The aponeuroses for covering are general or partial.
_Aponeuroses for General Covering._
They are found around the limbs, whose muscles they tie down. The
arm, the fore-arm and the hand, the thigh, the leg and the foot, are
provided with them.
_Forms._
They are, in their conformation, analogous to the form of the limb,
which they in part determine, and which they especially maintain, by
preventing the displacement of the subjacent parts, a displacement
which would continually take place, from the laxity of the cutaneous
organ. Their thickness varies. In general, the greater the number of
the muscles they cover, the greater their thickness; hence why the
aponeurosis of the fascia lata is superior in this respect to the
brachial; why that of the fore-arm is thicker in front than behind; why
the plantar and palmar are so considerable, whilst hardly any fibres
are found on the back part of the foot and the hand. There are however
some exceptions to this rule; for example, the aponeurotic covering
of the posterior part of the leg is not in proportion to the power
of the gastrocnemii and solæus muscles; thus these muscles are more
than all the others exposed to displacements, frequently very painful,
which constitute cramp, and which it is necessary to distinguish from
the pains or numbness which result from the compression of one of the
nerves of the lower limbs, as of the sciatic, or the external plantar,
a compression produced by a bad position, or any other analogous cause.
Externally, the aponeuroses of the general covering are contiguous to
the integuments. A very loose texture unites them, so that the latter
can easily slide over them in external pressures. Immoveable between
these motions and those of the muscles, they entirely separate them; so
that the skin and the muscles that correspond to it, have not, in this
respect, any influence upon each other.
Within, these aponeuroses are in general loosely joined to the muscles
by cellular texture. Here and there they send between the different
muscular layers numerous elongations, which are afterwards attached
to the bone, and which, at the same time that they furnish points of
attachment, increase the solidity of the covering of the limb.
_Tensor Muscles._
The aponeuroses for general covering have almost all one or two
particular muscles that are inserted in them in whole or in part, and
which are destined to give them a degree of tension or relaxation
proportioned to the state of the limb. This arrangement is remarkable
in the insertion, 1st, of the great dorsal and pectoral muscles in
the brachial aponeurosis; 2d, of the biceps in that of the fore-arm;
3d, of the palmaris longus in the palmar; 4th, of the glutæus maximus
and of the fascia lata in the aponeuroses of that name; 5th, of the
semi-tendinosus, semi-membranosus and biceps in the tibial.
As in the great motions of the limbs, in which all the muscles are the
most liable to be displaced, these are necessarily in action, they
distend powerfully the aponeurosis, which thus reflects the motion that
is communicated to it, and resists especially every displacement. When
the limb is at rest, the tensor muscles cease their contraction, and
the aponeurosis is relaxed. I would observe, that the muscles attached
to the fibrous capsules, as to that of the humerus, for example,
perform for them the same functions, that the tensor muscles do for
their respective aponeuroses.
The colour of these last is a brilliant white; in this respect they
differ from all the fibrous organs thus far examined, and are analogous
to the tendons, from which they differ a little however in their
nature; in fact, they yield less quickly to maceration and ebullition;
their fibres are more stiff and resisting. There are no aponeuroses
exactly like the tendons, except those which are essentially formed by
their expansion or which are at their origin, as those spread upon the
anterior rectus of the thigh, those which are concealed in the fleshy
fibres of a muscle, and afterwards go out of it to become a tendon. In
certain parts of the limbs, as at the top of the arm, for example, the
aponeuroses of general covering are insensibly lost in the cellular
texture, without our being able to draw the line of demarcation. This
arrangement is almost peculiar to the fibrous system; at least I know
of no one which thus intermixes and loses its fibres in the cellular
texture; it is so much the more remarkable, as the nature of the two
textures is essentially different; they do not yield the same products,
and they have not the same organic arrangement.
The fibres of the general aponeuroses are only interlaced in two or
three directions; this interlacing is almost always very evident to
the naked eye. But I have observed that by plunging an aponeurosis
into boiling water, and leaving it there for some time, its fibres, in
the horny hardening they then undergo, become still much more evident.
This observation is moreover applicable to the whole fibrous system,
to its organs especially, whose texture but little apparent seems at
first view to be homogeneous. In this way, we distinguish very well the
fibres of the dura-mater.
_Functions._
The constant compression made upon the limbs by their aponeuroses,
besides the uses pointed out, has that of favouring the circulation
of the red and white fluids. Thus varices, which are very rare in the
deep veins which accompany the arteries, are extremely common in the
superficial ones placed beyond the influence of this compression, which
art imitates by the application of tight bandages, the effect of which
is so advantageous in many external diseases arising from the want of
tone, and the relaxation of the parts. I have uniformly observed that
the serous infiltrations always begin in the sub-cutaneous cellular
texture, that it is only in an advanced stage of dropsy, that we find
effusion in that which is under the aponeuroses, and that in general
it does not contain as much serum in proportion as the other. In
most of the great distensions of dropsical limbs, when the skin is
removed and the subjacent water has flown off, the limb covered by its
aponeurosis, is scarcely larger than in the ordinary state. The muscles
not protected by these coverings, like those situated on the sides of
the abdomen for example, become dropsical much more easily.
