Muscles of the Pelvis.—The muscles of the pelvis visible on the model are all situated on the posterior surface of that part of the skeleton, and form the buttock, or gluteal region. In front the anterior wall of the abdomen, descending to Poupart’s ligament and the pubis (Fig. 70, page 197), conceals the muscles which proceed from the interior of the pelvis towards the thigh—muscles of which a short sketch will be given along with those of the front of the thigh.

Of the muscles of the gluteal region two only are superficial and well marked on the model—viz., the gluteus maximus and gluteus medius.

The gluteus maximus muscle (Fig. 72, page 201) is the largest and thickest of all the muscles of the body. It is composed of large fleshy fibres directed obliquely from the sacro-iliac region towards the upper part of the femur. Its fibres arise from the posterior extremity of the crest of the ilium (4, Fig. 34, page 107), by an aponeurosis from the back of the sacrum, from the sacrum itself, and from the sacro-sciatic ligament. The coarse fleshy fibres are directed downwards and outwards through the gluteal region (Fig. 67). At the level of the great trochanter these fibres are for the most part inserted into a broad, thick, tendinous lamina, which is continuous with the aponeurosis of the thigh or fascia lata. The lower and deeper part of the muscle is inserted directly into the gluteal ridge of the femur (page 133). The gluteus maximus muscle presents four borders: (1) an internal border, corresponding to its origin, and convex inwards, and (2) an external border, also slightly convex at its insertion. This border corresponds to the line along which the fleshy fibres terminate; it forms a prominence, which curves posteriorly round the great trochanter. When we have enumerated the muscles lying beneath the gluteus maximus, it will be more easy to understand how the muscular structure of the gluteal region as a whole is so prominent that the great trochanter actually forms on the model the centre of a depressed area, bounded behind and above by the prominence of the gluteal muscles, and in front by the tensor muscle of the fascia lata (see Muscles of the thigh). (3) The inferior border of the gluteus maximus is thick, and forms an oblique elevation, beneath which the posterior muscles of the thigh emerge; it is this which helps to form the lower limit of the prominence of the buttock. In the erect position the lower limit of the gluteal region is indicated by a transverse fold (the fold of the nates). This line runs diagonally across the lower border of the gluteus maximus, and is produced not by the edge, but by the weight of the relaxed muscle, and the fat which covers it. (4) The superior border is thin (Fig. 74, page 217), and is continuous with the fascia lata, which covers the gluteus medius, so that its prominence is little marked on the model, being more or less lost in the area corresponding to the position of the gluteus medius muscle.

The gluteus maximus is an extensor of the thigh on the pelvis. In the upright position it supports the pelvis behind and prevents it from inclining forwards. It is by its action that the trunk is raised so as to be in the same line as the upright lower limb. The gluteus maximus is therefore the muscle of the upright position, and we observe the large size which it presents in the human subject compared with its small proportions in animals which do not adopt the biped attitude.

The gluteus medius muscle is situated both above and beneath that of the gluteus maximus—that is to say, the postero-inferior part is covered by the preceding muscle, but its antero-superior part is superficially placed. This latter part (Fig. 74, between 22 and 23) is, however, covered by the thick fascia lata of the thigh, which binds down the muscle to the dorsum ilii. Arising from the anterior three-fourths of the crest of the ilium, and from the dorsum ilii as well as from the fascia over it, the fibres of the gluteus medius descend, converging towards the great trochanter, into the external surface of which they are inserted by a thick aponeurotic tendon. The fleshy fibres cease a little above the great trochanter so as to form a prominence along a curved line with its concavity downwards, which forms the superior boundary of the depression corresponding to the region of the great trochanter, referred to above. The gluteus medius, by its posterior fibres, acts like the gluteus maximus; by its anterior fibres it draws the thigh outwards, acting as an abductor.

The gluteus maximus and gluteus medius muscles conceal a series of deep muscles filling up the large space which we observe on the skeleton between the great trochanter and the pelvic wall. These muscles, which we only need to enumerate in order to understand the importance of the prominence of the buttock, are, taking them in order, as follows—the gluteus minimus, which lies exactly beneath the gluteus medius, and proceeds from the dorsum ilii to the superior and anterior border of the great trochanter; the pyriformis, which arises within the pelvis from the anterior surface of the sacrum, escapes from the pelvis through the great sciatic notch, and is directed obliquely towards the great trochanter, into the upper border of which its tendon is inserted; the obturator internus, which proceeds also from the interior of the pelvis, is reflected over the lesser sciatic notch and is inserted into the internal surface of the great trochanter along with two accessory muscles, the gemelli; and finally, the quadratus femoris, formed by short horizontal fibres, which extends from the outer part of the tuberosity of the ischium to the posterior surface of the femur between the two trochanters.

