Ultimate elements of which the body is composed—Proximate
principles—Fluids and solids—Primary
tissues—Combinations—Results—Organs, systems,
apparatus—Form of the body—Division into head, trunk, and
extremities—Structure and function of each—Regions—Seats
of the more important internal organs.
1. The ultimate elements of which the human body is composed are
azote, oxygen, and hydrogen (gaseous fluids); and carbon, phosphorus,
calcium, sulphur, sodium, potassium, magnesium, and iron (solid
substances). These bodies are called elementary and ultimate, because
they are capable of being resolved by no known process into more simple
substances.
2. These elementary bodies unite with each other in different
proportions, and thus form compound substances. A certain proportion
of azote uniting with a certain proportion of oxygen, hydrogen, and
carbon, forms a compound substance possessing certain properties.
Another proportion of azote uniting with a different proportion
of oxygen, hydrogen, and carbon, forms another compound substance
possessing properties different from the former. Oxygen, hydrogen, and
carbon, uniting in still different proportions without any admixture
of azote, form a third compound possessing properties different from
either of the preceding. The compounds thus formed by the primary
combinations of the elementary substances with each other are called
PROXIMATE PRINCIPLES.
3. Each proximate principle constitutes a distinct form of animal
matter, of which the most important are named gelatin, albumen, fibrin,
oily or fatty matter, mucus, urea, pichromel, osmazome, resin, and
sugar.
4. By chemical analysis it is ascertained that all the proximate
principles of the body, however they may differ from each other in
appearance and in properties, are composed of the same ultimate
elements. Gelatin, for example, consists (in 100 parts) of azote
16-988/1000, oxygen 27-207/1000, hydrogen 7-914/1000, carbon
47-881/1000 parts. The elementary bodies uniting in the above
proportions form an animal substance, soft, tremulous, solid, soluble
in water, especially when heated, and on cooling, which may be
considered as its distinctive property, separating from its solution in
water into the same solid substance, without undergoing any change in
its chemical constitution.
5. Again, albumen consists of azote 15-705/1000, oxygen 23-872/1000,
hydrogen 7-540/1000, carbon 52-888/1000, parts. The elementary bodies
uniting in these different proportions, there results a second
proximate principle, an adhesive fluid, transparent, destitute of smell
and taste, miscible in water, but when subjected to a temperature of
about 165°, converted into a solid substance no longer capable of being
dissolved in water. This conversion of albumen from a fluid, which is
its natural state, into a solid, by the application of heat, is called
coagulation. It is a process familiar to every one. The white of egg is
nearly pure albumen, naturally a glary and adhesive fluid: by boiling,
it is coagulated into a white and firm solid.
6. In like manner, fibrin consists of azote 19-934/1000, oxygen
19-685/1000, hydrogen 7-021/1000, carbon 53-360/1000 parts, forming
a solid substance of a pale whitish colour and firm consistence, the
peculiar character of which is its disposition to arrange itself into
minute threads or fibres.
7. On the other hand, fat or oil, which is a fluid substance of a
whitish yellow colour, inodorous, nearly insipid, unctuous, insoluble
in water and burning with rapidity, consists of a larger proportion of
hydrogen, a small proportion of oxygen, and a still smaller proportion
of carbon, without any admixture of azote.
8. From this account of the composition of the proximate principles,
which it is not necessary to extend further, it is manifest that all of
them consist of the same ultimate elements, and that they derive their
different properties from the different proportions in which their
elements are combined.
9. The ultimate elements that compose the body are never found in a
separate or gaseous state, but always in combination in the form of one
or other of the proximate principles.
10. In like manner, the proximate principles never exist in a distinct
and pure state, but each is combined with one or more of the others. No
part consists wholly of pure albumen, gelatin, or mucus, but albumen is
mixed with gelatin, or both with mucus.
11. Simple or combined, every proximate principle assumes the form
either of a fluid or of a solid, and hence the most general and obvious
division of the body is into fluids and solids. But the terms fluid
and solid are relative, not positive; they merely express the fact
that some of the substances in the body are soft and liquid compared
with others which are fixed and hard; for there is no fluid, however
thin, which does not hold in solution some solid matter, and no solid,
however dense, which does not contain some fluid.
12. Fluids and solids are essentially the same in nature; they
differ merely in their mode of aggregation; hence the easy and rapid
transition from the one to the other which incessantly takes place in
the living body, in which no fluid long remains a fluid, and no solid a
solid, but the fluid is constantly passing into the solid and the solid
into the fluid.
13. The relative proportion of the fluids in the human body is always
much greater than that of the solids; hence its soft consistence and
rounded form. The excess, according to the lowest estimate, is as 6
to 1, and according to the highest, as 10 to 1. But the proportion is
never constant; it varies according to age and to the state of the
health. The younger the age, the greater the preponderance of the
fluids. The human embryo, when first perceptible, is almost wholly
fluid: solid substances are gradually but slowly superadded, and even
after birth the preponderance is strictly according to age; for in
the infant, the fluids abound more than in the child; in the child,
more than in the youth; in the youth, more than in the adolescent; in
the adolescent, more than in the adult; and in the adult, more than
in the aged. Thus, among the changes that take place in the physical
constitution of the body in the progress of life, one of the most
remarkable is the successive increase in the proportion of its solid
matter: hence the softness and roundness of the body in youth; its
hard, unequal, and angular surface in advanced life; its progressively
increasing fixedness and immobility in old age, and ultimate inevitable
death.
14. The fluids are not only more abundant than the solids, but they
are also more important, as they afford the immediate material of the
organization of the body; the media by which both its composition and
its decomposition are effected. They bear nourishment to every part,
and by them are carried out of the system its noxious and useless
matter. In the brain they lay down the soft and delicate cerebral
substance; in the bone, the hard and compact osseous matter; and the
worn-out particles of both are removed by their instrumentality. Every
part of the body is a laboratory in which complicated and transforming
changes go on every instant; the fluids are the materials on which
these changes are wrought; chemistry is the agent by which they are
effected, and life is the governing power under whose control they take
place.
15. The fluids, composed principally of water holding solid matter in
solution, or in a state of mechanical division, either contribute to
the formation of the blood, or constitute the blood, or are derived
from the blood; and after having served some special office in a
particular part of the system, are returned to the blood; and according
to the nature and proportion of the substances they contain, are
either aqueous, albuminous, mucous, gelatinous, fibrinous, oleaginous,
resinous, or saline.
16. When the analysis of the different kinds of animal matter that
enter into the composition of the body has been carried to its ultimate
point, it appears to be resolvable into two primitive forms: first, a
substance capable of coagulation, but possessing no determinate figure;
and secondly, a substance having a determinate figure and consisting of
rounded particles. The coagulable substance is capable of existing by
itself; the rounded particles are never found alone, but are invariably
combined with coagulated or coagulable matter. Alone or combined with
the rounded particles, the coagulable matter forms, when liquid, the
fluids, when coagulated, the solids.
17. When solid, the coagulable substance is disposed in one of two
forms, either in that of minute threads or fibres, or in that of minute
plates or laminæ; hence every solid of the body is said to be either
fibrous or laminated. The fibres or laminæ are variously interwoven
and interlaced, so as to form a net-work or mesh; and the interspaces
between the fibres or laminæ are commonly denominated areolæ or cells
(fig. XVII).
18. This concrete substance, fibrous or laminated, is variously
modified either alone or in combination with the rounded particles.
These different modifications and combinations constitute different
kinds of organic substance. When so distinct as obviously to possess a
peculiar structure and peculiar properties, each of these modifications
is considered as a separate form of organized matter, and is called a
PRIMARY TISSUE. Anatomists and physiologists have been at great pains
to discriminate and classify these primary tissues; for it is found
that when employed in the composition of the body, each preserves its
peculiar structure and properties wherever placed, however combined,
and to whatever purpose applied, undergoing only such modification
as its local connexions and specific uses render indispensable.
Considering every substance employed in the construction of the body,
not very obviously alike, as a distinct form of organized matter, these
primary tissues may be said to consist of five, namely, the membranous,
the cartilaginous, the osseous, the muscular, and the nervous.
19. The first primary tissue is the peculiar substance termed MEMBRANE.
It has been already stated (16) that one of the ultimate forms of
animal matter is a coagulable substance, becoming concrete or solid
under the process of coagulation. The commencement of organization
seems to be the arrangement of this concrete matter into straight
thready lines, at first so small as to be imperceptible to the naked
eye. Vast numbers of these threads successively uniting, at length
form a single thread of sufficient magnitude to be visible, but still
smaller than the finest thread of the silkworm. If the length of these
threads be greater than their breadth, they are called fibres; if,
on the contrary, their breadth exceed their length, they are termed
plates or laminæ. By the approximation of these fibres or plates in
every possible direction, and by their accumulation, combination, and
condensation, is constituted the simplest form of organized substance,
the primary tissue called membrane.
20. Membrane once formed is extensively employed in the composition
of the body: it is indeed the material principally used in producing,
covering, containing, protecting, and fixing every other component
part of it. It forms the main bulk of the cartilaginous tissue; it
receives into its cells the earthy matter on which depend the strength
and hardness of the osseous tissue; it composes the canals or sheaths
in which are deposited the delicate substance of the muscular, and the
still more tender pulp of the nervous tissue; it gives an external
covering to the entire body; it lines all its internal surfaces; it
envelopes all internal organs; it enters largely as a component element
into the substance of every organ of every kind; it almost wholly
constitutes all the internal pouches and sacs, such as the stomach, the
intestines, the bladder; and all tubes and vessels, such as arteries,
veins, and lymphatics; it furnishes the common substance in which
all the parts of the body are, as it were, packed; it fills up the
interstices between them; it fixes them in their several situations; it
connects them all together; in a word, it forms the basis upon which
the other parts are superinduced; or rather the mould into which their
particles are deposited; so that were it possible to remove every other
kind of matter, and to leave this primary tissue unaltered in figure
and undiminished in bulk, the general form and outline of the body, as
well as the form and outline of all its individual parts, would remain
unchanged.
