We include under this heading of appendices: 1° the aponeuroses, which cover the muscles or even envelop them entirely

2° the fibrous sheaths, which hold their tendons against the bony gutters on which they slide; 3° the synovial sheaths and the serous bursae, which facilitate the sliding either of the tendons or of the muscular bodies themselves.

Aponeuroses

A set of fibrous membranes that surround the muscles are called fascias or fasciae and act to prevent them from moving laterally whenever they contract. These membranous tendons, which end in some large and thin muscles, such as the oblique muscles of the abdomen, are still called fascia tendons because of overextension. There are therefore two orders of fasciae: the aponeuroses of contention, and the aponeuroses of insertion. The latter, which are real tendons spread out in a membrane (see Abdominal muscles), cannot be discussed here. We will therefore only deal with the aponeuroses of contention or envelope aponeuroses.

The aponeurosis of the arm, seen in cross section.

1, humerus. — Two, brachial fascia. — 3, internal intermuscular aponeurosis. — 4, external intermuscular aponeurosis. — 5, anterior muscular compartment. — 6, posterior muscular compartment. — 7, skin. — 8, subcutaneous cellular tissue. — 9, 9, superficial veins. — 10, 10, superficial nerves. — 11, humeral artery and median nerve. — 12, ulnar nerve. — 13, deep humeral and radial nerve. 

General disposition and relations

Aponeuroses are found on the membranes, trunk, neck, head, in all points where a muscle is likely to move by contracting and therefore needs to be contained.

Member Aponeuroses

The aponeuroses present a remarkable development to the members. They affect the shape of hollow cylinders or sleeves, enveloping in all their extent the muscular masses that group around the bony levers. Each of these aponeuroses has two surfaces, one external, the other internal. - The outer surface is related to the skin which slides over it, thanks to the subcutaneous cellular tissue known as fascia superficialis. It is in this fascia superficialis, more or less rich in fat cells, that the vessels and nerves known as superficial travel. The inner surface rests on muscles that sometimes fit over it, as seen in the forearm and leg, but most often just join it together with loose connective tissue. From this deep surface of the fascia always detach a series of more or less resistant extensions which are directed towards the axis of the limb: some of them attach themselves to the bone, and, under the name of intermuscular partitions, divide the underlying muscles into distinct groups; the others, under the name of deep aponeuroses (the enveloping aponeuroses of the limb being the superficial aponeuroses), throw themselves onto the muscles themselves and onto the large vessels, constituting envelopes or sheaths for them; such are : the sheath of the biceps, the sheath of the vastus medialis, the sheath of the neck vessels, the sheath of the humeral and femoral vessels, etc. As it passes over the bony projections, the aponeurosis of the limbs usually attaches itself to these projections, as can be seen in the epitrochlea, the epicondyle and the two malleoli.

Aponeurotic lodges in the arm. Cross section of the right arm, lower segment of the cut.

H, humerus, - H', radial gutter.

1, 1', brachial aponeurosis. — 2, external intermuscular septum. — 3, internal intermuscular septum. — 4, biceps compartment. — 5, anterior brachial lodge. — 6, triceps compartment. — 7, humeral artery. — 8, median nerve. — 9, ulnar nerve. — 9', superior internal collateral artery. — 10, musculocutaneous nerve. — 11, radial nerve. — 12, deep humeral artery. — 13, basilic vein. — 14, internal cutaneous brachial nerve. — 15, internal cutaneous brachial accessory nerve. —— 16, cephalic vein.

The aponeuroses of the limbs not only give rise, on their deep side, to bundles of muscles; they sometimes receive the termination, either partial or total, of certain muscles which, for this reason, are called its tensor muscles.

The aponeurotic expansion of the biceps and the tensor of the fascia lata provide us with very clear examples of such an arrangement.

Finally, the aponeuroses of the envelope of the limbs have here and there more or less wide orifices, through which pass the various organs, vessels and nerves, which, from the subcutaneous layer, descend into the subaponeurotic layer or, vice versa, rise from the latter layer into the fascia superficialis. In this respect, we would like to mention the anterior and superior part of the femoral fascia, which, due to its numerous orifices, has been compared to a sieve and for this reason has been given the name of fascia cribriformis.