Muscles of the thigh.—The muscles of the thigh are arranged around the femur, and frequently in a direction so oblique that they appear in one portion, for example, on the front, and in another portion on the inner side of the thigh. We can, however, class them in four series—external, comprising the tensor vaginæ femoris (fasciæ latæ); anterior, comprising the sartorius and quadriceps extensor; internal, comprising the adductors; and posterior, comprising the biceps, the semi-membranosus, and the semi-tendinosus.

I. Outer side of the thigh. The tensor vaginæ femoris muscle (3, Fig. 81).—This muscle lies in the plane of the gluteus maximus (23, Fig. 74, page 217), and forms a distinct prominence in front of the great trochanter. It arises from the anterior superior spine of the ilium, and is directed downwards and slightly backwards on the outer side of the thigh, to terminate a short way below the level of the great trochanter by being inserted into the broad and thick aponeurosis—the fascia lata—which covers this region (4, Fig. 81). This aponeurosis is specially thickened by the addition of vertical fibres, which form a tendon of insertion known as the ilio-tibial band. This band descends to the outer side of the knee as a distinct prominent tendon, which is inserted finally into the outer tuberosity of the tibia (see page 135). This muscle rotates outwards the thigh and the whole lower limb, and contributes also to the flexion of the thigh on the pelvis; therefore, when the thigh is extended and not turned inwards, the tensor muscle forms beneath the iliac spine a long muscular elevation, but when it is in action its form becomes short and broad, and forms a characteristic globular mass. This contrast in the form of the tensor muscle in repose and in action has been beautifully shown on the Gladiator, in which there is contraction of the tensor of the right thigh and relaxation of that of the left.

The ilio-tibial band and the aponeurosis of the fascia lata cover, on the outer side of the thigh, the large fleshy mass of the vastus externus (a part of the quadriceps extensor muscle), which belongs to the anterior region of the thigh. This muscle, thus covered in, is shown throughout its entire extent on the surface. Though covered by the fascia lata, it forms a marked convexity on the outer side of the thigh, as a gentle curve in women, and a strongly bowed line in well-developed, muscular men.

II. Front of the Thigh. The sartorius muscle (5, Figs. 81, and 23, Fig. 70, page 197).—This is the longest muscle in the human body. It forms a narrow fleshy band, which arises from the anterior superior iliac spine, and is directed obliquely downwards and inwards. Crossing the front of the thigh obliquely, it descends round the inner side of the knee over the inner condyle of the femur (Fig. 86), describing a curve with the concavity forward. It finally terminates at the upper part of the inner surface of the shaft of the tibia in a flat tendon (19 and 20, Fig. 86) which sweeps forward to be inserted into that bone just below the inner tuberosity in front of and along with the gracilis and semi-tendinosus muscles.

The sartorius flexes and abducts the thigh on the pelvis, and flexes the leg on the thigh; so that it gives to the lower limb a position similar to that of a tailor when seated, hence the name of this muscle (sartor, a tailor). With regard to surface form, this muscle is indicated externally in a peculiar manner. When it contracts, only its superior extremity presents a superficial prominence; throughout the rest of its extent, the presence of the muscle, lying in the depressed groove between the quadriceps extensor in front and the adductor muscles on the inner side, is indicated during contraction by a large shallow furrow, particularly noticeable on the inner side of the thigh, at the junction of the upper two-thirds with the lower third.

Quadriceps extensor (6, 7, 8, Fig. 81).—The quadriceps extensor muscle belongs to the outer and inner regions of the thigh as well as to the front of the limb, but the element of the muscle of most importance in relation to external form—the rectus femoris—is situated anteriorly. The muscle, as its name implies, is composed of four portions: one, the rectus, in the middle; the vastus internus and the vastus externus, on each side; and the crureus, the deepest part, concealed by the other portions of the muscle.

The rectus femoris (21, Fig. 70, and 6, Fig. 81) is long and fusiform in shape—that is to say, larger at its centre than at its extremities. Its superior extremity, the more slender of the two, arises by a double tendon from the anterior inferior iliac spine and the dorsum ilii, and appears on the front of the thigh between the tensor fasciæ femoris and the sartorius muscles. Lying in the angular space which separates these two muscles (Fig. 81), the rectus descends vertically on the front of the thigh, and about four inches above the patella forms a broad tendon (Figs. 81 and 84), the borders of which give insertion on either side to the vastus internus and vastus externus muscles, while its deep surface gives insertion to the crureus muscle, and its base broadens out to be inserted into the patella. From the lower end of the patella a broad ligament—the ligament of the patella—arises (page 134), which is inserted into the tubercle of the tibia; it follows, therefore, that by means of the tendon of the rectus femoris, the patella, and the ligament of the patella, the quadriceps extensor muscle is eventually inserted into the tibia (Fig. 45, page 137).