21. The properties which belong to membrane are cohesion, flexibility,
extensibility, and elasticity. By its property of cohesion, the several
parts of the body are held together; by its combined properties of
cohesion, flexibility, and extensibility, the body in general is
rendered strong, light, and yielding, while particular parts of it
are made capable of free motion. But elasticity, that property by
which parts removed from their situation in the necessary actions of
life are restored to their natural position, may be regarded as its
specific property. The varied purposes accomplished in the economy
by the property of elasticity will be apparent as we advance in our
subject. Meantime, it will suffice to observe that it is indispensable
to the action of the artery in the function of the circulation; to the
action of the thorax in the function of respiration; to the action of
the joints in the function of locomotion: in a word, to the working
of the entire mechanism by which motion of every kind and degree
is effected. All these properties are physical, not vital; vital
properties do belong even to this primary form of animal matter; but
they are comparatively obscure. In the tissue with which organization
commences, and which is the least removed from an inorganic substance,
the properties that are prominent and essential are merely physical.
22. By chemical analysis, membrane is found to contain but a small
proportion of azote, the peculiar element of animal matter. Its
proximate principles are gelatin, albumen, and mucus. In infancy and
youth, gelatin is the most abundant ingredient; at a more advanced
period, albumen predominates[3]. Gelatin differs from albumen in
containing a less proportion of azote and a greater proportion of
oxygen; on both accounts it must be regarded as less animalized. Thus
animalization bears a certain relation to organization. The simplest
animal tissue is the least animalized, and the least of all at the
earliest period of life. Not only are the physical and mental powers
less developed in the young than in the adult, but the very chemical
composition of the primary tissue of which the body is constructed is
less characteristic of the perfect animal.
23. Membrane exists under several distinct forms; a knowledge of the
peculiarities of which will materially assist us in understanding the
composition of the body. The simplest form of membrane, and that which
is conceived to constitute the original structure from which all the
others art produced, is termed the _cellular_. When in thin slices,
_cellular membrane_ appears as a semi-transparent and colourless
substance; when examined in thicker masses, it is of a whitish or
greyish colour. It consists of minute threads, which cross each other
in every possible direction, leaving spaces between them, and thus
forming a mesh or net-work (fig. XVII.), not unlike the spider's web.
The term cells, given to these interspaces, is employed rather in a
figurative sense than as the expression of the fact; for there are no
such distinct partitions as the term cell implies. The best conception
that can be formed of the arrangement of the component parts of
this structure is, to suppose a substance consisting of an infinite
number of slender thready lines crossing each other in every possible
direction (fig. XVII.). The interspaces between these lines during
life, and in the state of health, are filled with a thin exhalation of
an aqueous nature, a vapour rather than a fluid, rendering and keeping
the tissue always moist. This vapour consists of the thinner part of
the blood, poured into these interstitial spaces by a process hereafter
to be described, termed secretion. When occupying those spaces, it
makes no long abode within them, but is speedily removed by the process
of absorption. In health, these two operations exactly equal each
other; but if any cause arise to disturb the equilibrium, the vapour
accumulates, condenses and forms an aqueous fluid, which distends the
cells and gravitates to the most depending parts. Slightly organized
as this tissue is, and indistinct as its vita functions may be, it
is obvious that it must be the seat of at least two vital functions,
secretion and absorption.
[Illustration: Fig. XVII.
A single film of the cellular tissue lifted up and slightly distended.]
24. It is certain that the interspaces or cells of this membrane have
no determinate form or size, that they communicate freely with each
other, and that this communication extends over the whole body; for if
a limb which has been infiltrated be frozen, a thousand small icicles
will be formed, assuming the shape of the containing cells, some of
which are found to be circular and others cylindrical, and so on. If
air or water escape into any particular part of the body, it is often
effused over the whole extent of it, and butchers are observed to
inflate animals by making a puncture in some part where the cellular
tissue is loose, and from this one aperture the air is forced to the
most distant parts of the body.
25. Cellular membrane, variously modified and disposed, forms the
main bulk of all the other solid parts of the body, constituting
their common envelope and bond of union, and filling up all their
interstices. It is dense or loose, coarse or fine, according to its
situation and office. Wherever it is subject to pressure, it is dense
and firm, as in the palm of the hand and the sole of the foot; around
the internal organs it is more loose and delicate, and it becomes finer
and finer as it divides and subdivides, in order to envelope the soft
and tender structures of the body.
[Illustration: Fig. XVIII.
A portion of cellular tissue, very highly magnified, showing the
strings of globules of which its ultimate fibres are by some supposed
to consist.]
26. According to some who have carefully examined with the microscope
its component threads, they consist of minute particles of a globular
figure (fig. XVIII.); other microscopical observers regard the cellular
threads as coagulated or condensed animal substance, perfectly
amorphous (without form).
27. Every part of this tissue is penetrated by arteries, veins,
absorbents, and nerves, endowing it with properties truly vital, though
in a less degree than any of the other primary tissues; and varied and
important as the uses are which it serves in the economy, the most
manifest, though certainly not the only ones, are those which depend
upon its physical properties of cohesion, flexibility, extensibility,
and elasticity.
[Illustration: Fig. XIX. 1, A portion of adipose tissue; 2, minute bags
containing the fat; 3, a cluster of the bags, separated and suspended.]
28. The tissue which contains the fat, termed the _adipose_, is
the second form of membrane; it is obviously a modification of the
cellular, from which it differs both in the magnitude of its fibres,
whence it constitutes a tougher and coarser web, and in their
arrangement; for it is so disposed as to form distinct bags in which
the fat is contained. Adipose tissue consists of rounded packets,
separated from each other by furrows (fig. XIX. 2, 2); each packet
is composed of small spheroidal particles (fig. XIX. 2, 2); each
particle is again divisible into still smaller grains, which, on minute
inspection, present the appearance of vesicles filled with the adipose
matter (fig. XIX. 3).
29. The cells of the cellular tissue, as has been shown (24), are
continuous over the whole body; but each adipose vesicle is a distinct
bag, having no communication whatever with any other (fig. XIX. 2, 2).
The cellular tissue is universally diffused; but the adipose is placed
only in particular parts of the body; principally beneath the skin, and
more especially between the skin and the abdominal muscles, and around
some of the organs contained in the chest and abdomen, as the heart,
the kidneys, the mesentery, and the omenta. In most of these situations
some portion of it is generally found, whatever be the degree of
leanness to which the body may be reduced; while in the cranium, the
brain, the eye, the ear, the nose, and several other organs, there is
none, whatever be the degree of corpulency. The uses of the fat, which
are various, will be stated hereafter.
30. The third form of membrane is termed the
_serous_. Like the adipose, _serous membrane_ is a modification of
the cellular, and, like it also, it is limited in its situation to
particular parts of the body, that is, to its three great cavities,
namely, the head, the chest, and the abdomen. To the two latter it
affords an internal lining, and to all the organs contained in all
the three cavities, it affords a covering. By its external surface it
is united to the wall of the cavity or the substance of the organ it
invests; by its internal surface it is free and unattached; whence this
surface is in contact only with itself, forming a close cavity or shut
sac, having no communication with the external air. Smooth and polished
(fig. XX.), it is rendered moist by a fluid which is supposed to be
exhaled in a gaseous state from the serum of the blood; and from this
serous fluid the membrane derives its name.
[Illustration: Fig. XX.
A portion of intestine, showing its external surface or serous coat.]
31. Though thin, serous membrane is dense, compact, and of great
strength in proportion to its bulk: it is extensible and elastic;
extensible, for it expands with the dilatation of the chest in
inspiration; elastic, for it contracts with the diminished size
of the chest in expiration. In like manner, it stretches with the
enlargement of the stomach during a hearty meal, and contracts as
the stomach gradually diminishes on emptying itself of its contents.
It is furnished with no blood-vessels large enough to admit the
colouring matter of the blood; but it is supplied with a great number
of the colourless vessels termed exhalents, with the vessels termed
absorbents, and with a few nerves. It indicates no vital properties,
but those which are common to the simple form of the primary tissue.
Its specific uses are to afford a lining to the internal cavities; to
furnish a covering to the internal organs; by its polished and smooth
surface, to allow a free motion of those organs on each other, and by
the moisture with which it is lubricated, to prevent them from adhering
together, however closely, or for however long a period they may be in
contact.
32. The fourth form of membrane, the _fibrous_, named from the obvious
arrangement of its component parts, consists of longitudinal fibres,
large enough to be visible to the naked eye, placed parallel to each
other, and closely united. Sometimes these fibres are combined in such
a manner as to form a continuous and extended surface, constituting
a thin, smooth, dense, and strong membrane, such as that which lines
the external surface of bones termed PERIOSTEUM, or the internal
surface of the skull (dura mater). At other times, they form a firm and
tough expansion (aponeurosis) which descends between certain muscles,
separating them from each other, and affording a fixed point for the
origin or insertion of neighbouring muscles; or which is stretched
over muscles, and sometimes over even an entire limb, in order to
confine the muscles firmly in their situation, and to aid and direct
their action (fig. XXVII.). Fibrous membrane also constitutes the
compact, strong, tough, and flexible bands used for tying parts firmly
together, termed LIGAMENTS, principally employed in connecting the
bones with each other, and particularly about the joints; and lastly,
fibrous membrane forms the rounded white cords in which muscles often
terminate, called TENDONS (fig. XXV., XXVI.), the principal use of
which is to connect the muscles with the bones, and to serve as cords
or ropes to transmit the action of the muscle to a distant point, in
the accomplishment of which purposes their operation appears to be
entirely mechanical.
