they are organs that have an outstanding ability to shrink, i.e. to reduce their length. The contraction takes place under the influence of stimuli, either mechanical, like a blow, or electrical, chemical, or systemic movements, as a result of a stimulus from the nerve cells of the brain or spinal cord. 

– muscles made of smooth muscle, contracting slowly and slowly returning to their original length – they are managed by the autonomic nervous system 

– cardiac muscle, which consists of striated muscle fibers, independent of conscious nerve stimuli 

– muscles made of striated muscle fibers, rapidly shrinking under the influence of central nervous system stimuli – subject to our will. 

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The subject of the science of muscles (myology) is the last group of muscles that cause skeletal movements or determine the position of only bones in relation to others. For this reason, these types of cherry make up the system of active organs, in contrast to the system of passive motion organs, which is the skeleton. With the muscles of the first two groups and the viscera with transversely striated viscera. 

The skin muscles (musculi cutanei) are located directly under the skin and attach to it with one or both of their ends. Skin muscles strongly developed in many animals, in man are in a residual condition and survived only on the head and face, on the neck and on the palm of the hand. 

Skeletal muscles (musculi skeleti) are located under superficial fascia. In the vast majority of both ends, they are attached to the skeleton, where their name comes from. Only a small number of striated muscles connects to the organs of the senses, such as the muscles of the eyeball and the ossicles, or is in close relation to some of the viscera, such as, for example, tongue, throat, larynx or rectum. 

FT fibers are white and shrink faster. This is associated with ATP activity – the myofibrillary phase. 

ST-type fibers are red, they are distinguished by a denser network of capillaries surrounding them, a greater number of mitochondria and a higher content of myoglobin, triglycerides and also oxygen-based energy transformation enzymes. The content of enzymes associated with the glycolytic cycle is higher in FT type fibers. 

The described differences indicate that ST-type fibers are capable of using aerobic processes. They have a larger so-called oxidation potential than FT type fibers. They are also better provided with oxygen due to better vascularity and higher content of myoglobin, which has the ability to bind oxygen. Metabolism of FT muscle fibers is characterized by a greater severity of anaerobic (glycolytic) processes. 

The effect of metabolic differences are differences in the speed of development of fatigue and the nature of work between types of fibers. ST fibers are able to work longer than FT fibers. 

When it comes to stimulating muscle fibers, ST fibers are stimulated by larger motoneurons and a lower threshold of excitability, while FT fibers by smaller moto-neurons with a high threshold of excitability. 

Long muscles are found mainly on the limbs, where they usually form a few layers. 

In broad muscles, the dimensions of length and width are much larger than the thickness, they are usually flattened and thin. Most of them participate in the production of the walls of the great cavities of the chest, abdomen and pelvis. 

Short muscles occur in the area where the movements are slight, but require a lot of strength, we see them, e.g. around the spine or around some joints 

Finally, the fourth group would be mixed muscles, which can not be included in any of the above basic groups. An example is the abdominal muscle, which is also a long and wide muscle. (The mixed group may also include circular muscles that form muscle rings around the body’s openings, the so-called Spars, such as, for example, the ocular muscle of the eye or the sphincter.) 

Muscles are attached with their ends to the surface, which are the points of the muscle attachment. The surfaces of the trailer are varied. Some muscles attach to the inner surface of the skin, these are skin muscles. Others like, for example, tongue and lip muscles, attach to the mucous membrane. Some muscles connect to the fascia or the joint capsule. 

However, the vast majority of striated muscles are attached with both ends to the two parts of the skeleton, which the muscle usually brings together during contraction. Each muscle has at least two points of the attachment, one of which is called the beginning (orgio) or muscle starter, the other – the trailer (insertio) or the end / muscle trailer of the muscle. The beginning of the muscle can be called the point of attachment more permanent (punctum fixum), and the end point is the more mobile point (punctum mobile). However, in the changed conditions, both points may change. A fixed point becomes then a moving point and vice versa. 

The muscle is attached to the attachment site either directly or via the tendon. In the first case, muscle fibers run to the surface of the attachment and end up anchoring to it. In the case of the second connective tissue sheath that surrounds the muscle fibers, the tendon extends to the tendon that extends the muscle up to its point of attachment. The tendon / tenon is a significant part of the muscle, connects it to the skeleton and transfers its work to the skeleton. The shape of the tendons is very different, some are cylindrical, other flattened. Some occur in the form of a wide flat membrane called aponeurosis. In this embodiment, e.g., tendons of both oblique muscles and abdominal transverse muscle exist. In general, the shape of the tendon is adapted to a certain extent to the shape of the muscle. 

The mutual ratio of muscle strands and tendons is very different, it is very important for muscle work. In some muscles, the direction of the tendon fibers is a straightforward extension of the muscle fiber direction. Such a system is usually found in flat muscles (mm.plani), e.g. broad abdominal muscles, or in intercostal muscles. A similar ratio of muscle to tendon is also found in spindle muscles (mm.fusiformes) in which the fibers run parallel. And here the direction of the tendon fibers is an extension of the direction of the muscle fibers. The long but relatively few fibers of such a muscle can perform extensive but not very powerful movement. The degree of shortening of the muscle depends on the length of its fibers, and the strength of the muscle – on the number of fibers. 

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A completely different arrangement is in the muscles in which the fibers reach obliquely to the tendon. In the piedmont muscle (m.unipennatus) the muscle fibers after a short course come to the tendon on one side, in the feathered muscle (m.bipennatus) they connect with the tendon on both sides. Both of these types of muscles have numerous but short fibers and can perform not very extensive but strong movements. 

Some muscles in addition to the initial tendon and end tendon may contain in their middle part the tendon intermedium (tendo intermedium), which divides the muscle into two parts, i.e. into two bivrosis (m.dwubozmścowy, m.digastricus). Other muscles can be divided completely or partially by short tendons, tendonous tendons (intersectiones tendineae). Such muscle arises from segmental parts, becoming a functionally higher product. 

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