The vastus internus muscle (8, Fig. 81) is a very large fleshy mass, which covers the inner side of the femur. Arising from the inner lip of the linea aspera of the femur, from the edge of the bone (spiral line) above, and from the internal supra-condyloid ridge below, its muscular fibres are directed downwards and forwards, to be inserted into the deep surface and the inner border of the tendon of the rectus femoris, into the side of the patella, and into the capsule of the knee-joint. It conceals the crureus and partly blends with that muscle. The outline of the vastus internus is easily discernible on the surface, forming a somewhat triangular, convex, and rounded contour; bounded internally by the furrow of the sartorius, externally by a vertical line where it joins the tendon of the rectus femoris, and below by its lower border, it forms a well-marked rounded line sweeping outwards over the internal condyle of the femur to the side of the patella. These details are of great importance in regard to the contour of the region above the patella, and are in marked contrast, as we shall see, with the arrangement presented at the same level by the lower part of the vastus externus.

The vastus externus (7, Fig. 81) lies external to the rectus femoris, and conceals the crureus, with which its deep surface is partially blended. Its origin, which underlies the tensor fasciæ femoris and the ilio-tibial band, is from the upper half of the shaft of the femur, from the outer lip of the linea aspera. Its fibres sweep downwards and slightly forwards to be inserted into the tendon of the rectus, the patella, and the capsule of the knee-joint. The line along which it is inserted into the rectus tendon describes a curve of which the convexity looks towards the supero-external angle of the patella, from which it is separated by a considerable interval (Figs. 81 and 85). Consequently on the surface the flat surface above the patella forms a sort of triangle, of which the borders are very different, the inner border being vertical, the outer one oblique, curved, and higher in position; the base of the triangle corresponds to the patella and the lateral parts of the capsule of the knee-joint; its truncated summit corresponds to the inferior extremity of the fleshy part of the rectus; and the borders of this flat surface, formed by the fleshy insertions of the vasti muscles, become very prominent when the leg is forcibly straightened on the thigh.

The crureus muscle, deeply placed, is the fourth element in the quadriceps muscle. It arises from the upper three-fourths of the shaft of the femur, externally and in front, and forms a thick, fleshy mass, responsible to a great extent for the general contour of the front and sides of the thigh, though it is not actually noticeable directly on the surface. It is almost wholly concealed by the three previous muscles. It is partially blended on each side with the vasti muscles; and it is inserted as well into the deep surface of the tendon of the rectus femoris. It is hardly necessary to point out, since it is clear from its anatomical arrangement, that the quadriceps muscle, passing by means of the patella and the patellar ligament to the tubercle of the tibia, is essentially the extensor muscle of the leg.

It is to be noted that the two vasti muscles have a very different effect in the shaping of the contours of the thigh. The vastus externus has its greatest prominence well above the patella on the outer side: the most prominent part of the vastus internus is formed by its lowest fibres, as they sweep across over the inner condyle of the femur.

III. The inner side of the thigh. The adductor muscles.—We understand by the adductors the numerous muscles that occupy the inner part of the thigh, and which, passing down from the pubis and ischium along the entire length of the femur, fill up the triangular space which appears in the skeleton between the inner surface of the femur and the pubic part of the hip-bone. Some of the muscles of this series take the special name of adductors. We will study in succession three muscles clearly visible on the model—viz. the pectineus, the adductor longus, and the gracilis; afterwards we will mention briefly the muscles almost hidden by the preceding—namely, the adductor brevis and the adductor magnus muscles.

The pectineus muscle (22, Fig. 70, and 11, Fig. 81), the first and shortest of the muscles of this region, is a broad fleshy band which extends from the horizontal ramus of the pubis to the upper part of the shaft of the femur (to a rough line passing from the linea aspera to the lesser trochanter). The lower part of this muscle is hidden by the sartorius, and the upper part does not show itself clearly beneath the skin, being covered by the large vessels of the thigh, and also in most cases by a quantity of fat. The pectineus forms the middle part of the floor of a triangular space in the upper third of the thigh, with its apex below, known in surgical anatomy as Scarpa’s triangle, which is bounded externally by the sartorius muscle. The base of the triangle corresponds to the groin, and is formed by Poupart’s ligament. The outer part of the floor of the triangle is formed by a large muscle, of which the greater part is situated in the cavity of the abdomen. This is the ilio-psoas muscle (1 and 2, Fig. 81), which arises from the lateral parts of the lumbar vertebræ (psoas) and from the iliac fossa (iliacus), passes beneath Poupart’s ligament (page 199), and after traversing Scarpa’s triangle is inserted into the lesser trochanter of the femur. This muscle is not visible superficially except just below and internal to the anterior superior iliac spine, where it forms a slight bulging beneath the skin.

