When the amount of this kind of deformity is slight, the inlet of the pelvis forms a triangle, with its angles rounded off, and the sacrum for its base. In a higher degree of it, the sides of the triangle become convex, and encroach upon the pelvic cavity, and after a time the base projects inward too; the three sides then unite at very acute angles, and the upper aperture of the pelvis acquires the shape of a heart on playing-cards. The deformity may even go farther; both sides of the triangle, or it may be one only, approximating to the base so much as to bring the promontory into contact, and even into union with them, just above the acetabula.

The triangular pelvis is the extremest instance of contraction. The ilia are so compressed from before backward that their venter becomes a narrow fissure; the symphysis pubis forms a beaklike prominence; the arch of the pubes narrows considerably, and may even be obliterated; the tubera ischii approach each other; and as the promontory and the lumbar part of the spine sink down into the cavity of the pelvis, the sacrum beneath presents a curved anterior surface, or may be bent at an angle.

The triangular pelvis is, for the most part, a result of mollities ossium; but it is an error to ascribe it exclusively to that disease. Mollities ossium may undoubtedly be the cause of every decided and advanced degree of triangular pelvis, but minor degrees of it are sometimes due to rickets. In extreme cases there is less inclination of the pelvis than usual, and it is sometimes even raised above the horizontal line.

The form and position of the pelvis undergo an interesting change after so-called coxalgia: for the dislocation of the femur upward and backward which takes place in that disease, when it occurs only on one side, destroys the symmetry of the pelvis in a remarkable manner, and renders it oblique; and the same change occurs when the dislocation is the result of violence. The general characters of the deformity are, that the whole os innominatum becomes wasted and small, the ilium assumes nearly a vertical direction, the upper aperture becomes enlarged, and the pelvis acquires an abnormal inclination. In other respects its characters present many varieties which may depend upon the extent of the displacement, upon the condition of the joint as to mobility or anchylosis after the diseased process has ceased, or supposing the limb to have been movable, upon its having been used afterwards or not.

a. In cases of coxalgia on both sides, in which a layer of more or less dense fibrous tissue binds a remnant of the head of the femur, or the stump of its neck, to the upper margin of the wasted acetabulum, and in which, therefore, some mobility of the limb remains, the ossa innominata are attenuated, especially at the pubes and ischium, and the whole cavity of the pelvis is enlarged. The latter change is partly due to the attenuation of the bones; but it can be traced, at particular parts of the pelvis, to other circumstances. The inlet is enlarged by the curve of the linea innominata becoming shallower, and by the disappearance of the ilio-pectineal eminence, and of the angle at the sacro-iliac synchondrosis; the cavity of the pelvis increases in size symmetrically, at the expense of the basis of the wasted acetabula; and the very striking dilatation of the outlet is due to the separation of the ischia from each other. This separation of the ischia is attributed by Hülshof to the action of those rotating muscles of the femur which arise from the tuber ischii; for as the whole weight of the body falls on them, they draw the ischia outwards, and consequently, asunder. The angle beneath the pubes thus becomes obtuse and may even be changed into a shallow arch. The ilia assume nearly a vertical position, partly in consequence of the abdominal muscles attached to them being stretched by the sinking of the pelvis, and perhaps also, from the pressure of the displaced head, or stump of the femur, against the upper part of their outer surface.

The wasting and enlargement thus described, diminish the height of the pelvic cavity, and the length of its axis; the pelvis itself is depressed, and has a greater inclination than natural, and the lumbar part of the spine forms an arch, directed forward, which varies in degree, according to the amount of the inclination of the pelvis.

