The femur (Figs. 39 and 43), or bone of the thigh, is one of the long bones. It is the largest bone in the skeleton, and consists, like all the long bones, of a shaft and two extremities (Fig. 39). In this chapter we have to consider its upper end, and its articulation with the hip-bone.

The upper extremity of the femur is composed of a head, a neck, and of two tuberosities (the great and small trochanters) placed at the junction of the neck with the body of the bone. The head of the femur (5, Fig. 43) is regularly rounded, and forms three-fourths of a sphere. Its spherical surface, turned inwards, upwards, and forwards, is smooth and covered with cartilage, except at a pit (6, Fig. 43) seen a little below and behind its centre, which gives attachment to the ligamentum teres—a strong fibrous band which connects the femur to the acetabulum.

The neck of the femur (7, Fig. 43), connecting the head with the shaft, is directed downwards and outwards in the form of a cylinder, flattened a little from front to back, to become attached to the upper extremity of the shaft of the femur at an obtuse angle, looking downwards and inwards (Fig. 39). This angle which the axis of the neck makes with that of the body varies in different individuals. In the adult male it is about 135 degrees; in the female it is less open—that is, it approaches nearer to a right angle. This helps to increase the transverse diameter of the hips in the female. Again, in both sexes this angle approaches gradually to a right angle with the advance of age—a change that assists in shortening the total height in the aged. At the junction of the neck of the femur with the body of the bone are the two tuberosities, or trochanters—one situated above and to the outer side, called the great trochanter (3, Fig. 39; and 8, Fig. 43); the other situated below and to the inner side, at the junction of the neck and the shaft, called the lesser trochanter (10, Fig. 43). The great trochanter is of large size, is quadrilateral in shape, and projects beyond the upper border of the neck. It presents on its surfaces and borders numerous impressions, for the insertion of the muscles of the buttock. The lesser trochanter, on the contrary, is smaller in size, mammillated or conical in shape, and gives attachment to the psoas and iliacus muscles only.

The Hip-joint.—The ilio-femoral or coxo-femoral articulation is formed by the reception of the head of the femur into the cavity of the acetabulum, in which it exactly fits. The edge of the cavity is surrounded by the cotyloid and transverse ligaments, which clasp the head of the femur and serve to deepen the cavity and narrow its orifice. The articulation is a ball and socket joint (enarthrosis). As we should expect from the form of the articular surfaces, every possible description of movement can take place: the head of the femur can glide in all directions in the cavity in which it is received, producing the movement of abduction (the inferior limb being carried outwards, away from the middle line), adduction (towards the middle line), flexion (forwards, the anterior surface of the thigh being brought towards that of the abdomen), extension (backwards), and rotation, of the femur, outwards and inwards. But these movements are variously influenced by the disposition of the ligaments of the joint. Some are limited, others are very extensive.

The ligamentous apparatus of the hip-joint consists of a thick capsule of fibrous tissue, which arises from the margin of the cotyloid cavity and encloses the head and neck of the femur. The disposition of this capsule is very different in front and behind.

The capsule is composed of superficial longitudinal fibres stretching from acetabulum to femur, and thickened, except posteriorly, to form strong ligaments; and deeper, circular fibres, which alone constitute the back part of the capsule—consequently the capsule is thinner and weaker at the back, where the circular fibres appear superficially—constituting the zona circularis, or ligament of Bertini. The posterior portion of the capsule consequently cannot at any time become tense. The movement of flexion, indeed, may be continued as far as possible without causing tension of the back of the capsule of the joint. It is for this reason we speak of flexion of the thigh as unlimited, and, in fact, it may be continued until the anterior surface of the thigh comes in contact with the abdomen.

