic ligaments; although
more is required when a man supports upon his back any extra weight or
burden.
_The Ligamentum Nuchæ_ (paxwax) is a large and powerful ligament
composed of yellow elastic tissue. It is highly developed in
quadrupeds, and is attached between the spinous processes of the
cervical vertebræ and the occipital crest, a vertical ridge on the back
of the skull. In man it is a rudimentary structure (as the head is
poised on top of the vertebral column) and forms a membranous partition
separating and giving partial attachment to the muscles of either side
at the back of the neck.
_Curves of the Vertebral Column._--The vertebral column is subject
to a slight lateral curvature, generally towards the right side. Its
chief curves, however, are antero-posterior, and are four in number
(Fig. 8): two, the thoracic and sacral curves, concave forwards, are
primitive embryonic curves; two, cervical and lumbar, convex forwards,
are secondary in their origin. The convexity forwards of the cervical
region is to be connected with the raising upwards of the head on the
trunk; the convex lumbar curve is due to the straightening of the lower
limb, which in the course of development is brought into line with the
vertebral axis.
These curves (except the pelvic or sacral curve) are to be associated
with a difference in the thickness in front and behind of the vertebral
bodies, and of the intervertebral discs in the different regions of
the spine.
In most animals the vertebral column has but two curves, one the
cervical curve, which is convex inferiorly, the other the dorso-lumbar,
which is concave inferiorly.
We have now to examine the influence that the vertebral column has in
moulding the external form of the body, and to see if the length of the
column can be made use of for a system of proportion.
It is evident, in the first place, that the posterior portions of
the vertebræ only can affect the outline of the body, the anterior
portions, the bodies of the vertebræ, being deeply hidden in the cavity
of the thorax and abdomen. Therefore, in the skeleton the posterior
surface of the vertebral column (Fig. 9) presents itself under the
aspect of a median crest, formed by a series of spinous processes, the
_spinal crest_, on each side of which is a groove bounded laterally
by the series of transverse processes (the _vertebral furrow_). In
the living subject these grooves are filled up by powerful and thick
muscles, which project in such a manner that in the erect position
the back presents a furrow in the median line bounded on each side by
these muscles, at the bottom of which furrow the bony structure of the
vertebral column is shown only by a series of projections placed one
beneath the other, like the beads of a necklace, each one being formed
by the summit or free extremity of a spinous process. These projections
are well seen in the thoracic region, in which the curvature of the
column is convex backwards, and they show themselves still more
clearly when the subject bends forward, and thereby increases this
curvature. They are not visible in the cervical region, where the
ligamentum nuchæ projects to the surface, and a bed of powerful muscles
covers them; but we have seen that the seventh cervical, or _vertebra
prominens_--along with the sixth also in many cases--is remarkable for
the projection which its spinous process makes. Finally, in the lumbar
region these projections are but little marked, the spinous processes
here being short and terminated not by points, but by straight borders
(Fig. 8).
[Illustration: FIG. 9.
VERTEBRAL COLUMN (posterior view).--1, 1, cervical transverse
processes;--2, 2, thoracic transverse processes;--3, 3, lumbar
transverse processes;--7, 8, 9, 10, spinous processes;--11, 11,
articular surfaces for occipital bone of skull;--12, odontoid
process of the axis;--13, 14, sacrum and coccyx.]
The measurements of the vertebral column are useful, on the one hand,
as absolute measurements of length and height, and, on the other hand,
in giving the ratio of its length to the stature of the subject.
The height of the vertebral column in the average adult man is from
twenty-three to twenty-four inches, being five for the cervical region,
eleven for the thoracic, and seven inches for the lumbar region. But as
the length of the vertebral column does not serve as a common measure
for the total height of the body or for its different parts, it cannot
be used as the basis of a system of proportion. A German zoologist,
Carus, has advanced the idea that the length of the column forms one
third of the height; but this proposition is not exact. On the other
hand, it is not easy to measure the column from the atlas as far as the
last lumbar vertebra, without taking account of the sacrum and coccyx.
It will be more frequently found that the length of the trunk, from the
superior limit of the thorax to the inferior limit of the pelvis, gives
a measurement more easy to take, and more useful for comparing the
general proportions of the body.
[Illustration: FIG. 10.
OUTLINE OF THE BACK AND SHOULDERS.]
It is enough to say here that the proportion of the vertebral column
to the height varies according to age and sex, and according as the
stature of the individual is very great or very little; the vertebral
column is, in fact, in comparison with the height, longer in the infant
and in the female than in the adult male; it is also much longer in
proportion to the height in subjects of short stature than in tall
people. The cause of difference of stature between men and women,
infants and adults, long people and short, is principally due to the
length of the lower extremities--a question which will be dealt with in
a subsequent chapter.