Muscle auxiliary apparatus

Muscles, when contracting, perform their function with the participation and assistance of anatomical structures, which should be considered as auxiliary apparatus of muscles. These include fascia, tendon sheaths, synovial bags and muscle blocks.

Fascia (fascia) is a connective tissue covering of a muscle. By forming sheaths for muscles, fascia separates them from each other, creates support for the muscle belly during its contraction, eliminates friction between muscles. Having a sheath-like structure, fascia in pathology limits the spread of pus, blood during hemorrhage, and makes it possible to carry out "sheath" local anesthesia. Between the surface of the muscle, its membrane (epimisium) and fascia there is a thin layer of loose cellulose. In some places (on the shin, forearm), fascia serve as the place where muscles begin, and then it is difficult to separate the muscle from the fascia.

A distinction is made between proper fascia (fasciae propriae), which form a connective tissue sheath for a given muscle; superficial fascia (fasciae superficiales), which cover the muscles from above; deep fasciae (fasciae profundae), which separate one muscle group from another. Each region has its own fascia (for example, the shoulder - fascia brachii, the forearm - fascia antebrachii). If the muscles lie in several layers, then between the adjacent layers there are fascia plates: between the superficial muscles - the superficial plate (lamina superficialis), between the deep muscles - the deep plate (lamina profunda). The superficial fascia (plate) is located under the skin, separates the muscles from the subcutaneous base (cellulose), envelops the muscles of a particular part of the body (for example, the muscles of the limbs). Between the muscle groups (usually of different functional purposes) there are intermuscular septa (septa intermuscularia), connecting the superficial fascia with the bone (periosteum). In places where fascia connect to each other, thickenings are formed, the so-called fascial nodes, which play a significant role in strengthening the fascia and protecting vessels and nerves from compression. Fascia, intermuscular partitions firmly grow together with the periosteum of bones, form a soft base for muscles and other organs, participating in the formation of a soft frame, or soft skeleton.

The structure of the fascia, developing from embryonic connective tissue during muscle formation, depends on the functions of the muscles, the pressure that the muscles exert on the fascia during their contraction. In those places where the muscles partially begin on the fascia, the fascia is well developed, dense, reinforced with tendon fibers and in appearance resembles a thin wide tendon (wide fascia of the thigh, fascia of the leg). However, this is not a tendon, not an aponeurosis, as they were incorrectly called, but a fascia of the tendon type. Muscles that perform a smaller load have a fragile, loose fascia, without a specific orientation of connective tissue fibers. Such thin, loose fascia are called felt-type fascia.

In some places, formations are observed that are thickenings of the fascia. These include the tendinous arch (arcus tendineus), which is formed as a local thickening of the fascia over the underlying vascular-nerve bundle or other anatomical formation. In the area of some joints (ankle, wrist), where the muscles and tendons change their direction according to the structure of the limb, the fascia is also thickened. Attaching to bone protrusions, the fascia forms fibrous bridges - tendon holders (retinacula). The holders prevent the tendons from shifting to the sides and give them the desired direction when the muscles contract.

The channels formed between the retinacula and the underlying bones, in which the long thin tendons of the muscles pass, are called osteofibrous. The tendons in such channels are surrounded by dense fibrous connective tissue, forming a fibrous sheath of the tendons (vagina fibrosa tendinum). Such a fibrous sheath can be common to several tendons or divided by fibrous bridges into several independent sheaths for each tendon.

The tendon moves in its fibrous sheath (osseous-fibrous canal) with the participation of the synovial sheath, which eliminates friction of the moving tendon against the motionless walls of the canal. The synovial sheath is formed by the synovial membrane, the synovial layer (stratum synoviale), which has two plates (sheets) - internal and external. The internal (visceral) plate (lamina visceralis) envelops the tendon on all sides, grows together with it, with its connective tissue sheath - peritendinium. The external (parietal) plate (lamina parietalis) is adjacent from the inside to the walls of the fibrous sheath (osseous-fibrous canal). Between the visceral and parietal (wall) plates there is a narrow gap containing a small amount of mucus-like fluid - synovium.

The visceral and parietal plates of the synovial membrane pass into each other at the ends of the tendon sheath, as well as along the entire length of the sheath, forming the mesentery of the tendon - mesotendinium (mesotendineum). The mesotendinium consists of two sheets of the synovial membrane connecting its visceral and parietal plates, contains blood vessels and nerves that feed the tendon. During muscle contraction, its tendon, covered with the visceral plate of the synovial membrane, slides freely along the outer (parietal) plate, like a piston inside a cylinder, due to the presence of synovium in the slit-like cavity of the synovial sheath. The synovial layer can surround one tendon or several if they lie in the same fibrous sheath (canal).

In places where a tendon or muscle is adjacent to a bony protrusion, there are synovial bags that perform the same functions as the synovial sheaths of tendons - they eliminate friction.

The synovial bursa (bursa synovialis) has the shape of a flattened connective tissue sac containing a small amount of synovial fluid. The walls of the synovial bursa are fused with the moving organ (muscle, tendon) on one side and with the bone or another tendon on the other. The sizes of the bursa vary from a few millimeters to several centimeters. The cavity of the synovial bursa located near the joint can communicate with the joint cavity. Often the synovial bursa lies between the tendon and a bony protrusion that has a groove covered with cartilage for the tendon. Such a protrusion is called a muscular trochlea (trochlea muscularis). The trochlea changes the direction of the tendon, serves as a support for it and simultaneously increases the angle of attachment of the tendon to the bone, thereby increasing the lever for applying force. The same function is performed by sesamoid bones that develop in the thickness of some tendons or are fused with the tendon. These sesamoid bones include the pisiform bone in the hand, as well as the patella, the largest sesamoid bone.

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