Muscles are characterized by their shapes, insertions and relationships. These characteristics are sometimes subject to variation.

Shape

Muscles come in a variety of shapes and forms. Considered according to the respective ratio of their three dimensions, they are divided, like bones, into three classes: long muscles, large muscles and short muscles.

Long muscles

Long muscles meet at the limbs, where they usually form several layers. - The superficial muscles are always the longest and usually pass in front of several joints: such are the biceps at the arm, the semitendinosus at the thigh, which fit, on the one hand, on the belt, and will attach, on the other hand, on the forearm and the leg. - The deep muscles, which are shorter than the previous ones, are inserted by their two extremities on the two neighbouring skeletal segments and therefore pass in front of a single joint: these are the anterior brachialis which goes from the humerus to the ulna, the popliteus which extends from the femur to the tibia, etc., etc., etc.

Large muscles

Large muscles are those in which two diameters predominate: length and width. They are usually flattened and very thin.

Most of the large muscles occupy the walls of the large thoracic, abdominal and pelvic cavities, which they help to form. These muscles are, moreover, very variable in their external configuration: some are triangular (large pectoral, small pectoral); others are quadrilateral, ribbed like the rectus abdominis, rhomboid like the rhomboid, etc. The large muscles are usually flattened and very thin. These are flat, like the trapezius and the great pectoral; these are curved so as to be concave on one side, convex on the other, thus forming segments of hollow cylinders: the large muscles of the abdomen (great oblique, small oblique and transverse) belong to the latter group. As for their edges, they are rectilinear, curved or serrated; there is even a group of muscles that take their name from this last character, the large serrated and the small serrated, so called because they are inserted on the ribs by a series of digitations, which together form regular serrations.

There are still large muscles at the head (frontal and occipital), at the neck (skinner), around the digestive tract (buccinator, pharyngeal constrictors). Finally, among the large muscles, the diaphragm, a vast muscular partition between the thorax and the abdomen.

Short muscles

Short muscles are found around the joints, around the spine and especially in the spinal gutters, everywhere in a word where movements have little range of motion and require a lot of strength. As for their shape, they are just as variable as the large muscles: here triangular (supercostals), there quadrilateral (crural square), farther ribbed (intertransverse), etc., etc. To the group of short muscles, the so-called semi-orbicular muscles should be attached. These semi-orbicular muscles are arranged on the periphery of an orifice, the oral orifice or the anal orifice for example, in the form of a semicircle or a half-ring. Each half-orbicular muscle is designed to occupy only one half of the orifice in question. However, by joining at its two ends with the semi-orbicular on the opposite side, it forms with the latter a complete ring, whose contraction has the effect of closing the orifice, hence the name sphincter (from the Greek word serrer) which has been given, in anatomy as well as in physiology, to these annular or orbicular muscles (orbicular of the eyelids, orbicular of the lips).

Mixed muscles

The boundaries between the three above-mentioned classes are not always clear-cut and we must recognise that there are muscles that have mixed characteristics and, as a result, are very difficult to classify. Such are the subhyoid muscles, which are both long and wide; the motor muscles of the eye, which are both short and yet elongated; the rectus abdominis and the pyramidal abdomen, which are both wide and long at the same time, etc., etc.

Muscle Insertions

Free in their middle part, the muscles attach themselves by their extremities to surfaces that are called their attachment points, or insertion points.

Variability of muscle insertion surfaces

Insertion surfaces are extremely variable. There are muscles that attach themselves to the deep face of the skin, these are the skincier s muscles. Others, such as the muscles of the tongue, like the muscles of the lips, attach themselves to the deep face of the mucous membranes. A certain number, inserted on the one hand on bony surfaces, are inserted on the other hand on an organ that they are intended to move: these are the eye muscles, which are carried from the bottom of the orbit to the eyeball. Some muscles still end, either on aponeuroses or on synovials, and are called tensor muscles of these aponeuroses and synovials. Of these are: the tensor of the fascia lata and the tensor of the knee synovium, one of which occupies the external face and the other the anterior face of the thigh.