The other movements of the thigh at the hip-joint are limited by the several longitudinal ligamentous bands of the capsule. In front, the capsule of the joint is put on the stretch when the thigh is carried backwards during extension, and the movement is checked when it arrives at a certain point. This is due to the ilio-femoral, or Y-shaped ligament of Bigelow, which extends from the acetabular margin to the front of the neck of the femur, dividing as it descends into two bands. It is the most important ligament of the hip-joint. Owing to the strength of this ligament, the movement of extension can be limited and checked at any given moment. Now if we try the experiment on ourselves, having flexed the thigh on the abdomen and then gradually extend it, we shall perceive that the movement is arrested at the moment when the axis of the thigh is in a continuous line with that of the body (or if the subject is upright, when the thigh is vertical). If we repeat this experiment on the dissected subject, with the joint prepared, we see that the ilio-femoral ligament is relaxed when the thigh is flexed on the pelvis, and becomes tense in proportion as the bone is extended, and that this tension arrives at its maximum when the femur is found in a line with the trunk, and the ilio-femoral ligament presents an insurmountable obstacle to any further extension.

It is true that a man in the upright position can move his thigh backward; but it is necessary to observe, if, for example, it is the right thigh which is carried backward, it is not in the right hip-joint that the movement takes place, but in the left; in other words, the trunk is flexed upon the left femur. We may therefore state that when the thigh is so extended as to be in the same continuous plane with the trunk, the thigh and trunk form one and the same piece, and the two parts are incapable of further extension; and, consequently, when one thigh is carried behind the vertical line, the trunk must be rotated to an equal extent on the opposite side.

The ilio-femoral band plays also an important part in the maintenance of the erect attitude. Being stretched in extension of the thigh, the head and neck of the femur rest against it, and so the erect position can be maintained without excessive muscular exertion. The ilio-femoral ligament also, aided by the pubo-femoral band of the capsule, presents an obstacle to abduction, or the movement of the thigh outwards, in the upright position, when the ligament is tense. When the thigh is slightly flexed on the pelvis, and the ligament is relaxed abduction of the thigh becomes comparatively easy (Fig. 40).

The movement of adduction, or bringing of the thighs together, is limited by the ilio-trochanteric band of the capsule, and by the ligamentum teres—a special ligament, not part of the capsule, contained within the joint. This movement becomes very easy if the joint is slightly bent. If we try the experiment on an articulation in which the ilio-femoral ligament is cut, we perceive that in the position of extension the movement of adduction is just as difficult as if the ligament were intact. This is due to the presence of the ligamentum teres. This ligament is attached by one extremity to the rough depression below and behind the head of the femur, and by the other extremity, which is bifurcated, to the margins of the acetabular notch. We have already seen that in the pelvis of a man in the upright position this notch looks directly downwards (page 106); the ligament in this position is also vertical, and is put on the stretch, as the two ligaments, so to speak, help to suspend the pelvis on the heads of the femora. Now, in the upright position, the femur being vertical, the movement of adduction could be produced only by a gliding movement of the head of the femur in the acetabular cavity; but this gliding movement cannot take place, as the head of the femur is kept in position by the tense, round, or suspensory ligament. If, on the contrary, the femur is slightly flexed, the round ligament will be found relaxed; this permits the gliding of the head in the cavity, and at the same time permits of adduction, a movement which may now be accomplished with ease. An experiment which proves these facts without giving an anatomical demonstration of them (obtainable only from a prepared subject) is very easy to accomplish upon oneself. If one stands perfectly upright, rigid, with the body thrown backwards as far as possible, it will be seen that it is almost impossible to bring the two thighs and knees close together. Adduction is almost entirely lost in this position, so that we should be unable to bring the knees together and crush a fragile body, such as an egg, placed between them. But if, on the other hand, we flex the thighs a little, or the trunk is bent on the thighs, adduction becomes extremely easy, and now we can knock the knees together without any difficulty.