33. The fifth form of membrane, the _mucous_ (fig. XXI.), derives its
name from the peculiar fluid with which its surface is covered, called
mucus, and which is secreted by numerous minute glands, imbedded in
the substance of the membrane. As serous membrane forms a shut sac,
completely excluding the air, mucous membrane, on the contrary, lines
the various cavities which are exposed to the air, such as the mouth,
the nostrils, the wind-pipe, the gullet, the stomach, the intestines,
the urinary organs, and the uterine system. Its internal surface, or
that by which it is attached to the passages it lines, is smooth and
dense; its external surface, or that which is exposed to the contact
of the air, is soft and pulpy, like the pile of velvet (fig. XXI.). It
bears a considerable resemblance to the external surface of the rind of
the ripe peach.
[Illustration: Fig. XXI.
A portion of the stomach, showing its internal surface or mucous coat.]
Unlike all the other tissues of this class, the mucous membranes are
the immediate seat of some of the most important functions of the
economy; in the lung, of respiration; in the stomach, of digestion; in
one part of the intestine, of chylification; in another, of excretion;
while in the mouth and nose, they are the seat of the animal functions
of taste and smell; and they are highly organized in accordance with
the importance of the functions they perform.
34. The last form of membrane which it is necessary to our present
purpose to particularize, is that which constitutes the external
covering of the body, and which is called the _skin_. The skin is
everywhere directly continuous with the mucous membranes that line the
internal passages, and its structure is perfectly analogous. Both the
external and the internal surface of the body may be said therefore to
be covered by a continuous membrane, possessing essentially the same
organization, and almost identically the same chemical composition.
The skin is an organ which performs exceedingly varied and important
functions in the economy, to the understanding of which it is necessary
to have a clear conception of its structure; some further account of it
will therefore be required; but this will be more advantageously given
when the offices it serves are explained.
[Illustration: Fig. XXII. Portions of cartilage, seen in section.]
35. Such is the structure, and such are the properties, of the
first distinct form of organized matter. The second primary tissue,
termed the CARTILAGINOUS (fig. XXII.), is a substance intermediate
between membrane and bone. The nature of its organization is not
clearly ascertained. By some anatomists, it is regarded as a uniform
and homogeneous substance, like firm jelly, without fibres, plates,
or cells; others state that they have been able to detect in it
longitudinal fibres, interlaced by other fibres in an oblique and
transverse direction, but without determinate order. All are agreed
that it is without visible vessels or nerves: not that it is supposed
to be destitute of them, but that they are so minute as to elude
observation. Its manifest properties are wholly mechanical. It is
dense, strong, inextensible, flexible, and highly elastic. It is
chiefly by its property of elasticity that it accomplishes the various
purposes it serves in the economy. It is placed at the extremities of
bones, especially about the joints, where, by its smooth surface, it
facilitates motion, and, by its yielding nature, prevents the shock or
jar which would be produced were the same kind and degree of motion
effected by a rigid and inflexible substance. Where a certain degree
of strength with a considerable degree of flexibility are required, it
supplies the place of bone, as in the spinal column, the ribs and the
larynx.
[Illustration: Fig. XXIII.
Membranous portion of bone; the osseous portion being so completely
removed, that the bone is capable of being tied in a knot.]
36. The third distinct form of organized matter is termed the
OSSEOUS tissue. Bone is composed of two distinct substances, an animal
and an earthy matter: the former organic, the latter inorganic. The
animal or organic matter is analogous both in its nature and in its
arrangement to cellular tissue; the earthy or inorganic matter consists
of phosphoric acid combined with lime, forming phosphate of lime. The
cellular tissue is aggregated into plates or laminæ, which are placed
one upon another, leaving between them interspaces or cells, in which
is deposited the earthy matter (phosphate of lime). If a bone, for
example, the bone called the radius, one of the bones of the fore-arm,
be immersed in diluted sulphuric, nitric, muriatic, or acetic acid, it
retains its original bulk and shape; it loses, however, a considerable
portion of its weight, while it becomes so soft and pliable, that it
may be tied in a knot (fig. XXIII.). In this case, its earthy matter
is removed by the agency of the acid, and is held in solution in the
fluid; what remains is membranous matter (cellular tissue). If the
same bone be placed in a charcoal fire, and the heat be gradually
raised to whiteness, it appears on cooling as white as chalk; it is
extremely brittle; it has lost much of its weight, yet its bulk and
shape continue but little changed. In this case, the membraneous matter
is wholly consumed by the fire, while the earth is left unchanged (fig.
XXIV.). Every constituent atom of bone consists, then, essentially
of animal and earthy matter intimately combined. A little more than
one-third part consists of animal matter (albumen), the remaining
two-thirds consist of earthy matter (phosphate of lime); other saline
substances, as the fluate of lime and the phosphate of magnesia, are
also found in minute quantity, but they are not peculiar to bone.
[Illustration: Fig. XXIV.
Earthy portion of bone.]
37. In general, the osseous tissue is placed in the interior of the
body. Even when bone approaches the surface, it is always covered
by soft parts. It is supplied with but few blood-vessels, with
still fewer nerves, with no absorbents large enough to be visible,
so that though it be truly alive, yet its vital properties are not
greatly developed. The arrangement of its component particles is
highly curious; the structure, the disposition, and the connexion of
individual bones afford striking examples of mechanism, and accomplish
most important uses in the economy; but those uses are dependent
rather upon mechanical than vital properties. The chief uses of bone
are— 1. By its hardness and firmness to afford a support to the soft
parts, forming pillars to which the more delicate and flexible organs
are attached and kept in their relative positions. 2. To defend the
soft and tender organs by forming a case in which they are lodged and
protected, as that formed by the bones of the cranium for the lodgment
and protection of the brain (fig. XLVII.); by the bones of the spinal
column for the lodgment and protection of the spinal cord (fig.
XLVIII.); by the bones of the thorax (fig. LIX.), for the lodgment and
protection of the lungs, the heart, and the great vessels connected
with it (fig. LIX.). 3. By affording fixed points for the action of the
muscles, and by assisting in the formation of joints to aid the muscles
in accomplishing the function of locomotion.
38. All the primary tissues which have now been considered consist of
precisely the same proximate principles. Albumen is the basis of them
all; with the albumen is always mixed more or less gelatin, together
with a minute quantity of saline substance: to the osseous tissue is
superadded a large proportion of earthy matter. With the exception
of the mucous, the organization of all these tissues is simple;
their vital properties are low in kind and in degree; their decided
properties are physical, and the uses they serve in the economy are
almost wholly mechanical.
[Illustration: Fig. XXV.
Portion of a muscle; showing (_a_) the muscular fibres and their
parallel direction; and (_b_) the termination of the fibres in tendon.]
39. But we next come to a tissue widely different in every one of
those circumstances, a tissue consisting of a new kind of animal
matter, and endowed with a property not only peculiar to itself,
but proper to living substance, and characteristic of a high degree
of vital power. MUSCULAR TISSUE, the fourth distinct form of animal
matter, commonly known under the name of flesh, is a substance
resembling no other in nature. It consists of a soft and pulpy
substance, having little cohesive power, arranged into fibres which
are distinctly visible to the naked eye, and which are disposed in a
regular and uniform manner, being placed close and parallel to each
other (fig. XXV.). These fibres are every where pretty uniformly the
same in shape, size, and general appearance, being delicate, soft,
flattened, and though consisting only of a tender pulp, still solid
(fig. XXV.). When examined under the microscope, fibres, which to the
naked eye appear to be single threads, are seen to divide successively
into smaller threads, the minutest or the ultimate division not
exceeding, as is supposed, the 40,000th part of an inch in diameter. On
the other hand, the fibres which are large enough to be visible to the
naked eye, are obviously aggregated into bundles of different magnitude
in different muscles, but always of the same uniform size in the same
muscle (fig. XXV.).
[Illustration: Fig. XXVI.
Two portions of muscle; one of which, _a_, is covered with membrane;
the other, _b_, is uncovered; _c_, the muscular fibres terminating in
tendon.]
40. The ultimate thread, or the minutest division of which the
muscular fibre is susceptible, is called a filament; the smallest
thread which can be distinguished by the naked eye is termed a fibre
(fig. XXVI.); and the bundle which is formed by the union of fibres
is denominated a fasciculus. The proper muscular substance is thus
arranged into three distinct forms progressively increasing in
size,—the filament, the fibre, and the fasciculus. The filament, the
fibre, the fasciculus, as well as the muscle itself, formed by the
aggregation of fasciculi, is each inclosed in its own distinct sheath
of cellular membrane (fig. XXVI. _a_).
[Illustration: Fig. XXVII.
Portion of a muscle enclosed in a sheath of fascia or aponeurosis.]
41. The composition of the ultimate filament has been very carefully
examined by many distinguished physiologists with microscopes of high
magnifying power. Under some of these microscopes the filament appears
to consist of a series of rounded particles or globules of the same
size as the particles of the blood when deprived of their colouring
matter, so that it looks like a string of pearls (fig. XXVIII.), each
globule being commonly stated to be about the 2000th part of an inch
in diameter. But it is now pretty generally agreed that this globular
appearance of the ultimate muscular fibre vanishes under the more
improved microscopes of the present day, and, as viewed by the latter,
appears as a peculiar pulpy substance arranged into threads of extreme
minuteness, placed close and parallel to each other, intersected by a
great number of delicate lines passing transversely across the muscular
threads (fig. XXIX.),
[Illustration: Fig. XXVIII.
Ultimate fibres of muscle, very greatly magnified; showing the strings
of globules of which they are supposed by some to consist.]