Fixed point and mobile point

But most of the muscles of the relationship life fit with their two ends on two pieces of the skeleton, which they bring closer together when they enter into contraction. Each muscle has at least two points of insertion: of these two points, one is called the mobile point, the other the fixed point. These two expressions are self-explanatory: when the muscle is shortened by the contraction, the mobile point starts to move and approaches the fixed point, which does not move. Thus, in the flexion of the forearm on the arm, the biceps which determines this movement has as fixed point the scapula and as mobile point the bicipital tuberosity of the radius. It is worth noting that, for the same muscle, each of its two insertion points can, depending on the case, play the role of a fixed point: the geniohyoid, for example, which extends from the hyoid bone to the geniohyoid processes of the lower jaw, will lower the jaw if the hyoid bone is previously fixed by the subhyoid muscles, or will raise the hyoid bone, if the jaw is previously immobilized by the contraction of its elevating muscles. On the other hand, there are muscles that are attached by both ends to two mobile points, which walk towards each other whenever the muscle that unites them enters into contraction: as an example of the latter mode of insertion, we will recall the lateral fibres of the diaphragm, which are attached by one of their ends to the phrenic centre (mobile point) and by the other end to the ribs (also mobile point).

Modes of muscle insertion, tendons

The muscle is inserted in two different ways: directly or through a tendon.

1. In the first case, the muscle fibre continues to its insertion surface and ends up attaching to it.
2. In the second case, the connective tissue surrounding the fleshy fibres of the muscular body is attached to a tendon, which extends the muscle to its point of attachment. Tendons thus become an important, if not essential, part of the muscle of the relationship life. Their shape is highly variable: some are cylindrical, others are flattened; some, like the tendon of the small plantar foot, are remarkable for their length; others, on the contrary, are very short. Finally, there are some which spread out in the form of large membranes and which have been improperly called aponeuroses, insertion aponeuroses: such are the large tendons which extend to the white line the two oblique muscles and the transverse muscle of the abdomen. However, whatever the shape of the tendons, their nature is always the same: they are fibrous formations, whitish in colour, very resistant and almost inextensible, which means that the muscle, when contracting, acts without delay and without loss of force on the lever it is intended to set in motion.

Original insertion and terminal insertion

A distinction is usually made between the two insertions of the muscle in the original insertion (Ursprung of the German anatomists) and terminal insertion (Ansatz of the German anatomists). Thus the anterior brachialis has its original insertion on the humerus and its terminal insertion on the ulna. They can still be distinguished between superior and inferior insertion, proximal and distal insertion.

The muscle sometimes originates from two or more muscle bodies, each with its own tendon. It is called biceps, triceps or quadriceps, depending on whether it has two, three or four original heads: these are the biceps and triceps brachialis, biceps and quadriceps crural .

The terminal insertion can be done in the same way by two, three or even more tendons (W. Gruber's bicaudal or tricaudal muscle): the common flexors of the fingers, for example, each end in four tendons, intended for the last four fingers.

The mode of continuity of the tendon with the muscular body, as well as its mode of attachment to the bone, will be indicated later.

Macroscopic relationships of muscles with their tendons. Texture of the muscles.
Considered in their reciprocal relations or, if you like, in their arrangement, the muscle bundles and tendons present a host of varieties, which can be reduced to two main types: either the muscle bundles and their tendon continue end to end, presenting exactly the same direction (end to end insertion); or the first implant themselves laterally on the second, forming a more or less open angle with it (lateral insertion).

Butt to butt insertion

End-to-end insertion (A) of the muscle bundle over the tendon bundle is relatively rare. It is usually only observed for large muscles, whose insertions are linear and are made over a large area: examples include the large muscles of the abdomen, the intercostals, the thyrohyoid, the clavicular bundle of the sternocleido-mastoid, etc.

Lateral insertion

In lateral insertion, the muscle bundles implant obliquely on their tendon like the barbs of a feather on their common stem. The muscle is said to be penniform (feather, penna, feather-shaped), when the muscle bundles are inserted on both sides of the tendon (B); it is said to be semi-peniform, when the muscle bundles implant on one side only, the other side remaining free (D and E). As a variety of the semi-penniform muscle, we will report those muscles whose bundles end on a flattened tendon and more or less rolled up in the shape of a horn or half horn (C).

 

Various ways of arranging the muscle fibres on their tendons : A, broad muscle (the tendon bundles have the same direction as the muscle fibres); B, penniform muscle (the muscle bundles fall obliquely on both sides of the tendon); G, penniform muscle at the top, ending at the bottom on a horn-shaped tendon: D and E, two types of semi-penniform muscle.

1, muscular body. — 2, upper or proximal insertion. — 3, inferior or distal insertion. — 4, original tendon. — 5, terminal tendon.

