The striated muscles each consist of two distinct parts: 1° a red, soft, contractile part, constituting the muscle proper; 2° a whitish, firm, non-contractile part, forming the tendon.
The muscle itself is made up of fibres, the striated fibres that can be broken down into fibrils. Fibrils of variable length (on average 4 to 6 centimetres in length) are grouped into bundles, the primary bundles.
These are in turn arranged in secondary bundles, then tertiary and even quaternary bundles when the muscle is voluminous. The muscular element is always associated with the connective element. The connective element, which is lamellar, penetrates the very thickness of the bundles, where it is called endomysium, and separates the bundles from each other by forming partitions, which are called perimysium or interfascicular connective tissue. This tissue is the path taken by the vessels that enter or leave the muscle (arteries, veins and lymphatics). (See Histology.)
Mode of arrangement of muscle fibres, as seen on a cross-section of the muscle. (The figure shows us only part of the section).
1, primitive beams joined together to form: 2, a secondary beam; 3, a tertiary beam. - 4, external perimysium, surrounding the muscle. - 5, 5, first-order partitions, delimiting tertiary bundle 3; 6, 6, various second-order partitions delimiting secondary bundles.
Tendons are made up of a combination of fibres, the tendon fibres, which are a modality of connective tissue. These fibres are grouped into primary, secondary and tertiary bundles. Like the muscle bundles, they are penetrated and separated from each other by loose connective tissue which is called internal and external peritonium. Sometimes the tendon may undergo cartilage transformation in certain places, especially near its bony insertion. In the elderly, the tendon may ossify over a certain area.
Striated muscles, like all highly active organs, have extremely rich vascularization. It has been well studied by Hyrtl, by Ranvier and, more recently (1888), by Spalteholz. Each muscle usually receives multiple arterial branches which penetrate it at the most diverse points and at a highly variable angle of incidence.
Cross-section of an adult male tendon (according to Stöhr)
1. Connective sheath of the tendon or external peritenonium. - 2. first-order septum dividing into second-order septum, constituting the peritenonium. - 3, tendon bundles; the small black dots seen on the bundles represent connective cells. - 4, blood vessels.
Once they reach the thickness of the muscular body (where they always occupy the bays of the internal perimysium), these arterial branches divide and subdivide into a multitude of branches and ra-muscles which, by anastomosing with each other, form a rich network around each secondary bundle, which we will call the perifascicular network. This network is very irregular, but most of its branches are arranged in a longitudinal direction, that is, parallel to the axis of the muscle bundles.
Contracted muscle bundles of the human tongue with their capillary network (after Pouchet and Tourneux).
From the perifascicular network, a multitude of arterioles, terminal arterioles, then start out, which engage in the very thickness of the secondary bundles and resolve there, in the interval between the primitive bundles or muscle fibres, into a rich network of true capillaries, the intrafascicular or interfibrillar network. This network has a characteristic arrangement, so characteristic, says Kölliker, that it is enough to have seen it once to recognize it always in the following. It is formed by long capillaries, running parallel to the fibres and joined together from distance to distance by very short transverse anastomoses: it is therefore a rectangular mesh network, oriented in such a way that the long side of each mesh corresponds to the longitudinal axis of the fibre. Moreover, each fibre alone has several longitudinal capillaries and, as these longitudinal capillaries are all anastomosed to each other, the result is that the fibre is as if contained in a sort of vascular net, which envelops it both all around its perimeter and in its entire extent. It should be added that the longitudinal vessels, which are fairly regularly rectilinear when the muscle is at rest, become more or less flexible when the muscle has shortened due to the fluence of the contraction.
Vessels of striated muscles (after Spalteholz).
Arterial vessels are represented in solid lines; venous vessels in striated or dotted lines.
From the interfibrillar capillary network, veinlets, 'primitive' veinlets, are born, which are directed towards the surface of the secondary bundles and then unite, in the perimysial walls, with similar veinlets to form increasingly voluminous veins.
As shown in the preceding figure, which represents a preparation of Spalteholz, these primitive veinlets travel perpendicular to the muscle fibres, as do the terminal arterioles to which they correspond. This same figure also shows us that the terminal arterioles and the primitive venules, although following a similar direction, are independent of each other and even alternate quite regularly: indeed, going through the preparation from top to bottom, one encounters first an arteriolus, then a venule, then an arteriolus, again a venule, and so on.
Further on, in the second-order and first-order conjunctive partitions, the veins, on the contrary, follow the path of the arteries and each of them presents, according to its volume, one or two satellite veins. The arterial branches which penetrate into the muscle are generally each accompanied by two venous branches which, after a more or less long extra-muscular path, flow into the neighbouring veins.
In rabbit red muscles, Ranvier found fusiform dilatations on the venous capillaries, especially on the transverse anastomoses of the inter-fibrillar network, in which blood collects. These are all pockets and reservoirs where the muscle, at the time of its contraction and while circulation is momentarily suspended, would draw, as if from a kind of reserve, the materials necessary for its functioning. The physiological attribute of red muscles is that they contract slowly but persistently, and this is why we would find the pockets in question in the muscle to provide it with the oxygen it needs to keep itself contracted.
Kolliker, for a long time now, has encountered on some large muscles lymphatic vessels, 0 mm, 5, on average, accompanying the blood vessels that penetrate their thickness.
For his part, Sappey saw lymphatic vessels escaping from the pectoralis major, gluteus maximus and adductor major; he was even able, on the diaphragm, to follow them into the interstices of the main bundles, which they entwine with their anastomoses.