42. With the exception of the organs of sense, the muscular tissue is
more abundantly supplied with arteries, veins, and nerves, than any
other substance of the body. Every ultimate thread or filament appears
to be provided with the ultimate branch of an artery, vein, and nerve.
These vessels are seen ramifying on the surface of the delicate web of
membrane that incloses the pulp, but cannot be traced into it.
[Illustration: Fig. XXIX.
The appearance of the ultimate muscular fibres and of their transverse
lines, as seen under the microscope of Mr. Lister, when the object is
magnified 500 diameters.]
43. The proximate principle of which the muscular pulp is composed
is fibrin. From the pulp, when inclosed in its sheath of membrane,
albumen, jelly, various salts, and a peculiar animal extract called
osmazome, are also obtained; but these substances are probably derived
from the membranous, not the muscular, matter. Fibrin contains a larger
proportion of azote, the element peculiar to the animal body, and by
the possession of which its chemical composition is distinguished from
that of the vegetable, than any other animal substance.
[Illustration: Fig. XXX.
Portion of the trunk of a nerve; dividing into branches.]
44. Muscular tissue possesses a slight degree of cohesion, a high
degree of flexibility and extensibility, but no degree of elasticity;
for although muscle, considered as a compound of muscular substance and
membrane, be highly elastic, yet this property is probably altogether
owing to the membranous matter in which it is enveloped. Its peculiar
and distinctive property is vital, not physical, and consists in the
power of diminishing its length, or of contracting or shortening itself
on the application of a stimulus. This property, which is termed
contractility, is the great, if not the sole source of motion in the
body. Without doubt, elasticity and gravity, under the generating and
controlling powder of contractility, aid in accomplishing various
kinds of motion. Thus membranes, tendons, ligaments, cartilages, and
bones, by their physical and mechanical properties, modify, economize,
facilitate, concentrate and direct the motive power generated by the
pure muscular substance; but still the only real source of motion in
the body is muscular tissue, and the only mode in which motion is
generated is by contractility. This will be more fully understood
hereafter.
[Illustration: Fig. XXXI.
Ultimate fibres of nerve, very highly magnified; showing the strings of
globules of which they consist.]
45. The last primary tissue, termed the NERVOUS, is equally distinct
in nature and peculiar in property. It consists of a soft and pulpy
matter, of a brownish white colour (fig. XXX.). According to some,
the nervous, like the muscular pulp, is composed of minute globules,
arranged in the same manner like a string of pearls (fig. XXXI.);
according to others, it consists of solid elongated threads, of a
cylindrical form, differing in thickness from that of a hair to the
finest fibre of silk. The pulp, whatever its form of aggregation, is
inclosed in a sheath of delicate cellular tissue. This external or
containing membrane is called the neurilema, or the nerve-coat; the
internal or contained substance, the proper nervous matter, is termed
the nerve-string. The nerve-string, enveloped in its nerve-coat,
constitutes the nervous filament. As in the muscle, so in the nerve,
many filaments unite to form a fibre, many fibres to form a fasciculus,
and many fasciculi to form the large cord termed a nerve. Moreover,
as in the muscle, so in the nerve, the filament, the fibre, the
fasciculus, the nervous cord itself, are each enveloped in its own
distinct sheath of cellular membrane; but the arrangement of the
nervous fibres differs from that of the muscular in this, that though
the nervous fibres are placed in juxtaposition, yet they do not,
like the muscular, maintain through their entire course a parallel
disposition, but cross and penetrate each other, so as to form an
intimate interlacement (fig. XXXII.).
[Illustration: Fig. XXXII.
Nervous fibres, deprived of their neurilema and unravelled, showing the
smaller threads, or filaments, of which the fibres consist.]
46. The nervous pulp is at least as liberally supplied with
blood-vessels as the muscular; the vessels are spread out upon the
nerve-coat, in which they divide into innumerable branches of extreme
minuteness, the distribution of which is so perfect, that there is
not a particle of nervous matter which is not supplied both with an
arterial and a venous vessel. Hence the neurilema is not merely a
sheath containing and protecting the nervous pulp, but it affords an
extended mechanical surface for sustaining the arterial vessels, from
which the pulp is probably secreted, and certainly nourished.
47. Albumen, in conjunction with a peculiar fatty matter, constitutes
the chief proximate principles of which the nervous tissue is composed.
To these are added a small proportion of the animal substance termed
osmazome, a minute quantity of phosphorus, some salts, and a very large
proportion of water; for out of one hundred parts of nervous substance,
water constitutes as much as eighty. Its peculiar vital property is
sensibility; and as all motion depends on the contractility of the
muscular fibre, so all sensation depends on the sensibility of the
nervous substance.
48. Such are the primary tissues, or the several kinds of organized
matter of which the body is composed; and from this account it is
obvious that they consist of three only—namely, the concrete matter
forming the basis of membrane, the pulpy matter forming the proper
muscular substance, and the pulpy matter forming the proper nervous
substance. Of these three kinds of animal matter the component parts of
the body consist. In combining to form the different structures, these
primary substances are intermixed and arranged in a great variety of
modes; and from these combinations and arrangements result either an
organ, a system, or an apparatus.
49. As filaments unite to form fibres, and fibres to form tissues, so
tissues unite to form organs: that is, bodies having a determinate
size and figure, and capable of performing specific actions. The
cellular, the muscular, and the nervous tissues are not organs;
membranes, muscles, and nerves are organs. The tissue, the simple
animal substance, is merely one of the elements of which the organ
is composed; the organ is compounded of several of those simple
substances, arranged in a determinate manner, and moulded into a given
shape, and so constituting a specific instrument. The basis of the
muscle is muscular tissue; but to this are added, invariably, membrane,
often tendon, and always vessels and nerves. It is this combination
that forms the specific instrument called a muscle, and that renders it
capable of performing its specific action. And every such combination,
with its appropriate endowment, constitutes an organ.
50. Organs are arranged into groups or classes, according as they
possess an analogous structure, and perform an analogous function; and
this assemblage constitutes a SYSTEM. All the muscles of the body,
for example, whatever their size, form, situation, or use, have an
analogous structure, and perform an analogous function, and hence are
classed together under the name of the muscular system. All the bones,
whatever their figure, magnitude, density, position, or office, are
analogous in structure and function; and hence are classed together
under the name of the osseous system. For the same reason, all the
cartilages, ligaments, vessels and nerves, form respectively the
cartilaginous, ligamentous, vascular and nervous systems.
51. An APPARATUS, on the contrary, is an assemblage of organs, it
may be differing widely from each other in structure, and exercising
various and even opposite functions; but all nevertheless concurring
in the production of some common object. The apparatus of nutrition
consists of the organs of mastication, deglutition, digestion,
absorption, and assimilation. Among the individual organs which concur
in carrying on these functions may be reckoned the lips, the teeth,
the tongue, the muscles connected with the jaws, the gullet, the
stomach, the duodenum, the small intestines, the pancreas, the liver,
the lacteal vessels, the mesenteric glands, and the lungs. Many of
these organs have no similarity in structure, and few have any thing
analogous in function; yet all concur, each in its appropriate mode and
measure, to the conversion of the aliment into blood. In the apparatus
of respiration, in that of circulation, of secretion, of excretion;
in the apparatus of locomotion, in the apparatus of sensation, and
more especially in the apparatus of the specific sensations,—vision,
hearing, smell, taste, touch, organs are combined which have nothing
in common but their concurrence in the production of a common end:
but this concurrence is the principle of their combination; and the
individual organs having this conjoint operation, taken together,
constitute an apparatus.
52. A clear idea may now be affixed to the terms structure and
organization. Structure may be considered as synonymous with
arrangement; the disposition of parts in a determinate order; that
which is constructed or built up in a definite mode, according to a
determinate plan. The arrangement of the threads of the cellular web
into areolæ or cells; the combination of the primary threads into
fibres or laminæ; the disposition of the muscular pulp into filaments,
placed parallel to each other; the investment of the filaments in
membraneous sheaths; the combination of the filaments, included in
their sheaths, into fibres; the aggregation of fibres into fasciculi;
and the analogous arrangement and combination of the nervous pulp,
are examples of structure. But when those structures are applied to
particular uses; when they are so combined and disposed as to form
a peculiar instrument, endowed with a specific function; when the
cellular fibres, for example, are so arranged as to make a thin, dense,
and expanded tissue; when to this tissue are added blood-vessels,
absorbents, and nerves; when, in a word, a membrane is constructed,
an organ is formed; when, in like manner, to the muscular and the
nervous fibres, arranged and moulded in the requisite mode, are added
blood-vessels, absorbents, and nerves, other organs are constructed
capable of performing specific functions: and this is organization—the
building up of organs—the combination of definite structures
into special instruments. Structure is the preparatory process of
organization; the one is the mere arrangement of the material; the
other is the appropriation of the prepared material to a specific use.
53. The term organization is employed in reference both to the
component parts of the body, and to the body considered as a whole. We
speak of an organized substance and of an organized body. An organized
substance is one in which there is not only a definite arrangement
of its component parts (structure), but in which the particular
arrangement is such as to fit it for accomplishing some special
use. Every organized substance is therefore essentially a special
organ; limited in its object it may be, and perhaps only conducive
to some further object; but still its distinctive character is, that
it has a peculiar structure, fitting it for the accomplishment of
some appropriate purpose. On the other hand, an organized body is a
congeries of organs—the aggregate of the individual organs. Attention
was directed in the early part of this work to one peculiar and
essential character, by which such an organized is distinguished from
an unorganized body. Between the individual parts of the organized body
there is so close a relation, that no one of them can be removed or
injured, or in any manner affected without a corresponding affection of
the whole. The action of the heart cannot cease without the cessation
of the action of the lung; nor that of the lung without that of the
brain; nor that of the brain without that of the stomach; in a word,
there is no organ in whatever distant nook of the system it be placed,
or however apparently insignificant its function, that is not necessary
to the perfection of the whole. But into whatever number of portions an
unorganized body may be divided, each portion retains the properties of
the mass, and constitutes in itself a perfect existence; there being
no relation between its individual parts, excepting that of physical
attraction: on the contrary, each component part of an organized body,
being endowed with some appropriate and specific power, on the exercise
of which the powers of all the other parts are more or less dependent,
the whole must necessarily suffer if but one part fail.