Aponeuroses of the trunk and neck

On the trunk and neck, the aponeuroses are similar to those on the limbs, but are much thinner, with the exception, however, of the aponeurosis of the vertebral gutters, which attaches more to the insertion aponeuroses than to the contention aponeuroses.

Aponeurosis of the head

At the head, the fasciae do not form a continuous sheet. This is because there is a special muscle system here, the skin muscles, which attach to the skin, at least at one of their extremities. Therefore, there is no fibrous blade between these muscles and the skin. Isolated fasciae cover the temporal, masseter and even the buccinator (see these muscles).

Physical Characteristics

If we now look at fasciae from the point of view of their physical characteristics, we see them appear to us as whitish membranes, sometimes with a pearly appearance. Although they are very flexible, they are very resistant and almost inextensible, which is in perfect harmony with their functions.

Thickness

As for their development, it varies, as Cruveilhier wisely points out, with that of the underlying muscles. Aponeuroses," he says, "have a thickness, and therefore a strength, strictly proportionate to the strength and resistance of the muscles they sheathe or insert: The femoral fascia is therefore singularly stronger than the brachial fascia, and the thickness of the fasciae increases from the upper to the lower part of the limbs, and the powerful vastus externalis muscle has a stronger contentive fascia than the muscles of the posterior region and those of the inner thigh. It can therefore be considered as a law without exception that the aponeurotic system constantly follows, in its development, the same phases as the muscular system. »

We have seen, in arthrology, that a number of fibrous formations around joints, which are considered ligaments, are in fact remnants of lost muscles. This is exactly the same for certain fibrous blades, which are improperly classified as envelope aponeuroses: these are the middle cervical aponeurosis, the intermediate aponeurosis of the posterior serrations, the clavicle-pectoral aponeurosis, etc. These are pseudoaponeuroses, representing, like pseudo-ligaments, muscles or portions of muscles that have atrophied and reduced to a fibrous state during phylogenetic development. These pseudoaponeuroses, on the other hand, if we examine them in the embryo, still present us with muscular elements and, on the other hand, these muscle bundles sometimes persist in the adult in the state of abnormality.

From a histological point of view, the fasciae belong to the connective tissue. They therefore present us with two orders of elements, fibres and fixed cells, to which are joined, as accessory elements, a certain number of elastic fibres. (see Muscle tissue histology)

Vessels and Nerves

Aponeuroses have a very rich vascularization, quite different from that granted by the ancient authors.

Arteries

Very numerous arteries, equipped with their three tunics, detach themselves from the subcutaneous trunks to penetrate, by their external face, the aponeuroses of the limbs envelope and form, in their superficial layer, a network with very tight meshes, in their deep layer a network with looser meshes.

Veins

The veins accompany the arteries. They are, for the most part, tributary of the subcutaneous veins.

Lymphatics

Lymphatic networks with polygonal meshes were reported in the fasciae in 1872 by Ludwig and Schweiger-Seidel. In turn, Mr. -and Mrs. Hoggan, in 1879, in their memoir on the lymphatics of striated muscles, reported lymphatics belonging specifically to the fasciae. Their mode of origin and termination is not yet elucidated.

Nerves

The existence of nerves in the fascia has not been disputed since Sappey's research (1866) and Tschiriew's more recent work (1879). According to Sappey, aponeurotic nerves emanate, for the most part, from subcutaneous nerves. Tschiriew endeavoured to demonstrate, above all, that the fascia receive their nerves from the sensory branches of the underlying muscles.

Fibrous tendon sheaths

We will give this name to fibrous formations, which develop, in the manner of bridges or arches, above the bony gutters in which the tendons slide. Together with these bone grooves, they form osteo-fibrous channels of variable length, but closed on all sides. These fibrous ducts, as we understand it, have the effect of keeping the tendons firmly applied against their gutter, while allowing them to slide freely. Some of them, because of their direction and under certain given conditions, become true reflection pulleys.