B. In cases in which similar changes have been produced by coxalgia in the articular structures of the hip, but on one side only, the os inno-minatum of that side becomes thin, and in consequence of some flattening and sinking of the linea arcuata, and of a distortion of the sacral and lumbar vertebrae, which has yet to be described, the inlet of the pelvis is enlarged on the same side. The capacity of the pelvis is also increased, but the size of the outlet varies, according as the limb connected with the diseased joint has been used or not. If it have been used, the corresponding tuber ischii projects outward, and the dimensions of that side of the outlet are increased; but if not, the weight of the body, in walking and standing, will have been sustained, in whole or in part, by the sound limb, while the actions of the diseased joint will have been assisted, or quite supplied, by the use of a stick or crutch. The tuber ischii, in that case, is not everted; but on the contrary, the wasting which accompanies the process of repair, the shrinking of the acetabulum, the angular bending of the os innominatum, and, in many cases, the inward direction which is given to the femur while the patient is lying in bed, and which is afterwards retained, all tend to force the tuber ischii inward, especially towards the coccyx, and thus to contract the outlet of the pelvis on the side of the disease.

Moreover, the ilium on this side becomes so nearly vertical, that the distance between its anterior superior spine and the upper border of the symphysis pubis, is manifestly lessened: the height of the same half of the pelvis is diminished; it is depressed, that is to say, its inclination is greater than that of the other side; and the sacrum is both inclined backward on the side of the diseased joint, and rotated towards it. The two halves of the pelvis, therefore, are not symmetrical; and the error of symmetry will increase in proportion as the acetabulum of the sound joint is driven inwards and upwards, to receive the weight which the curved spinal column directs upon it, - in proportion, that is, as the pelvis becomes contracted on the sound side, while it is enlarged on the diseased. The difference between the two sides will increase, too, so long as the bones continue soft.

As the curvature of the vertebral column, which has been referred to, is for the purpose of compensating for the obliquity of the pelvis, it is directed forward and towards the side opposite to the coxalgia, that is, towards the healthy, or narrower, half of the pelvis. It is a curvature which compensates for the deviation of the sacrum, and if it be itself considerable, it is.followed by a curvature in the dorsal region towards the opposite side. In this, again, we find a correspondence with the rule laid down as to the relation of deformities of the pelvis to primary lateral curvatures, that the pelvis is contracted, etc, on the side opposite to the dorsal curve.

y. When the morbid process has ended in complete anchylosis, the linea arcuata and the bottom of the acetabulum, on the diseased side, become not only flattened but angular; in fact, the whole os innominatum, wasted in all its separate parts, bends at an angle projecting outward, which is formed in the seat of the old acetabulum, - in the osseous cicatrix corresponding to the previous disorganization. The ilium inclines inward and forward, the ischium inward and backward, the promontory encroaches on the healthy side of the pelvis; and as the symphysis pubis is drawn by the angular distortion of the innominatum towards the side of the disease, the linea arcuata of the healthy side runs in a straight direction forward to reach the displaced symphysis.

§. Under certain circumstances, which have not as yet been clearly ascertained, the pelvis is found to have all the anomalies, already mentioned, as results of coxalgia, but to be elevated, and to have less inclination on the diseased side; although, at the same time, the extent of the consecutive luxation is not greater than that which has been presumed in the foregoing description, nor has the process terminated in a different manner. (Compare Guérin).

E. On the other hand, it certainly happens after complete dislocation upward and backward upon the outer surface of the ilium, whether it be the result of violence, or have occurred in the course of coxalgia, that the pelvis is raised, and has less inclination on the diseased side. This peculiarity was attributed by Guérin to the action of the stretched psoas and iliacus muscles: and there may be observed, at the base of the anterior inferior spine of the ilium, a more or less distinct impression or furrow, which has been made by the pressure of the common tendon of those muscles. The same condition is found when the disease terminates in anchylosis, after having destroyed the head and neck of the femur. The small trochanter is drawn upward against the border of the acetabulum, and is also turned backward by the inversion of the limb.

Besides these deformities of the cavity and the outlet of the pelvis, which either accompany, or may be inferred from the abnormal states we have already considered, there are several others of independent origin. They depend chiefly upon the sacrum, whether it be too much flattened or too much excavated, upon unusual projection of the coccyx inwards, upon excessive width or narrowness of the arch of the pubes, etc.