54. From the whole, then, we see that the human body is a congeries of
organs; that those organs are constructed of a few simple tissues; and
that all its parts, numerous, diversified, and complex as they are, are
composed of but three primary forms of animal matter variously modified
and combined.
[Illustration: Fig. XXXIII.
Muscles of the back and shoulders; showing their symmetrical
disposition.]
55. But though by the analysis of its component parts, this machine,
so complex in its construction, and so wonderfully endowed, may be
reduced to this state of simplicity; and although this analytical view
of it be highly useful in enabling us to form a clear conception of the
nature of its composition; yet it is only by considering its individual
parts such as they actually are, and by studying their situation,
connexion, structure, and action, that we can understand it as a whole,
and apply our knowledge of it to any practical use.
56. Viewing then the human body as a complicated whole, as a congeries
of organs made up of various combinations of simple tissues, it may
be observed, in reference to its external configuration, that it is
rounded. This rounded form is principally owing to the large proportion
of fluids which enter into its composition. The roundness of the face,
limbs, and entire surface of the child, are in striking contrast to
the unequal and irregular surface of the old man, whose humours are
comparatively very much smaller in quantity.
57. The length of the human body exceeds its breadth and thickness;
the degree of the excess varying at different periods of life, and
according to the peculiar constitution of the individual. In the
extremities, the bones, muscles, vessels, and nerves, are especially
distinguished by their length.
[Illustration: Fig. XXXIV.
Front view of the skeleton. 1. the head; 2. the trunk; 3. the superior
extremities; 4. the inferior extremities.]
[Illustration: Fig. XXXV.
Back view of the skeleton. 1. the head; 2. the trunk; 3. the superior
extremities; 4. the inferior extremities.]
58. The form of the human body is symmetrical, that is, it is
capable of being divided into two lateral and corresponding halves.
Suppose a median line to pass from the vertex of the head through the
centre of the spinal column (fig. XXXIV. 1, 2); if the body be well
formed, it will be divided by this line into two exactly equal and
corresponding portions (fig. XXXV. 1). This symmetrical disposition
of the body is not confined to its external configuration. It is
true of many of the internal organs; but principally, as has been
already stated, of those that belong to the animal life. The brain and
the spinal cord are divisible into two exactly equal halves (figs.
XLVIII. _d_, and XLIX. 1, 2, 3); the organs of sense are double and
symmetrical: the muscles of one side of the body exactly correspond to
those of the other (fig. XXXIII.); the two hands and arms and the two
lower extremities are alike (figs. XXXIV., XXXV.); but for the most
part, the organs of the organic life, the stomach, the intestines, the
liver, the spleen, for example, are single, and not symmetrical.
59. The human body is divided into three great portions, the head, the
trunk, and the extremities (figs. XXXIV. and XXXV. 1, 2, 3, 4).
60. By the HEAD is meant all that part of the body which is placed
above the first bone of the neck (fig. XXXIV. 1). It is of a spheroidal
figure, broader and deeper behind than before, somewhat like an egg in
shape, with the broad end behind; it is flattened at its sides (figs.
XXXV. 1, and XXXVI. 2, 4). Its peculiar figure renders it at once
stronger and more capacious than it could have been had it possessed
any other form. It is supported by its base on the spinal column, to
which it is attached by the peculiar structure termed a joint (fig.
XXXIV.), and fastened by ligaments of exceeding strength.
61. The head contains the central organ of the nervous system; the
organs of the senses, with the exception of that of touch; and the
organs of mastication. It comprehends the cranium and the face. Both
are composed partly of soft parts, as the teguments, namely, skin, fat,
&c., and muscles; and partly of bones.
[Illustration: Fig. XXXVI.
1. Frontal bone; 2. parietal bone; 3. occipital bone; 4. temporal bone;
5. nasal bone; 6. malar bone; 7. superior maxillary bone; 8. inferior
maxillary bone.]
[Illustration: Fig. XXXVII.
Bones of the skull, separated; front view. 1. Frontal bone; 2. portions
of the parietal bones; 3. malar or cheek bones; 4. nasal bones; 5.
superior maxillary or bones of the upper jaw; 6. the vomer; 7. the
inferior maxillary or bone of the lower jaw.]
[Illustration: Fig. XXXVIII.
Bones of the skull separated; side view. 1. Frontal bone; 2. parietal
bone; 3. occipital bone; 4. temporal bone; 5. nasal bone; 6. malar
bone; 7. superior maxillary bone; 8. the unguis; 9. the inferior
maxillary bone.]
[Illustration: Fig. XXXIX.
Bones forming the base of the skull; viewed from the inside. 1.
Occipital bone; 2. temporal bones; 3. sphenoid bone; 4. ethmoid bone;
5. superior maxillary bones, or bones of the upper jaw; 6. malar or
cheek bones; 7. foramen magnum.]
62. The bones of the cranium are eight in number, six of which are
proper to the cranium, and two are common to it and to the face. The
six bones proper to the cranium are the frontal (fig. XXXVII. 1), the
two parietal (fig. XXXVI. 2), the two temporal (fig. XXXVIII. 4), and
the occipital (fig. XXXVIII. 3); the two common to the cranium and face
are the ethmoidal (fig. XXXIX. 4), and the sphenoidal (fig. XXXIX.
3). The frontal bone forms the entire forepart of the vault (fig.
XXXVII. 1); the two parietal form the upper and middle part of it (fig.
XXXVIII. 2); the two temporal form the lower part of the sides (fig.
XXXVIII. 4); the occipital forms the whole hinder part, together with a
portion of the base (figs. XXXVIII. 3, XXXVI. 3, XXXIX. 1); while the
ethmoidal forms the forepart, and the sphenoidal the middle part of the
base (fig. XXXIX. 3, 4).
[Illustration: Fig. XL.
2 1
Portions of the bones of the cranium; showing the corresponding
inequalities in their margins: which margins, when in apposition,
constitute the mode of union termed suture. 1. External surface of the
bone; 2. internal surface.]
[Illustration: Fig. XLI.
1
Fig. XLII.
2
1. Side view of the adult skull, showing the several bones united by
suture; 2. side view of the fœtal skull, showing the bones imperfectly
ossified, separated to some extent from each other, the interspace
being occupied by membrane. The small size of the face compared with
that of the cranium is strikingly apparent.]
63. These bones are firmly united together. The union of bones is
technically called an _articulation_ or _joint_. All joints are either
immoveable or moveable. The union of the bones of the cranium affords
an example of an immoveable articulation. Prominences and indentations,
like the teeth of a saw, are formed in the margins of the contiguous
bones (figs. XXXVIII. and XL.). At these inequalities of surface, which
are exactly adapted to each other (figs. XXXVIII. and XL.), the two
bones are in immediate apposition in such a manner as to preclude the
possibility of motion, and even to render the separation extremely
difficult. This mode of articulation is termed a _suture_. There are
certain advantages in constructing the cranium of several distinct
bones, and in uniting them in this peculiar mode. 1. The walls of the
vault are stronger than they could have been had they been formed of a
single piece. 2. In the fœtus, the bones are at some distance from each
other (fig. XLII.); at birth, they yield and overlap one another; and
in this manner they conduce to the security and ease of that event. 3.
Minute vessels pass abundantly and securely through the interstices of
the sutures to and from the interior of the cranium; in this manner,
a free communication is established between the vessels within and
without this cavity. 4. It is probable that the shock produced by
external violence is diminished in consequence of the interruption of
the vibration occasioned by the suture; it is certain that fracture is
prevented by it from extending as far as it would do in one continued
bony substance.
[Illustration: Fig. XLIII.
Section of the skull. 1. Cavity of the cranium occupied by the brain;
2. cut edge of the bones of the cranium, showing the two tables of
compact bone and the intervening spongy texture called diploë.]
64. The vault of the cranium forms a cavity which contains the
brain (fig. XLIII. and XLVIII.) The size of this cavity is invariably
proportioned to that of the organ it lodges and protects. The form and
magnitude of the cavity, and consequently the shape and size of the
cranium, depend upon the brain, and not of the brain upon the cranium.
The soft parts model and adapt to themselves the hard, and not the hard
the soft. The formation of the brain in the fœtus is anterior to that
of the case which ultimately contains it; and the hard bone is moulded
upon the soft pulp, not the pulp upon the bone. At every period of
life, on the inner surface of the cranium there are visible impressions
made by the convolutions of the brain, and the ramifications of the
arteries (figs. XXXIX. 1, 2, and XL. 2), and on its external surface
are depressions occasioned by the action of the external muscles. Nor
does the modifying power of the brain over the bones of the cranium
terminate at birth. The formation of bone, always a slow process,
is never completed until the child has attained its third or fourth
year, and often not until a much later period. At this tender age, the
bones, which in advanced life are hard and rigid, are comparatively
soft and yielding, and consequently more readily receive and retain
the impression of the convolutions and of the other projecting parts
of the brain, by which they are sometimes so deeply marked, that an
attentive examination of the inner surface of the cranium is of itself
sufficient to determine not only that some part, but to indicate the
very part of the brain which has been preternaturally active. At this
tender age, pressure, internal or external, general or partial, may
readily change the form of the cranium. If, by a particular posture,
the head of a child be unequally balanced on the spine, the brain will
press more on that side of the cranium than on the other; the organ
will expand in the direction to which it inclines; that portion of it
will become preternaturally developed, and consequently the balance of
its functions will be disturbed. An awkward way of standing or sitting,
perhaps contracted inadvertently and kept up by habit; a wry neck; any
cause that keeps the head constantly inclined to one side, may produce
this result, examples of which and of its consequences will be given
hereafter.