The hip-joint is remarkable for the fact that upon it we can most conveniently demonstrate a law which applies also to all the other articulations, but of which we have reserved the explanation until now—namely, the law relative to the influence of atmospheric pressure in maintaining articular surfaces in contact. Up to the present, in studying the articulations, we have spoken of the form of the articular surfaces, and from their outline we have been able to deduce the nature of the movements permitted in the joint; we have spoken of the ligaments which surround the joint, and from their arrangement we have been able to infer the limits imposed upon these movements. But we have not spoken of the conditions which enable one articular surface to glide upon another without separating, and which maintain the surfaces in intimate and permanent apposition. It might be thought that this function devolves on the ligaments, but this would be an error; it is atmospheric pressure which keeps up this contact.

The head of the femur is fitted very exactly into the hollow of the acetabulum. In the first place, the head of the femur is in close contact with the cavity, the non-articular portion of which is filled up by adipose tissue. In the second place, the cotyloid ligament closely encircles the base of the head of the femur, and may be compared to the edges of a cupping-glass. Now, as a vacuum exists between the two articular surfaces, and the air cannot penetrate between them, they adhere very closely to each other, merely allowing one to glide on the other; but if by any means air is allowed to obtain access to the interior of the joint the bones naturally fall apart. The experiments which explain those facts were first demonstrated by the brothers Weber.

We have felt it important to dwell here, once for all, on the important part which atmospheric pressure plays in the mechanism of joints. Analogous experiments show that this pressure plays comparatively the same part in other articulations in maintaining their articular surfaces in contact.

Measurements and Proportions of the Hips.—To return to the study of the region of the hips, we must now examine the transverse dimensions of this region, and the external forms which result from the presence of bony prominences, especially the great trochanter of the femur.

The transverse distance which separates the one great trochanter of the femur from the other should be compared with the distance between the heads of the humeri; in other words, we must compare the diameter of the hips with that of the shoulders.

What strikes us most in this comparison, at the first glance at a series of skeletons, is the great projection which the hips form in the female. In order to express this difference, various formulæ have been proposed. In one such formula the trunk is regarded as a figure more or less regularly oval, of which one extremity corresponds to the shoulders, the other to the hips, and in the two sexes the diameter is greater at one end than at the other. The ancients expressed the formula in the following manner:—In the male and in the female the trunk represents an ovoid—that is to say, an oval similar to that of a figure of an egg having a greater and a smaller extremity. In the male this figure has its greater end above, while in the female the lower is the larger end. According to this formula, in the female the diameter of the hips exceeds that of the shoulders, while in the male it is the diameter of the shoulders which exceeds that of the hips. The formula, as regards the female at any rate, is evidently exaggerated. In fact, Salvage and Malgaigne proposed the following formula:—Allowing that the trunk of the male is an ovoid, with the greater extremity superior, the trunk of the female forms an ellipse—that is to say, a figure in which both extremities are of the same dimensions; so that while in the male the diameter of the shoulders exceeds that of the hips, in the female the diameter of the hips is just equal to it.

Now this last formula also exaggerates the real proportions of the hips in the female. The correct formula is as follows:—In the male, as well as in the female, the trunk represents an ovoid with the greater diameter above; but while in the male the difference between the greater extremity and the smaller is very considerable, in the female this difference is very slight. We can see by actual measurements that in the female the diameter of the hips, though always less, differs very little from that of the shoulders. In the male, the distance from the head of one humerus to the corresponding part on the opposite side (inter-humeral diameter) is on the average 15¼ inches, and the measure taken from one great trochanter to the other (inter-trochanteric diameter) is 12¼ inches; a difference between the two diameters of about one-fifth. In the female, the inter-humeral diameter is on the average 13¾ inches; the inter-trochanteric diameter is 12½ inches; therefore there is between the two diameters a difference of about one-twelfth. These figures also serve to demonstrate that the diameter of the shoulders is greater in the male than in the female (15 to 14), and that inversely the diameter of the hips is greater in the female than in the male (12½ to 12¼); so that, accordingly, if a man and a woman of average stature are supposed to throw their shadow on the same portion of a screen, the shadow of the shoulders of the male would cover a much larger surface than the shadow of the shoulders of the female; and, on the contrary, the shadow of the hips of the woman would exceed the shadow of the hips of the man, but only to a very small extent.