The fibrous sheaths of the tendons occupy the extremities of the limbs. We will find them further on in the palm of the hand, on the sole of the foot, on the palmar surface of the phalanges, where they give way to the tendons of the flexor muscles. We will also find them around the wrist and the elbow-foot, in the form of transverse ribbons, called annular ligaments (annular ligaments of the carpus and tarsus).

Sappey divides them into simple sheaths and compound sheaths. The simple sheaths allow only one tendon to pass through, or at most two tendons that are intimately joined; to this variety belong the sheaths of the palmar face of the phalanges. Compound sheaths are common to several tendons: from their deep face, vertical partitions escape, which are fixed on the other hand to the underlying bones, thus dividing the space between the fibrous formation and the skeleton into a more or less considerable number of secondary sheaths, in each of which a tendon is housed. The posterior annular carpal ligament is the perfect type of this second variety.

The fibrous sheath of the flexors, seen on the cross section of a finger.

1, phalanx. — Two, periosteum. — 3, flexor tendons, surrounded by the visceral lamina of the synovium. — 4, fibrous sheath of the flexors, lined medially by the parietal leaf of the synovium. — 5, subcutaneous cellular tissue. — 6, skin. — 7, collateral nerve. — 8, collateral artery. — 9, extensor tendon.

Histologically, the fibrous sheaths of the tendons belong, like the fascia and the tendons themselves, to connective tissue formations. Their basic elements are extremely dense fibrous bundles, joined together by loose connective tissue. The fibrous tissue is mixed with fixed connective tissue cells, a number of elastic fibres and some fat cells. It should be added that to the bundles of fibrous sheaths, tendinous bundles from neighbouring muscles are sometimes added: this is how the anterior annular ligament of the carpus is reinforced both by the terminal tendon of the lesser palmaris and by the tendons of origin of the thenar and hypothenar muscles.

The fibrous sheaths of the tendons have vessels and nerves like the fascia. They affect them in the same way as in the fascia.

Synovial tendon sheaths

The synovial sheaths of the tendons or tendon synovials are thin membranes, difficult to isolate, belonging to the serous class, just like the articular synovials. Their role is to promote the sliding of tendons in the osteo-fibrous slides through which they pass.

General Disposition

Afin d'obtenir une image précise du comportement des synoviales du tendon, deux ordres de coupes doivent être effectués sur les tendons et leur gaine fibreuse, l'un longitudinal et l'autre transversal :

Longitudinal cuts

On longitudinal sections, we see the synovium enveloping the tendon in the entire portion of it that responds to its sheath: then, at either end of this sheath, reflecting outwards, reaching the inner face of the osteo-fibrous groove and covering it evenly throughout.

The synovium thus presents two sheets, both cylindrical, leaning against each other and sliding over each other: an inner sheet (4"), sheathing the tendon, is the visceral sheet; an outer sheet (4'), lining the osteo-fibrous ramus internally, is the parietal sheet. These two layers continue at the top and bottom, forming a sort of annular cul-de-sac, reminiscent of the ring-shaped cul-de-sac, which can be seen, after a successful injection of the elbow, all around the neck of the radius. Between the parietal bundle and the visceral leaflet is a cavity (4'"), closed on all sides and almost virtual like that of the serosa: it contains a very small quantity of a creamy and flowing liquid, which presents the greatest analogies with the articular synovium.

Transverse sections

If we now examine a cross-section, we recognize successively: 1° in the centre, the section of the tendon; 2° at the periphery, the bone groove and the fibrous arch which, by completing it, transforms it into an osteo-fibrous canal; 3° between the tendon and its osteo-fibrous duct, the two circular visceral and parietal laminae, both arranged concentrically and intercepting the synovial cavity between them. 

Diagram showing the arrangement of the tendon synovials, views: A, in longitudinal section: B, C. D, in cross section.

1, tendon. - 2, his fibrous sheath. - 3, bone. - 4, synovial tendon, with 4', its parietal leaflet, 4", its visceral leaflet; 4'", its cavity. - 5, mesotendon with its vessels.