65. Tracing them from without inwards we see, then, that the various
coverings afforded to the brain, the central organ of the animal life,
seated in its vaulted cavity, are: 1. The tegument, consisting of the
skin and of cellular and adipose membrane. 2. Beneath the tegument,
muscles, in the forepart and at the vertex, comparatively slender and
delicate; at the sides and posteriorly, thick, strong, and powerful
(fig. XLIV.). 3. Beneath the muscles, a thin but dense membrane, termed
the pericranium, lining the external surface of the cranial bones. 4.
Beneath the pericranium, the bony substance of the cranium, consisting
of two firm and hard bony plates, with a spongy, bony structure, called
diploë, interposed between them (fig. XLIII. 2). 5. Immediately in
contact with the inner surface of the bony substance of the cranium,
and forming its internal lining, the dense and strong membrane, called
the _dura mater_, not only affording a general covering to the brain,
but sending firm partitions between individual portions of it (fig.
XLVIII. _c._). 6. A serous membrane lining the internal surface of
the dura mater, and reflected over the entire surface of the brain,
termed the arachnoid tunic. 7. A thin and delicate membrane in
immediate contact with the substance of the brain, descending between
all its convolutions, lining all its cavities and enveloping all its
fibres, called the pia mater. 8. An aqueous fluid, contained between
the arachnoid membrane and the pia mater. Skin, muscle, pericranium,
bone, dura mater, arachnoid membrane, pia mater, and aqueous fluid,
superimposed one upon another, form, then, the covering and defence of
the brain; so great is the care taken to protect this soft and tender
substance.
66. The bones of the _face_ consist of fourteen, namely, the two
superior maxillary or jaw-bones (fig. XXXVII. 5), the two malar or
cheek bones (fig. XXXVII. 3), the two nasal bones (fig. XXXVII. 4),
the two palate bones, the two ossa unguis (fig. XXXVIII. 8), the two
inferior turbinated bones, the vomer (fig. XXXVII. 6), and the inferior
maxilla or the lower jaw (fig. XXXVII. 7.) This irregular pile of bones
is divided into the superior and inferior maxilla or jaws; the superior
maxilla being the upper and immoveable portion of the face; the
inferior maxilla being the lower and moveable portion of it. Besides
these bones, the face contains thirty-two teeth, sixteen in each
jaw. The bones of the upper jaw are united together by sutures, and
the union is so firm, that they have no motion but what they possess
in common with the cranium. The lower jaw is united by a distinct
articulation with the cranium (figs. XXXIV. and XXXV.).
67. Besides the bones and the teguments, the face contains a number of
muscles, which for the most part are small and delicate (fig. XLIV.),
together with a considerable portion of adipose matter; while, as has
been stated, the face and head together contain all the senses, with
the exception of that of touch, which is diffused, more or less, over
the entire surface of the body.
[Illustration: Fig. XLIV.
Muscles of the face.]
68. The second great division of the body, termed the TRUNK, extends
from the first bone of the neck to that called the pubis in front, and
to the lower end of the coccyx behind (fig. XXXIV. 2). It is subdivided
into the thorax, the abdomen, and the pelvis (fig. XLV.).
69. The _thorax_ or _chest_ extends above from the first bone of
the neck, by which it is connected with the head, to the diaphragm
below, by which it is divided from the abdomen (figs. XLV. and LXI.).
It consists partly of muscles and partly of bones; the muscular and
the osseous portions being in nearly equal proportions. Both together
form the walls of a cavity in which are placed the central organs of
circulation and respiration (fig. LX. 2, 5). The chief boundaries of
the cavity of the thorax before, behind, and at the sides, are osseous
(fig. XLV.); being formed before, by the sternum or breast-bone (fig.
XLV. 6); behind, by the spinal column or back bone (fig. XLV. 2, 4);
and at the sides, by the ribs (fig. XLV. 7). Below, the boundary is
muscular, being formed by the diaphragm (fig. LXI. 2), while above the
thorax is so much contracted (fig. XLV.), that there is merely a space
left for the passage of certain parts which will be noticed immediately.
70. The figure of the thorax is that of a cone, the apex being above
(fig. XLV.), through the aperture of which pass the tubes that lead
to the lungs and stomach, and the great blood-vessels that go to and
from the heart (fig. LX.). The base of the cone is slanting, and is
considerably shorter before than behind, like an oblique section of the
cone (fig. XLV.).
71. The osseous portion of the walls of the thorax is formed behind by
the spinal column, a range of bones common indeed to all the divisions
of the trunk; for it constitutes alike the posterior boundary of the
thorax, abdomen, and pelvis (fig. XLV. 2, 4, 6). It is composed of
thirty distinct bones, twenty-four of which are separate and moveable
on one another, and on this account are called true vertebræ (fig. XLV.
2, 4); the other five, though separate at an early period of life, are
subsequently united into a single solid piece, called the sacrum (fig.
XLV. 5). The bones composing this solid piece, as they admit of no
motion on each other, are called false vertebræ (fig. XLV. 5). To the
extremity of the sacrum is attached the last bone of the series, termed
the coccyx (fig. XXXV.).
72. From above downwards, that is, from the first bone of the neck to
the first bone of the sacrum, the separate bones forming the column
progressively increase in size; for this column is the chief support
of the weight of the head and trunk, and this weight is progressively
augmenting to this point (fig. XLV. 2, 4). From the sacrum to the
coccyx, the bones successively diminish in size, until, at the
extremity of the coccyx, they come to a point (fig. XXXV.). The spinal
column may therefore be said to consist of two pyramids united at their
base (fig. XLV. 4, 5). The superior pyramid is equal in length to about
one third of the height of the body, and it is this portion of the
column only that is moveable.
73. The two surfaces of the spinal column, the anterior and the
posterior, present a striking contrast (figs. XXXIV. and XXXV.). The
anterior surface, which in its whole extent is rounded and smooth, is
broad in the region of the neck, narrow in the region of the back, and
again broad in the region of the loins (fig. XLV. 2, 4.). It presents
three curvatures (fig. XLV. 2, 4); the convexity of that of the neck
being forwards, that of the back backwards, and that of the loins again
forwards (fig. XLV. 2, 4).
74. From the posterior surface of the column, which is every where
irregular and rough, spring, along the median line, in regular series,
strong, sharp, and pointed projections of bone (fig. XXXV.), which
from being sharp and pointed, like elongated spines, are called
spinous processes, and have given name to the whole chain of bones.
These processes afford fixed points for the action of powerful
muscles. Extending the whole length of the column, from the base of
the skull to the sacrum, on each side of the spinous processes, are
deep excavations, which are filled up with the powerful muscles that
maintain the trunk of the body erect.
75. From the lateral surfaces of the column likewise spring short but
strong projections of bone, termed transverse processes, which also
give attachment to powerful muscles (fig. XLVI.).
[Illustration: Fig. XLV.
Bones of the trunk. 1. Spinal column; 2. the seven cervical vertebræ;
3. the twelve dorsal vertebræ; 4. the five lumbar vertebræ; 5. the
sacrum; 6. the sternum; 7. the true ribs; 8. the false ribs; 9. the
clavicle; 10. the scapula; 11. the ilium; 12. the ischium; 13. the
pubes; 14. the acetabulum; 15. the brim of the pelvis.]
76. The separate bones of the series have a kind of turning motion on
each other; hence each is called a vertebra, and the name of vertebral
column is often given to the entire series, as well as that of spinal
column. That portion of the column which forms the neck consists of
seven distinct bones, called cervical vertebræ (fig. XLV. 2); that
portion which forms the back consists of twelve, called dorsal vertebræ
(fig. XLV. 3); that portion which forms the loins consists of five,
called lumbar vertebræ (fig. XLV. 4). Between each of these classes of
vertebræ there are specific differences, but they need not be described
here: all that is necessary to the present purpose is an account of the
structure which is common to every vertebra.
77. By inspecting fig. XLVI. 1, it will be seen that the upper and
under edges of each vertebra consist of a ring of bone, of a firm
and compact texture, rendering what may be called the body of the
vertebra exceedingly strong (fig. XLVI. 3). This ring of bone forms a
superficial depression (fig. XLVI. 2), for the reception of a peculiar
substance, immediately to be described, which is interposed between
each vertebra (fig. XLVII. 2).
78. The anterior surface of the body of the vertebra is convex (fig.
XLVI. 3); its posterior surface is concave (fig. XLVI. 4); from the
posterior surface springs a bony arch (figs. XLVI. 5 and LIII. 1),
which, together with the posterior concavity, forms an aperture of
considerable magnitude (fig. XLVI. 6), a portion of the canal for the
passage of the spinal cord (figs. XLVII. 3, and XLIX. 3).
[Illustration: Fig. XLVI.
View of some of the vertebræ, which by their union form the spinal
column.
_a._ A vertebra of the neck; _b._ a vertebra of the back; a vertebra of
the loins.
1. Ring of compact bone forming, 3, the body of the vertebra; 2.
superficial depression for the reception of the intervertebral
cartilage; 3. anterior surface of the body of the vertebra; 4.
posterior surface; 5. bony arch; 6. opening for the passage of the
spinal cord; 7. opening for the passage of the spinal nerves; 8.
articulating processes by which the vertebræ are joined to each other;
9. two dorsal vertebræ united, showing the arrangement of, 10, the
spinous processes; 11. a portion of a rib articulated with the side of
the vertebra.]