As each muscle has two extremities and almost always two tendons, one original tendon, the other terminal tendon; as, on the other hand, each of these two tendons can have, in relation to the muscular bundles, a particular disposition, one immediately sees the innumerable varieties which the striated muscles present in their constitution: such a muscle which, at its initial end, continues end to end with its original tendon, terminates at its opposite end in a lateral insertion; such another, whose bundles are inserted at the top on the periphery of a central tendon (C), terminates at the bottom inside a tendon shaped like a horn, etc.; such a muscle, whose bundles are inserted at the top on the periphery of a central tendon (C), terminates at the bottom inside a tendon shaped like a horn, etc.; such a muscle, whose bundles are inserted at the top on the periphery of a central tendon (C), terminates at the bottom inside a tendon shaped like a horn, etc.; such a muscle, whose bundles are inserted at the top on the periphery of a central tendon (C), terminates at the bottom inside a tendon shaped like a horn. , and so on A very common arrangement is the semi-penniform arrangement at either end of the muscle. In this case, the two tendons are generally wide and flattened and, on the other hand, they are oriented in opposite directions, i.e. if one of them, the original tendon for example, is located on the superficial side of the muscular body, the other, the terminal tendon, spreads out on its deep side. The fleshy bundles go obliquely from one to the other and it should be noted that these fleshy bundles, for two muscular bodies of equal length, are the shorter and the more numerous (see the two muscles D and E in comparison) the more extended their tendon blades are.

Such an arrangement has an enormous influence on the mechanical action of the muscles. Physiology, as we know, teaches us, on the one hand, that the degree of shortening of a muscle, at the time of contraction, depends on the length of its fibers, and, on the other hand, that its energy is related to the number of fibers which constitute it. With this in mind, let us look at the two muscles D and E (Fig. 723). These two muscles have exactly the same length, the same width, the same thickness, and therefore the same volume. However, in muscle D, the fibres are three times longer and three times less numerous than in muscle E. As a result, under the influence of contraction, the second muscle will shorten three times less than the first, but it will develop three times more energy, i.e. lift a triple weight. From this it may be concluded that, in assessing the mechanical action of a given muscle, account must be taken not only of its volume, but also of the relationship between its fibres and the tendons on which they originate or end.

Intermediate tendons

In addition to their original tendon and their terminal tendon, some muscles have a third tendon, called intermediate tendon, in their middle part.

This intermediate tendon naturally divides the muscular body into two portions or bellies and, as a result, the muscle thus configured takes the name digastric muscle (from the Greek two and belly). Examples of digastric muscle include the digastric neck and omohyoid.

The meaning of the intermediate tendon in the digastric muscles is not always easy to identify. For some of them at least, notably the omohyoid, this intermediate tendon must be considered as representing, in the same way as the aponeurotic intersections of the rectus abdominis, vestiges of the metamerization of the body.

Muscle relations

Muscles have the most diverse ratios. These ratios vary, moreover, for each of them and, therefore, hardly lend themselves to general considerations. We will simply indicate here that the muscles are more or less intimately related:

With the bones

With the bones (sometimes cartilage), on which they are inserted and which they more or less envelop, separating them from the external integument;

With the joints

With the joints, which they cover, cross, bypass or even penetrate (tendon of the long portion of the biceps penetrating the shoulder joint) ;

With the fasciae

With the fasciae, which envelop them in all their extent or only on a part of their surface;

With the vessels...

With the vessels, which are attached to them and sometimes even pass through them: such is the great adductor of the thigh, perforated by the femoral vessels. The arteries, which are a little voluminous, are always connected to several muscles; but, among these muscles, there is one, generally, which follows the path of the vessel more exactly than all the others: in anatomy, and more particularly in operative medicine, it is given the name of satellite muscle of the artery. Thus, the biceps brachialis is the satellite muscle of the humeral artery, the dressmaker is the satellite muscle of the femoral artery, the sternocleidomastoid is the satellite muscle of the primitive carotid, etc.

With the nerves

With the nerves, which, like the vessels, are intimately attached to them and sometimes even cross them: such is the sternocleidomastoid, which is pierced by the spinal nerve; such is also the coraco-brachial, which is crossed from side to side by the musculocutaneous nerve of the brachial plexus. Each muscle receives one or more nerves, which provide it with voluntary motor stimuli or reflexes. The presence of multiple nerves for a single muscle can be explained, depending on the case, by one or other of these two facts: either the muscle in question derives from several metamers (as is the case for the rectus abdominis, which the aponeurotic intersections divide into several segments, each of these segments having its own innervation); or it is a complex muscle resulting from the fusion, during phylogenetic development, of two or three primitively distinct muscles. As an example of the latter process, I will recall our anterior leg, which, despite its apparent unity, actually represents two perfectly distinct muscles in lower apes, the anterior leg proper and the long abductor of the big toe (see Myology); this muscle, as an indication of its primitive duplicity, still shows us multiple innervation and double insertion of its lower tendon.