Mr. and Mrs. Hoggan (Journ. de l'Anatomie, 1879) have described lymphatic networks in the diaphragm, in the triangular of the sternum, in the large muscles of the abdomen, etc., and have even been able to follow them to the interstices of the main bundles, which they entwine with their anastomoses.
The existence of lymphatics in striated muscles is therefore not in doubt, for it is rational to admit that, if they have not been found up to now in all muscles, the cause is not in their absence, but in their tenacity and especially in the difficulties of injecting them. Their mode of origin and their intramuscular path are still virtually unknown. Recently Aagard has taken up this study again. He has shown that the lymphatics of the tendons and the muscular connective tissue communicate (Aagard, Ueber die Lymphgefässe der Zunge, der Quergestrefen Muskelgewebes... Wiesbaden, Bergmann, ed. 1913).
Tendons, like all fibrous formations, are not very active organs and are relatively poor in vessels. Their vascularisation is, in any case, much less rich than that of the muscle bundles to which they follow.
Vascular network of the semitendinosus muscle of the rabbit (according to Ranvier).
Here and there we see a system of fusiform bulges going from vessel to vessel.
The arteries, which originate from the nearest trunks, branch off into the outer peritenonium and form, within the thickness of this common envelope, a first network of irregular meshes. A multitude of arterioles escape from this network and, following the inter-fascicular spans, reach the interior of the tendon. Along the way, they divide, subdivide and anastomose to give rise to "long series of arches, arranged in a single, double or triple row, which offer the most elegant arrangements and almost infinite varieties" (Sappey). Eventually these vessels resolve into a capillary network, whose meshes advance between the primitive bundles, but never penetrate their thickness. Nutrition in the primitive bundle, as in the muscle fibre, is therefore carried out at a distance.
From the capillary network are born veins which, following an inverse path to that of the arteries, travel along the interfascicular conjunctive partitions, to reach the general envelope and from there flow into the veins of the neighbourhood.
The question of tendon lymphatics has not yet been clearly elucidated. Once denied by Ch. Robin, by Sappey and by Kôlliker, they have since been described by many histologists, among whom we will quote Ludwig and Schweiger-Seidel (1872). Budge (1877), Lôwe (1878), Mays (1879), Schiefferdecker (1890). According to these last histologists, there are real lymphatic channels in the interfascicular walls, formed by a simple endothelial layer and arranged for the most part parallel to the tendon bundles. Moreover, these vessels are connected to each other by transverse or oblique anastomoses, so as to form a network, the deep network. The trunks and trunks emanating from this deep network are directed towards the outer surface of the tendon and there form a second network, the superficial network, constituted like the first, by vessels reduced to their endothelium. How are these lymphatic channels born in the thickness of the tendon? How do they end outside it? These two questions, in the current state of our knowledge, must remain unanswered.
Nerves of striated muscles and tendons
The muscle is approached at one or more points by nerve threads which constitute a motor and a sensory device.
It should be noted that the nerve fillets in the interior of the striated muscle are not distributed in networks, but that there are areas that appear to be particularly rich in nerves, while others are poor. Histology shows, moreover, that the motor device is not distributed evenly throughout the muscle. This is probably related to the way in which the nerve acts on the muscle itself.
Motor endplate in lizards (after Boeck).
1, striated muscle. -2, myelin fibre (with 2', annular constriction), going to the end plate. - 3, amyelin accessory fibre. - 4, network of nerve fibrils. - 5, sole nuclei.
The motor nerve fibre, myelin, grows in a granular protoplasma, rich in nuclei, forming a plate, the motor plate, intermediate between the nerve fibre and the muscle substance itself. There is therefore no anatomical continuity between the nerve tissue and the muscle; only physiological continuity. This functional junction zone is called the synapse and exists at all nerve endings, whether motor, glandular or interneuronal.
Apart from the motor fibres which belong to the voluntary nervous system, the muscle receives sympathetic fibres which seem to be particularly affected to muscle tone, while the former seem to be intimately linked to the contraction itself.
Neuromuscular spindle (Policard).
1, spindle axial fibre; united in 7 with the capsule and of normal appearance in 8. - 2, laminated capsule. - 3, myelin nerve fibre. - 4, Henle sheath of the nerve. - 5, spiral nerve expansions. - 6, tree expansions. - 9, spindle fibre motor plate.
Mode of nerve endings in the musculo-tendinous corpuscles (after Golgi).
1, musculo-tendinous corpuscle. - 2, limit of its endothelial lining. - 3, tendon. - 4, muscle fibres. - 5, myelin nerve fibre, with 5', its entry into the musculo-tendinous corpuscle. - 6, 6, its myelin ramifications. - 7, point where the nerve fibre loses its myelin to become cylindraxial. - 8, 8, terminal arborizations.
The sensing device comes in a variety of forms. First of all, the muscle contains morphological formations which are special to it and which are undoubtedly functionally specialized: these formations constitute the neuromuscular spindles. Each neuromuscular spindle consists of an axis of fine, striated muscle fibres grouped in bundles (Weissmann's bundles) which are surrounded at some distance by a loose connective tissue capsule. A lymphatic-like fluid fills this capsule and bathes the neuro-muscular elements. The sensitive nerve fibre, the last element of the spindle, passes through the capsule and wraps itself in rings or spirals around the axial muscle fibres, which lose their striation at this nerve contact.
Other sensory devices, more commonplace, in that they are also found in the articular ligaments, in the periosteum and in the tendons, include Ruffini's and Pacini's corpuscles.
The tendons also present, like the muscle of the special formations, the neurotendinous Golgi corpuscles. This richness and variety of sensory apparatus in the muscle feels necessary to give an individual a sense of attitude, coordination of automatic acts and ensure the synergy of voluntary contractions. (For more details, see Treatises on Histology and Physiology).