79. Both the upper and under edges of the arch form a notch (fig. XLVI.
7.), which, together with a corresponding notch in the contiguous
vertebra, completes another aperture rounder and smaller than the
former, but still of considerable size (fig. XLVI. 7.), the passage of
the spinal nerves (fig. XLVII. 3).
80. From both the upper and under sides of the arch proceed two short
but strong projections of bone (fig. XLVI. 8.), termed the articulating
processes, because it is chiefly by these processes that the vertebræ
are connected together. From the beginning to the end of the series,
the two upper processes of the one vertebra are united with the two
lower processes of the vertebra immediately above it (fig. XLVI. 9),
and around the edges of all the articulating processes are visible
rough lines, which mark the places to which the articulating ligaments
are attached.
81. No vertebra, except the first, rests immediately upon its
contiguous vertebra (fig. XLV. 2, 4). Each is separated from its fellow
by a substance of a peculiar nature interposed between them, termed the
intervertebral substance (figs. XLVII. 2, and L. 2). This substance
partakes partly of the nature of cartilage, and partly of that of
ligament. It is composed of concentric plates, formed of oblique fibres
which intersect each other in every direction. This substance, for
about a quarter of an inch from its circumference towards its centre,
is tough, strong, and unyielding; then it becomes softer, and is
manifestly elastic; and so it continues until it approaches the centre,
when it becomes pulpy, and is again inelastic. The exterior tough and
unyielding matter is for the firmness of the connexion of the several
vertebræ with each other; the interior softer and elastic matter is for
the easy play of the vertebræ upon each other; the one for security,
the other for pliancy. And the adjustment of the one to the other is
such as to combine these properties in a perfect, manner. The quantity
of the unyielding substance is not so great as to produce rigidity;
the quantity of the elastic substance is not so great as to occasion
insecurity. The firm union of its solid matter renders the entire
column strong; the aggregate elasticity of its softer substance renders
it springy.
[Illustration: Fig. XLVII.
1. One of the Lumbar vertebræ. 2. Intervertebral substance. 3. A
portion of the spinal cord in its canal.]
82. The column is not constructed in such a manner as to admit of an
equal degree of motion in every part of it. Every thing is contrived
to give to that portion which belongs to the neck freedom of motion,
and, on the contrary, to render that portion which belongs to the back
comparatively fixed. In the neck the mechanism of the articulating
processes is such as to admit of an equal degree of sliding motion
forwards, backwards, and from side to side, together with a turning
motion of one bone upon another; at the same time, the intervertebral
substance between the several vertebræ is thick. In consequence of this
mechanism, we can touch the breast with the chin, the back with the
hind head, and the shoulders with the ear, while we can make the head
describe more than a semicircle. But, in the back, the articulating
processes are so connected as to prevent the possibility of any motion,
either forwards or backwards, or any turning of one vertebra upon
another, while the intervertebral substance is comparatively thin (fig.
XLV. 2, 4). That portion of the column which belongs to the back is
intended to afford a fixed support for the ribs, a support which is
indispensable to their action in the function of respiration. In the
loins, the articulating processes are so connected as to admit of a
considerable degree of motion in the horizontal direction, and from
side to side, and the intervertebral substance here progressively
increases in thickness to the point at which the upper portion of the
column is united to the sacrum (fig. XLV. 2, 4), where the degree of
motion is extensive.
83. The canal for the spinal cord, formed partly by the concavity in
the posterior surface of the vertebra, and partly by the arch that
springs from it (fig. XLVI. 6.), is lined by a continuation of the
dense and strong membrane that constitutes the internal periosteum
of the cranium, the dura mater (fig. XLVIII. _c_), which, passing
out of the opening in the occipital bone, called the foramen magnum
(figs. XXXIX. 7, and XLIX. 3), affords a smooth covering to the canal
throughout its whole extent.
[Illustration: Fig. XLVIII.
_a._ The scalp, turned down.
_b._ The cut edge of the bones of the skull.
_c._ The external strong membrane of the brain (Dura Mater) suspended
by a hook.
_d._ The left hemisphere of the brain, showing its convolutions.
_e._ The superior edge of the right hemisphere.
_f._ The fissure between the two hemispheres.]
[Illustration: Fig. XLIX.
1. Hemispheres of the brain proper, or cerebrum; 2. hemispheres of the
smaller brain, or cerebellum; 3. spinal cord continuous with the brain,
and the spinal nerves proceeding from it on each side.]
84. The spinal cord itself, continuous with the substance of the
brain, passes also out of the cranium through the foramen magnum into
the spinal canal (fig. XLIX. 3), enveloped in the delicate membranes
that cover it, and surrounded by the aqueous fluid contained between
those membranes. The size of the spinal canal, accurately adapted
to that of the spinal cord, which it lodges and protects, is of
considerable size, and of a triangular shape in its cervical portion
(fig. XLIX. 3), smaller and rounded in its dorsal portion (fig. XLIX.
3), and again large and triangular in its lumbar portion (fig. XLIX. 3).
85. The spinal column performs several different, and apparently
incompatible, offices.
First, it affords a support and buttress to other bones. It sustains
the head (fig. XXXIV. 1); it is a buttress to the ribs (fig. XLVI. 7);
through the sternum and ribs it is also a buttress to the superior, and
through the pelvis, to the lower, extremities (fig. XXXIV. 2, 3, 4).
Secondly, it affords a support to powerful muscles, partly to those
that maintain the trunk of the body in the erect posture against the
force of gravitation, and partly to those that act upon the superior
and inferior extremities in the varied, energetic, and sometimes
long-continued movements they execute.
Thirdly, it forms one of the boundaries of the great cavities that
contain the chief organs of the organic life. To the support and
protection of those organs it is specially adapted; hence the surface
in immediate contact with them is even and smooth; hence its different
curvatures, convexities, and concavities, have all reference to their
accommodation; hence in the neck it is convex (fig. XLV. 2), in order
to afford a firm support to the esophagus, the wind-pipe, the aorta,
and the great trunks of the venous system (fig. LX. 3, 4); in the back
it is concave, in order to enlarge the space for the dilatation of
the lung in the act of inspiration (figs. XLV. 3, and LX. 5); in the
loins it is convex, in order to sustain and fix the loose and floating
viscera of the abdomen (figs. XLV. 4, and LX. 6, 7, 8, 9); in the
pelvis it is concave, in order to enlarge the space for lodging the
numerous delicate and highly-important organs contained in that cavity
(fig. XLV. 5).
Fourthly, it forms the osseous walls of a canal (figs. XLVI. 6, and
XLVII. 3) for the lodgment and protection of the soft and tender
substance of the spinal cord, one of the great central masses of the
nervous system, the seat of the animal life (fig. XLIX. 3).
Fifthly, it affords in its osseous walls secure apertures for the
passage of the spinal nerves (figs. XLVI. 7, and XLIX. 3), by which
impressions are transmitted from the organs to the spinal cord and
brain, in the function of sensation; and from the spinal cord and brain
to the organs in the function of volition.
86. For the due performance of these offices, it is indispensable that
it should be firm, rigid, strong, and yet to a certain extent readily
flexible in every direction. By what mechanism is it endowed with these
apparently incompatible properties?
87. By means of the ring of compact bone, which forms so large a part
of its body (fig. XLVI. 1) it is rendered firm, rigid, and strong. By
means of its numerous separate pieces, exactly adjusted to each other,
and dove-tailed into one another, an increase of strength is gained,
such as it would not have been possible to communicate to a single
solid piece. By the same mechanism, some degree of flexibility is also
obtained; each separate bone yielding to some extent, which, though
slight in a single bone, becomes considerable in the twenty-four.
88. But the flexibility required is much greater than could be
obtained by this expedient alone. A rigid and immoveable pile of bones,
in the position of the spinal column, on which all the other parts of
the body rest, and to which they are directly or indirectly attached,
would necessarily have rendered all its movements stiff and mechanical;
and every movement of every kind impossible, but in a given direction.
That the movements of the body may be easy, free, and varied; that it
may be possible to bring into play new and complex combinations of
motion at any instant, with the rapidity of the changes of thought,
at the command of the impulses of feeling, it is indispensable that
the spinal column be flexible in every direction, forwards, backwards,
and at the sides: it is equally indispensable that it be thus capable
of yielding, without injuring the spinal cord; without injuring the
spinal nerves; without injuring the thoracic and abdominal viscera; and
without injuring the muscles of the trunk and extremities. The degree
in which it possesses this power of flexibility, and the extent to
which, by the cultivation of it, it is sometimes actually brought, is
exemplified in the positions and contortions of the posture-master and
the tumbler. It is acquired by means of the intervertebral substance,
the compressible and elastic matter interposed between the several
vertebræ. So compressible is this substance, that the human body is
half an inch shorter in the evening than in the morning, having lost
by the exertions of the day so much of its stature; yet, so elastic is
this matter, that the stature lost during the day is regained by the
repose of the night. The weight of the body pressing in all directions
upon the spinal column; muscles, bones, cartilages, ligaments,
membranes, with all their vessels and all the fluids contained in
them; the weight of all these component parts of the head, trunk, and
extremities, pressing, without the cessation of an instant, during all
the hours of vigilance, upon the intervertebral substance, compresses
it; but this weight, being taken off during the night, by the recumbent
posture of the body, the intervertebral substance, in consequence of
its elasticity, regains its original bulk, and of course the spinal
column its original length.