Mesotendons

We saw earlier that the two synovial layers were joined together at the terminal cul-de-sacs. They are still connected at one or more intermediate points, and this is how. The tendon is joined to the osteo-fibrous slide, preferably to the bony part of the slide, by more or less developed connective tracts, affecting here the shape of simple filaments, and further on the shape of very small membranes. These connective tracts, whether membraniform or simply filiform, serve as support for the tendon's feeder vessels: the arterioles which go towards it and the venules which return from it (D, 5). However, at the point where this conjunctivo-vascular bundle approaches the parietal leaflet of the synovium (B), the latter reflects on it and envelops it from all sides to continue further with the visceral leaflet. These synovial folds thrown onto the vessels of the tendon and going from the parietal leaflet to the visceral leaflet have been given the name mesotendons: they are in fact reminiscent, by their arrangement, of the peritoneal fold or mesentery which connects the small intestine to the posterior wall of the abdomen. Thanks to this arrangement, the tendon vessels and the connective tissue that accompany them reach the tendon without passing through the serosa cavity. As a result, at the mesotendons, part of the tendon, the part where the vessels end, is not covered by the synovium. This part of the tendon surface, thus devoid of synovium, is usually minimal. Sometimes, however, it can be a quarter or even half the circumference of the tendon. In the latter case (B), the synovium, instead of forming a complete cylinder, has the appearance of a half cylinder or, if you like, a simple groove, covering the tendon and pressing it against the bone.

Relationship of the synovials to the articular synovials

The tendon synovials are primitively independent and most of them retain this independence in adults: such are the synovials of the flexors and extensors of the fingers, which, although they are very close to the wrist joint, are only contiguous with it. There are, however, a few which, during development, enter into communication with the synovium of the neighbouring joint: of these are the synovium of the popliteus, which communicates with the knee joint and the synovial sheath of the long portion of the biceps brachialis, which appears to be, in adults at least, a simple expansion of the shoulder synovium.

Structure

Viewed from a histological point of view, the tendon synovials are of the same type as the articular synovials (see Arthrology).

 Flexor tendons with their serous sheath (after Farabeuf).

A, the serosa passes to the superficial side of the tendon without covering its deep side. — B, the serosa covers the tendon over almost its entire circumference and forms a mesotendon behind it.

1, tendon. — 2, serous membrane. — 3, fibrous sheath, which has been incised and eroded in part of its extent. — 4, arterial branch from the collateral branches of the fingers. — o, 5, branches located between the two leaves of the mesotendon. 

They present us, like these last ones, two superimposed layers, one external moon, the other internal.

a) The outer layer is of a conjunctive nature. On the visceral layer, it is extremely thin and merges with the conjunctive sheath of the tendon or external peritenonium. On the parietal layer, it is much more developed; in some cases, it even has small excrescences, more or less loaded with fat, protruding into the synovial cavity and recalling exactly, by their nature and meaning, the synovial fringes of the joint cavities. Here again, it merges without any demarcation line with the surrounding connective tissue.

b) The inner layer is formed by flattened cells of connective origin, more or less similar to endothelial cells.

Vessels and Nerves

The synovial sheaths of the tendons have vessels and nerves. -- The arteries originate, for the most part, from those that irrigate the fibrous sheath. - The veins accompany the arteries. They are always larger than the arteries. - The nerves were reported by Sappey, both on the parietal and visceral leaflet. Us emanate, partly from the nerve branches which distribute themselves to the fibrous sheath, partly from the nerves of the tendon itself.

Serous bursae annexed to the muscles

In addition to the synovials we have just described, which surround the tendons like a double sheath, the muscles have other synovials, which are called serous bursae. These affect a vesicular shape and, instead of enveloping the organ, especially its periphery as the previous ones, they simply apply on one of its faces, thus separating them from the parts with which they are in contact.

Divisions and relations

The bursae are usually divided into two groups, depending on whether they are related to a tendon or a muscle, they are called tendon bursae or muscle bursae:

Tendon serous bursae.