_Aponeuroses for Partial Covering._
These aponeuroses are met with in insulated parts, in front of the
abdomen, on the head, the back, &c.; they are usually destined to
retain in place a certain number of muscles which they do not surround
on all sides, like the preceding, but with which they correspond
only in one direction. Their thickness is much less than that of the
preceding ones; it is adapted to the efforts that they are to support.
All have a tensor muscle which proportions their degree of relaxation
or of tension to the effort of the neighbouring muscles. The anterior
rectus, by means of its intersections, and the pyramidalis, perform
this office for the abdominal aponeuroses; the small posterior
dorsocostals do it for that which covers the muscles of the vertebral
foramina; the auricular, the frontal and the occipital for that of the
cranium.
The aponeuroses of covering, whose use is limited to one muscle only,
like that, for example, of the temporal, want the tensor muscle, and
have consequently the same degree of tension always; it is on this
account no doubt that they have a very compact and thick texture, as
that which I have just mentioned is an example.
In general, the use of all the aponeuroses of covering whether general
or partial, relative to the compression of muscles, is required by
the displacements of which they would be susceptible in contracting,
displacements evident, 1st, when we place the hand upon a muscle in
action, and which is destitute of aponeurosis, as the masseter; 2d,
when a wound having injured a considerable part of an aponeurosis of
covering, the subjacent muscles become accidentally contiguous to
the integuments; 3d, when in an animal we lay bare the muscles of a
limb, and leave only the cellular texture to confine them, and in this
state excite their contraction; 4th, in certain wounds of the muscles
happening at the instant of their contraction, it is difficult to
probe these wounds, because in their relaxation the muscles taking a
different position, the relations change between the parts that formed
the two edges of the wound.
_Of the Aponeuroses of Insertion._
We have divided into three species the aponeuroses of insertion.
_Aponeuroses of Insertion with a Broad Surface._
They are very numerous. Sometimes they arise from the expansion of
a tendon, as we see in those of the anterior rectus of the thigh;
sometimes, as in the masseter, they derive their origin immediately
from the bones. Sometimes it is on one side only that the insertion is
made; at others it is on both at the same time, and then they appear
like partitions placed between the fleshy fasciæ, which they serve at
the same time to separate and unite, as we observe in the muscles that
arise from each of the condyles of the humerus.
These aponeuroses always receive in a very oblique direction the
insertion of the fleshy fibres. Their mutual adhesion is intimate; I
shall speak of it in treating of the tendons.
They have the great advantage of multiplying prodigiously the points
of insertion, without requiring great osseous surfaces. The width
of the whole of the temporal fossa would not be sufficient for the
masseter, if it was inserted by separate fibres. By means of the
aponeurotic partitions which receive its fibres and are afterwards
fixed in the bone, its insertion is concentrated upon one of the edges
of the zygomatic arch. Thus in general, all the very strong muscles,
whose fibres are consequently very numerous, are crossed by similar
aponeuroses, as the deltoid, the pterygoids, &c. are a proof.
Almost all these aponeuroses are exactly like the tendons; many
are continuous with them and then their fibres remain in the same
direction. In general, it is a character of these aponeuroses not to
have their fibres crossed in different directions, like those of the
aponeuroses of covering; the reason of it is plain; the fleshy fibres
to which they give attachment being all nearly in one direction, or at
least not crossing, it is necessary that these should be like them as
they are continuous with them.
I have made an experiment which shows very evidently the identity of
the tendons with these aponeuroses; it consists in macerating for
some days a tendon; it then becomes supple; its fibres separate; by
stretching in the direction of its width, it forms a kind of membrane
which it would be impossible to distinguish from a true aponeurosis.
_Aponeuroses of Insertion in the form of an Arch._
They are much more rare than the preceding. When a great vessel passes
under a muscle, nature employs this means, so as not to interrupt
the insertion of the fleshy fibres. The diaphragm for the aorta, the
solæus for the tibial artery, exhibit an arrangement of this kind.
The insertion is made on the convexity, and the passage of the vessel
under the concavity of the arch, both extremities of which are fixed
in the bone. It has been thought for a long time that the arteries
could be compressed under these arches; and hence the explanation of
popliteal aneurisms, of apoplexy by the reflux towards the head of the
blood interrupted in the aorta, &c. But it is very evident that by
contracting, the fleshy fibres would enlarge the passage, instead of
contracting it, since the necessary effect of these contractions is to
enlarge in all directions the aponeurotic curve, an effect which would
be directly opposite, if their insertion was made at the concavity.
These aponeuroses are strongly interlaced and very resisting.
_Aponeuroses of Insertion with Separate Fibres._
They are a collection of an infinite number of small fibrous bodies
wholly distinct from each other, which seem to be detached from the
periosteum, as the threads of velvet go off from their common woof.
Each is continuous with a fleshy fibre; so that when by maceration we
remove all those fibres, these small bodies become floating and are
seen perfectly well, especially when the periosteum which has been
detached is plunged into water.
It is evident that this mode of insertion on the part of the muscles
requires always broad osseous surfaces, since each fibre has a place of
its own; we have an example of it in the superior part of the iliacus,
of the anterior tibialis, of the temporal, &c. If all the muscles were
inserted in this manner, ten times more surface in the skeleton would
not be sufficient to receive them.