89. But the flexibility acquired through the combined properties of
compressibility and elasticity is exceedingly increased by the action
of the pulpy and inelastic matter in the centre of the intervertebral
substance; this matter serving as a pivot to the vertebræ, facilitating
their motion on each other. Its effect has been compared to that of
a bladder partly filled with water, placed between two trenchers; in
this case, the approximation of the circumference of the two trenchers
on one side, would instantly displace a portion of the water on that
side, which would occupy the increasing space on the other, with the
effect of facilitating the change, in every possible direction, of
the position of the two trenchers in relation to each other. To this
effect, however, it is indispensable that the matter immediately around
this central pivot should be, not like itself, rigid and unyielding,
but compressible and elastic. It is an interesting fact, that since
this illustration was suggested, it has been discovered that this
very arrangement is actually adopted in the animal body. In certain
animals, in the very centre of their intervertebral substance, there
has been actually found a bag of water, with a substance immediately
surrounding the bag, so exceedingly elastic, that when the bag is cut,
the fluid contained in it is projected to the height of several feet in
a perpendicular stream.
90. But besides securing freedom and extent of motion, the
intervertebral substance serves still another purpose, which well
deserves attention.
Firmness and strength are indispensable to the fundamental offices
performed by the column; and to endow it with these properties, we
have seen that the external concentric layers of the intervertebral
substance are exceedingly tough and that they are attached to the
bodies of the vertebræ, which are composed of dense and compact bone.
But than dense and compact bone, nothing can be conceived better
calculated to receive and transmit a shock or jar on the application
of any degree of force to the column. Yet such force must necessarily
be applied to it in every direction, from many points of the body,
during almost every moment of the day; and did it actually produce a
corresponding shock, the consequence would be fatal: the spinal cord
and brain would be inevitably killed; for the death of these tender and
delicate substances may be produced by a violent jar, although not a
particle of the substances themselves be touched. A blow on the head
may destroy life instantaneously, by what is termed concussion; that
is, by the communication of a shock to the brain through the bones of
the cranium. The brain is killed; but on careful examination of the
cerebral substance after death, not the slightest morbid appearance
can be detected: death is occasioned merely by the jar. A special
provision is made against this evil, in the structure of the bones
of the cranium, by the interposition between its two compact plates
of the spongy substance called diploë (fig. XLIII. 2); and this is
sufficient to prevent mischief in ordinary cases. A great degree of
violence applied directly to the head is not common: when it occurs
it is accidental: thousands of people pass through life without ever
having suffered from it on a single occasion: but every hour, in the
ordinary movements of the body, and much more in the violent movements
which it occasionally makes, a degree of force is applied to the spinal
column, and through it transmitted to the head, such as, did it produce
a proportionate shock, would inevitably and instantly destroy both
spinal cord and brain. The evil is obviated partly by the elastic, and
partly by the non elastic properties of the matter interposed between
the several layers of compact bone. By means of the elastic property
of this matter, the head rides upon the summit of the column as upon a
pliant spring, while the canal of the spinal cord remains secure and
uninvaded. By means of the soft and pulpy portion of this matter, the
vibrations excited in the compact bone are absorbed point by point as
they are produced: as many layers of this soft and pulpy substance, so
many points of absorption of the tremors excited in the compact bone;
so many barriers against the possibility of the transmission of a shock
to the delicate nervous substance.
91. Alike admirable is the mechanism by which the separate pieces
of the column are joined together. If but one of the bones were to
slip off its corresponding bone, or to be displaced in any degree,
incurable paralysis, followed ultimately by death, or instantaneous
death, would happen; for pressure on the spinal cord in a certain part
of its course is incompatible with the power of voluntary motion, and
with the continuance of life for any protracted term; and in another
part of its course, with the maintenance of life beyond a few moments.
To prevent such consequences, so great is the strength, so perfect the
attachment, so unconquerable the resistance of that portion of the
intervertebral substance which surrounds the edge of the bodies of the
vertebræ, that it will allow the bone itself to give way rather than
yield. Yet such is the importance of security to this portion of the
frame, that it is not trusted to one expedient alone, adequate as that
might seem. Besides the intervertebral substance, there is another
distinct provision for the articulation of the bodies of the vertebræ.
Commencing at the second cervical vertebra, in its fore part, and
extending the whole length of the column to the sacrum, is a powerful
ligament, composed of numerous distinct longitudinal fibres (fig. L.),
which are particularly expanded over the intervals between the bones
occupied by the intervertebral substance (figs. L. 1, and LI. 2, 2).
This ligament is termed the _common anterior vertebral_, beneath which,
if it be raised from the intervertebral substance, may be seen small
_decussating_ fibres, passing from the lower edge of the vertebra
above, to the upper edge of the vertebra below (fig. L. 3), from which
circumstance these fibres are termed _crucial_.
[Illustration: Fig. L.
1. Common anterior ligament; 2. intervertebral substance. The anterior
ligament is removed to exhibit (3.) the crucial fibres passing over it.]
[Illustration: Fig. LI.
1. Portion of the occipital bone; 2. common anterior ligament.]
92. Corresponding with the ligament on the anterior, is another on
the posterior part of the spine (fig. LII. 1), which takes its origin
from the foramen magnum (fig. LII. 1); descends from thence, within the
vertebral canal, on the posterior surface of the bodies of the vertebra
(fig. LII. 1), and extends to the sacrum. This ligament is termed the
_common posterior vertebral_, which, besides adding to the strength of
the union of the bodies of the vertebræ, prevents the column itself
from being bent too much forward.
[Illustration: Fig. LII.
1. Posterior vertebral ligament.]
93. Moreover, the bony arches of the vertebræ (fig. LIII. 1) are
connected by means of a substance partly ligamentous, and partly
cartilaginous (fig. LIII. 2), which, while it is extremely elastic, is
capable of resisting an extraordinary degree of force.
[Illustration: Fig. LIII.
1. Arches of the vertebræ seen from within; 2. ligaments connecting
them.]
94. And in the last place, the articular processes form so many
distinct joints, each being furnished with all the apparatus of a
moveable joint, and thus possessing the ordinary provision for the
articulation of bones, in addition to the whole of the foregoing
securities.
95. "In the most extensive motion of which the spinal column is
capable, that of flexion, the common anterior ligament is relaxed; the
fore part of the intervertebral substance is compressed, and its back
part stretched; while the common posterior ligament is in a state of
extension. In the _extension_ of the column the state of the ligaments
is reversed; those which were extended being in their turn relaxed,
while the common anterior vertebral is now put upon the stretch. In the
_lateral inclination_ of the column, the intervertebral substance is
compressed on that side to which the body is bent. In the _rotatory_
motion of the column, which is very limited in all the vertebræ, but
more particularly in the dorsal, in consequence of their attachment to
the ribs, the intervertebral substance is contorted, as are likewise
all the ligaments. All the motions of the column are capable of being
aided to a great extent by the motion of the pelvis upon the thighs."
96. "The number and breadth of the attachments of these bones,"
says an accomplished anatomist and surgeon,[4] "their firm union by
ligament, the strength of their muscles, the very inconsiderable
degree of motion which exists between any two of them, and lastly, the
obliquity of their articular processes, especially in the dorsal and
lumbar vertebræ, render dislocation of them, at least in those regions,
impossible without fracture; and I much doubt whether dislocation even
of the cervical vertebræ ever occurs without fracture, either through
their bodies or their articular processes. The effects of each of these
accidents would produce precisely the same injury to the spinal marrow,
and symptoms of greater or less importance, according to the part of
the spinal column that is injured. Death is the immediate consequence
if the injury be above the third cervical vertebra, the necessary
paralysis of the parts to which the phrenic and intercostal nerves
are distributed causing respiration instantly to cease. If the injury
be sustained below the fourth cervical vertebra, the diaphragm is
still capable of action, and dissolution is protracted. The symptoms,
in fact, are less violent in proportion as the injury to the spinal
marrow is further removed from the brain; but death is the inevitable
consequence, and that in every case at no very distant period."
97. So the object of the construction of the spinal column being
to combine extent and freedom of motion with strength, and it being
necessary to the accomplishment of this object to build up the column
of separate pieces of bone, the connecting substances by which the
different bones are united are constituted and disposed in such a
manner as to prove absolutely stronger than the bones themselves. Such
is the structure of this important portion of the human body considered
as a piece of mere mechanism; but our conception of its beauty and
perfection would be most inadequate if we did not bear in mind, that
while the spinal column performs offices so varied and apparently so
incompatible, it forms an integrant portion of a living machine: it is
itself alive: every instant, blood-vessels, absorbents and nerves, are
nourishing, removing, renewing, and animating every part and particle
of it.
98. The anterior boundary of the thorax is formed by the bone called
the sternum, or the breast-bone, which is broad and thick at its upper,
and thin and elongated at its lower extremity (figs. XLV. 6, and
LIV.), where it gives attachment to a cartilaginous appendix, which
being pointed and somewhat like a broadsword, is called the ensiform
cartilage.
[Illustration: Fig. LIV.
Anterior view of the sternum.]
99. Its position is oblique, being near the vertebral column at the
top, and distant from it at the bottom (fig. XLV. 6). Its margins
are thick, and marked by seven depressions, for the reception of the
cartilages of the seven true ribs (fig. LIV). Its anterior surface is
immediately subjacent to the skin, and gives attachment to powerful
muscles, which act on the superior extremities: its posterior surface
is slightly hollowed in order to enlarge the cavity of the thorax (fig.
LV.).
[Illustration: Fig. LV.
Posterior view of the sternum.]
100. The thorax is bounded at the sides by the ribs, which extend like
so many arches between the spinal column and the sternum (fig. XLV. 7,
8). They are in number twenty-four, twelve on each side, of which the
seven upper are united to the sternum by cartilage, and are called true
ribs (fig. XLV. 7); the cartilages of the remaining five are united
with each other and are not attached to the sternum; these are called
false ribs (fig. XLV. 8): all of them are connected behind to the
spinal column (fig. XXXV.).