In the foregoing paragraphs we have dealt with the transverse hip-measurements as inter-trochanteric. There is, however, another method of measurement, which justifies to a certain extent the formulæ adopted by the authors previously mentioned; it consists in comparing on the skeleton in both sexes the diameter of the pelvis without the femora with the diameter of the shoulders without the humeri. In this instance the shoulders are represented by the inter-acromial, and the hips by the inter-iliac diameter (from one iliac crest to the other). Under these circumstances the exact measurements show that in the male the inter-acromial diameter is 12¾ inches, and the inter-iliac 11 inches; therefore, as in the other formula, the trunk, deprived of its members, still represents an ovoid, with its greater extremity superior. On the other hand, in the female, the inter-acromial diameter is 11½ inches, and the inter-iliac measurement is 12 inches; so that here the trunk, deprived of its members, represents an ellipse or an ovoid, with its greater extremity below, although the upper extremity differs very little in size from the lower. The fault of this method of measurement is that it does not explain things as they are. The artist does not contemplate the torso as otherwise than complete—that is to say, provided with the upper and lower limbs—and it is absolutely necessary to take into account the part which the head of the humerus and the great trochanter of the femur take in the formation of the contours of the shoulder and hips. We have thought it desirable, however, to demonstrate this mode of measurement, because of the clear illustration which it gives of the greater diameter of the female pelvis as compared with that of the male.

If we arrange in a table the figures given above for the inter-humeral, inter-trochanteric, inter-acromial, and inter-iliac diameters in the male and in the female, or if, better still, we represent those figures by lines intended to express, on the profile of a man and that of a woman, the proportionate value of the diameters of the region of the shoulders compared with the diameters of the pelvis and the hips, we obtain two figures which express in a striking manner all that has been pointed out (Figs. 41 and 42).

We see, in fact, that in the male subject (Fig. 41) the vertical lines (y and y) passing through the extremities of the inter-trochanteric (d d) and the inter-iliac (c c) diameters, both fall within the extremity of the inter-humeral (b b), and also the inter-acromial diameter (a a); on the contrary, in the female (Fig. 42) we find that these same vertical lines both fall within the extremities of the inter-humeral (b b), but on the outer side of the inter-acromial (a a) diameter.

The Great Trochanter.—A word is necessary concerning the influence of the great trochanter of the femur on the external form of the hip. Looking at the skeleton the great trochanter is seen to stand out so clearly and forms a projection so considerable that we should expect to see on the model a prominence corresponding to its shape. This, however, is not so. The gluteal muscles which proceed from the pelvis to be attached to the great trochanter are numerous and thick, and the fleshy bellies of the more superficial form a projection which is raised above the trochanter; while over the process they are replaced by tendons, more or less flattened, so that the trochanter is marked on the surface by a depression bounded by the muscles—the tensor vaginæ femoris in front, the glutei muscles above and behind. Below, the concave space corresponding to the great trochanter is continuous with the broad, flat surface which occupies the outer aspect of the thigh.

There are many analogous cases where osseous projections are frequently marked on the external figure by a depression, and the reason is always the same as that just explained, that these osseous projections give insertion to muscles, the fleshy bellies of which give place to tendinous expansions at a little distance from them, and form by their thickness a raised surface round the prominence; in a general way, then, we may say that, with a few exceptions (such as the malleoli of the ankle-joints), wherever an osseous surface is covered over only by the skin, the muscles which surround this surface arise above its level, and in consequence the bone is marked by a depression, more or less pronounced as the subject is more or less muscular. Similarly, the middle portion of the sternum is marked superficially by a depression limited on each side by the swelling of the great pectoral muscles, and the internal surface of the tibia forms a long and broad groove when the anterior and posterior muscles of the leg are well developed.