Tendon bursae are most often placed between a tendon and the underlying bone surface. This is the Achilles tendon bursa, which develops between this tendon and the highest part of the posterior surface of the calcaneus. Such are also: the bursa of the medial obturator, which is located between the tendon of this muscle and the small sciatic notch; the inferior bursa of the biceps, located between the distal tendon of this muscle and the bicipital tuberosity of the radius; the inferior bursa of the psoasiliac, located between the tendon of this muscle and the lesser trochanter, etc. Tendinous bursae may also be found between two neighbouring tendons (inter-tendinous bursae): such is the 

La bourse séreuse de l’obturateur interne.

1, ischium. — 2, small sciatic spine. — 3, internal obturator muscle, incised and strongly erect, to show its bursa serosa (stained blue), 3', its tendon. — 4, transverse ridges, responding to the inter—fascicular spaces of the obturator tendon. — 5, upper orifice of the subpubic canal with the vascular—nervous bundle engaging it. — 6, pubic spine. — 7, great sciatic nerve. — 8, small sciatic nerve. — 9, internal pudendal nerve. — 10, ischial artery. — 11, internal pudendal artery.

serous bursa which separates the tendon of the great dorsal and the great round from each other.

Muscle Serous Bursaries

Muscular or intermuscular bursae develop between two muscles that slide over each other: they are all the more considerable as these sliding movements are more frequent and more extensive. A bursa is found between the spinous and deltoid muscles; another bursa is seen between the gluteus maximus and the muscles of the thigh that attach to the ischium, etc. The bursa is also seen between the gluteus maximus and the muscles of the thigh that attach to the ischium.

Relationship of the serous bursal sacs to the articular synovials

Like the tendon synovials, the muscle bursae are primitively distinct from the joint synovials. However, there are always a number of them which, by virtue of their progressive enlargement, approach the neighbouring articular synovials, come into contact with them, and finally merge with them. Examples include the bursa of the subscapularis, which in adults communicates with the scapulohumeral joint; the greater bursa of the psoasiliac, which in some cases also merges with the coxo-femoral synovium, etc.

Structure

The bursae annexed to the muscles are similar in structure and meaning to subcutaneous bursae.

The bursae attached to the tendons are entirely reminiscent in their wall structure of the arrangement between the interarticular meniscus and the temporal condyle in the temporomandibular joint (Tourneux). They have a customary fibro-cartilaginous blade at their sliding surfaces, 60 to 400 g thick, resting on the one hand on the bone tissue and on the other hand on the tendon tissue. On its periphery, this fibro-cartilaginous blade is continuous: 1° by its superficial layer, with a clearly characterized synovium; 2° by its deep layer with the periosteum (on the bone) and with the conjunctive envelope of the tendon.

The serous bursae, either muscular or tendinous, contain in their interior a small quantity of a creamy and spinning liquid, the appearance of which is reminiscent of the synovium of the articular cavities.

Mode of origin of the serous cavities annexed to the muscles

It is generally accepted that the serous cavities annexed to the muscles develop at all the points where the muscle or its tendon slides on the underlying plane, and it is also accepted that their appearance is the consequence of the slide: under the influence of this slide, the connective trabeculae, which are tightly pulled, become thinner and disappear, leaving in their place a more or less considerable cavity, which is none other than the bursa serosa.

Superficial layer of the fibro-cartilaginous lining of the Achilles tendon bursa (Tourneux).

The cartilage cells, immersed in a homogeneous amorphous material, are provided with a refractive capsule and contain one or more fat droplets.