The inner part of the floor of Scarpa’s triangle is formed by the adductor longus, triangular in shape (10, Fig. 81); arising by a narrow tendon from the angle of the pubis, it is inserted, beneath the sartorius, into the middle portion of the linea aspera of the femur.

The gracilis is visible throughout its entire length on the inner surface of the thigh (9, Fig. 81). It forms a long slender fleshy band, broader above and narrower below. Arising from the symphysis and descending ramus of the pubis, it descends vertically; a little above the inner condyle of the femur it is replaced by a narrow tendon (15, Fig. 82), which sweeps round the condyle, behind the sartorius, and in front of the semi-tendinosus, to be inserted into the upper part of the inner surface of the shaft of the tibia behind the sartorius and above the semi-tendinosus.

Covered by the preceding muscles and placed more deeply, are the adductors, brevis and magnus, which fill up the space between the gracilis and the femur. The adductor brevis extends from the pubis to the upper part of the linea aspera; the adductor magnus is a thick, fleshy, triangular muscle which arises from the tuberosity and ramus of the ischium, and is inserted into the entire length of the linea aspera of the femur, so that its superior fibres are horizontal and its inferior fibres almost vertical in direction; among these last the most internal, which arise from the ischial tuberosity, form below a distinct tendon, which projects above the inner side of the knee-joint and is inserted into a tubercle (the adductor tubercle of the femur) placed above the inner condyle of the femur.

The action of all the muscles which we have just studied, except the pectineus, is to draw the thigh inwards towards the axis of the body; they also bring the knees close to each other, hence they are called the adductors of the thigh.

IV. The back of the thigh. The posterior muscles of the thigh (hamstring muscles).—These muscles, three in number, arise from the tuberosity of the ischium, and thus their origins are hidden beneath the gluteus maximus. They emerge below the lower border of that muscle and descend vertically; above the posterior surface of the knee (or ham) they divide into two masses, one, the external, formed by a single muscle, the biceps femoris, the other internal, formed by two muscles placed one on the other, the semi-tendinosus and the semi-membranosus.

The biceps femoris (4, Fig. 83) is so called because, like the biceps in the arm, it is formed above by two heads, a long head which arises from the tuberosity of the ischium, and a short head, more deeply placed, which arises from the linea aspera of the femur. These two heads unite in a stout tendon (12, Fig. 82) which the muscular fibres accompany for the greater part of its length, and which is directed downwards on the outer side of the knee, to be inserted into the summit of the superior extremity of the head of the fibula. This muscle flexes the leg on the thigh, and when it accomplishes this action its tendon becomes very prominent, forming the outer boundary of the region of the ham or popliteal space.

The semi-tendinosus (13, Fig. 82; 6, Fig. 83), visible throughout its entire extent (except at its origin, which is hidden beneath the gluteus maximus), has been so called because in a great part of its length, almost equal to its lower half, it is represented only by its tendon. It arises above from the ischium, and descends parallel to the long head of the biceps, to the inner side of which it is situated; towards the junction of the middle with the lower third of the posterior surface of the thigh, its fleshy belly becomes narrower, and is replaced by a long, thin tendon (13, Fig. 82) which inclines inwards, and passes behind the internal condyle of the femur, describing a slight curve with its concavity forwards behind the tendons of the sartorius and gracilis. With them (pages 258 and 264) it is inserted into the upper part of the internal surface of the shaft of the tibia (24, Fig. 86). This muscle flexes the leg, and in this movement the prominence of its tendon starts out as the internal boundary of the region of the ham.

The semi-membranosus is broader than the semi-tendinosus (7, Fig. 83). It is situated beneath it and projects beyond it on both sides. It is so called because its upper half is formed by a broad membranous tendon arising from the tuberosity of the ischium. The muscular fibres commence below the middle of the thigh and form a large fleshy belly, thick, broad, and short, which passes into a strong tendon (14, Fig. 82) inserted mainly into the posterior surface of the internal tuberosity of the tibia.

The fleshy belly of this muscle projects beyond the tendon of the semi-tendinosus on each side, and reaches to the middle line of the posterior surface of the thigh and knee, forming a large muscular prominence. When the leg is flexed on the thigh, the tendons of the biceps and semi-tendinosus become prominent, bounding a deep pit (the ham or popliteal space) corresponding to the upper part of the posterior surface of the knee, and the fleshy part of the semi-membranosus remains hidden in the bottom of this pit; but when the leg is extended on the thigh there is no longer a hollow, but the posterior surface of the knee presents, on the contrary, a prominent form, produced in the upper part by the fleshy mass of the semi-membranosus, and in the lower part by the muscles of the calf, to be described in the next chapter.