With the muscles
With the muscles, well, I mean the muscles of the neighbourhood, which are applied against their faces or against their edges. Between two contiguous muscles there is an aponeurosis or at least a layer of cells, which isolates them from each other and ensures their mutual independence both anatomically and physiologically.

Muscular abnormalities

All the anatomists who have dealt with the anatomical variations of the muscular system in man, Wood, Macalister, Turner, Gruber, Humphry, Testut, Le Double, etc., agree that these variations are very frequent. We consider, for our part, the muscles of man as being just as variable as his vessels, much more variable than his bones and nerves. To be convinced of this, it is enough to know these variations and to look for them.

Classification

The muscular anomalies seem to us to be divided, from a purely morphological point of view, into two large groups: 1° the supernumerary muscles; 2° the modifications brought about by the anomaly to the muscles which normally exist in the constitution of the human body.

1. Supernumerary muscles are entirely new formations that appear in any region at a more or less advanced stage of development. These are the presternal at the anterior aspect of the thorax, the cleido-tracheal at the neck region, the epitrochleo-olecranon at the elbow region, the radiocarpal at the wrist, the coccy-femoral at the gluteal region, etc.
2. The changes brought about by the abnormality to the ordinary muscles are related to their shape, constitution, insertions, relationships, with neighbouring muscles. These abnormalities are divided into four groups: 1° shape abnormalities; 2° constitution abnormalities; 3° insertion abnormalities; 4° relationship abnormalities.

Each of these groups presents, in turn, a number of varieties that we summarize in the following table:

Table showing abnormalities of the human muscular system

				Examples :
I. Supernumerary muscles.				Cleidotracheal. Epitrochleocubital.
Ordinary muscles, modified :	A) In their shape.			Small fusiform palmaris
	B) In their constitution	1° Duplication.	a) complete	Sternocleidomastoid, divided into two distinct clusters.
			b) partial	Tibialis anterior muscle sending a distinct beam to the first metatarsal..
		2° Fusion.	a) complete	Single-beam sternocleidomastoid muscle.
			b) partial	The two portions of the biceps joined together higher than usual.
		3° Appearance of new beams		Third head of the biceps muscle
		4° Disappearance of some beams		Biceps reduced to its coracoid portion.
		5° Fibrous muscle transformation		Peroneus tertius muscle transformed into a tendon
		6° Muscle disappearance		Absence of the small palmar
		7° Aponeurotic Intersections	a) added	On the sterno-cléido-mastoid mucle.
			b) removed	Disappearance of the intermediate digastric tendon
		8° Reversal of the muscle		Palmaris longus muscle upside down
	C) In their relations with surrounding muscles.	1° Normal isolation		The inner beam of the extensor digitorum brevis muscle
		2° Abnormal fusion	a) complete	Fusion of the Extensor carpi radialis muscle and the Extensor carpi radialis brevis muscle.
			b) partial	Same muscles.
			c) by anastomosis	Superficial flexor and deep flexor of fingers.
	D) In their insertions	1° Added.		Beam of the Sterno-maxillary muscle of the sterno-cléido-mastoid muscle
		2° Displaced.		Digastric muscle inserted at the jaw angle
		3° Decreased in extent.		Posterior Scalene from the 1st rib only.
		4° Increased in extent.		Pectoralis minor from 4 or 5 ribs.
		5° Deleted.		Muscle lost in cell tissue.

 

Morphological significance

Abnormalities of the muscular system, long neglected, have today taken their rightful place in human anatomy. Several foreign anatomists, including Henle and Quain, have given much attention to muscle abnormalities in their treatises on anatomy. We will follow their example and, in the different chapters of this book, we will add, following the classical description of a muscle, a note in small text indicating its main variations. We will always try to make this note as succinct as possible, but it seems to us indispensable. Muscular abnormalities, in fact, are not simple products of chance, games of nature, as was believed for a long time; they accidentally reproduce in man, most of them at least, anatomical dispositions that are constant and typical in the animal series, and they thus acquire, in anthropological anatomy, considerable importance (see, on this subject, Testut, Les anomalies musculaires chez l'homme expliquées par l'anatomie comparée, leur importance en anthropologie, un vol. gr. in-8, Paris, 1884).