However, this gliding theory is not applicable to all bursae, especially those that develop between the distal end of a tendon and the bone on which it is inserted. An example is the Achilles tendon bursa, which, as mentioned above, is located between the tendon and the upper posterior surface of the calcaneus. Under no circumstances do the two surfaces, the tendon and bone surfaces, slide on top of each other, so that the bursa between them does not slip. In the normal state, in the standing position (A), the Achilles tendon is pressed directly against the bone. But when the foot is extended, and this happens in walking whenever the gastrocnemius, by contracting, lifts the heel, the tendon (B) tends to move it away from the bone surface and separate from it by an angular space at the upper base, the opening of which is proportional to the degree of extension of the foot. Under these conditions, the loose connective tissue which originally joins the tendon to the upper part of the calcaneus is strongly pulled back and forth: as before, its trabeculae become longer, thinner and disappear, and so the bursa serosa is formed. In a sagittal section, the figure clearly shows that in the extended state of the foot, a fat packet, comparable to a synovial fringe (see Arthrology), moves backwards from the calcaneus to fill the gap created by the mutual separation of the bone surface and the tendon.

 

Sagittal section of the heel; A, with the foot at rest (in an upright position); B, with the foot in extension (in gait).

1, calcaneus. — 2, Achilles tendon. — 3, bursa serosa. — 4, adipose packet (in B you can see that this fat packet, in the extension of the foot, projects behind the calcaneus to fill the angular space which occurs at this moment between the bone and the tendon). — xx, horizontal plane. — yy, anterior—posterior axis of the calcaneus, horizontal in figure A, oblique in figure B. 

The same mechanism applies to the development of the bursa of the biceps brachialis: when the radius is in pronation and the tendon of this muscle is as if wrapped around the bicipital tuberosity; but if the biceps contracts, bringing the radius into supination, its tendon unwinds and is thus separated from the bone by a triangular space at the anterior base. It is this separation of the two surfaces which, frequently repeated, determines the resorption of the intermediate connective tissue and, consequently, the appearance of the bursa serosa. Here again, a fat packet fills the angular space that occurs between the tendon and the bone.

The formation of the serous cavities annexed to the muscles and tendons (muscular and synovial tendon bursae) is therefore the consequence of the intermittent, but frequently repeated tightening of the connective tissue trabeculae, whether this tightening occurs as a result of sliding from the muscular or tendon surface to the underlying surface, or as a result of these two surfaces moving away from each other.

Embryological research tells us that most muscle bursae and tendon synovials develop well before birth, at a time when the action of the muscles is really too small to reasonably explain the digging of these cavities. The mechanical theory invoked above nevertheless retains all its value, but it must be considered in phylogeny and not in ontogeny: the movements of the muscle and its tendon under the conditions indicated above were the real cause of the production of muscle and tendon bursae in those of our ancestors who did not yet have them. Currently, these bursae are an integral part of our constitution, either fetal or adult: they are, like the muscles themselves to which they are annexed, both inherited and hereditary formations.

Horizontal section of the posterior part of the foot of a rabbit foetus on the twenty-third day of incubation (according to Retterer).

1, calcaneus. — 2, talus. — 3, hail plantar, with its 3' fibrous expansions. — 4, Achilles tendon, with fibrous expansions 4'. — 5, serous bursa, placed in front of the Achilles tendon. — 6, bursa serosa, placed between the Achilles tendon and the tendon of the small plantar foot. — 7, deep flexor tendon with 7', its serous cavity. 

Retterer, who has carefully studied in rabbits the way in which tendon bursae develop, has established in principle that each tendon is initially surrounded, in all its length and all around, by reticulated connective tissue, i.e. by clusters of connective cells, whose protoplasma is differentiated: 1° into a fibrillar network; 2° into a perfectly homogeneous hyaloplasma. At the points where the peritendinous bursa will later exist, this reticular tissue undergoes a series of transformations, which can be summarised as follows: first of all, the hyaloplasma increases and gradually takes on the characteristics of the mucous substance; then the mucous substance becomes fluid, so that large empty or vacuous areolas are produced; finally, the fibrils of the reticulum and, with them, the nuclei of the connective cells undergo a progressive atrophy which leads to their complete disappearance. The final cavity is thus established. Once this cavity has been formed, the connective cells that spread out on its walls become elongated and arrange themselves in more or less regular series. At the same time, they flatten parallel to the wall itself and gradually take on the characteristics of epithelioid cells, characteristics which are sometimes sufficiently clear-cut that many authors have described them as true endothelial cells (see Arthrology).

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