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always evident, but the cell-wall less clearly marked, so that you often see contiguous nuclei without being able to trace the junction of the cell-walls; the cell-walls are best seen in the detached cells that float off at the edge of the specimen.”

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“The lung of the bird I have not yet examined, for I could not • obtain a bird fresh enough. However, having found the epithelium in human lung, I do not see what you want with the bird's lung."

Dr. Brittan brought the subject under the notice of the Bristol Microscopical Society—a society which includes all the leading medical men of Bristol and Clifton, and writes thus :

“As you know, it is impossible to show much to twenty men with six microscopes, especially when such minute matters are in question. I believe, however, that many were quite satisfied, and some I have promised to convince at leisure. I need not enter into further detail, but I repeat that I am assured of what I before stated—namely, that there is an epithelial lining to the air-vesicles." To this unprejudiced testimony of Dr. Brittan, I can, of course, add 39-xx.

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nothing. I think it will be conceded that I could not have adopted a fairer mode of attempting to settle the point in discussion. The reader has my drawings and those of Dr. Brittan before him, and can judge for himself whether the appearances (assuming the representations to be tolerably faithful) are or are not characteristic of a minute tesselated epithelium.

Ant. III.

The Mechanism of the Joints of the Sacrum. By CHARLES H. MOORE,

Surgeon to the Middlesex Hospital, Lecturer on Surgical Ana

tomy. The mechanical relations of the sacrum can only be entirely understood by studying all its connexions with adjoining bones. The shape of its articulating surfaces exhibits the directions in which it can move, the ligaments show from what movements it is restrained; but the connexions of the bone are so complicated, and its stability depends on circumstances so unique, that confusion still appears to rest on the minds of some anatomists with regard to it. Dr. Matthews Duncan has recently thrown much light on the subject,* though his views appear not unmixed with error. Indeed it is impossible that the anatomy of this part of the body can be completely understood, so long as the precise form and direction of the articulating surfaces, and the use of the ilio-lumbar ligaments, are disregarded or mistaken, and so long as the assertion of Cruveilhier is accepted, that the sacro-sciatic ligaments are termed ligaments “ rather on account of their fasciculated shape than from their use, which scarcely has reference to the union of the bones of the pelvis.”+

The inclination of the sacrum backward beneath the spinal column, causes it to present its upper articulating surface to the body of the fifth lumbar vertebra on an inclined plane downward and forward. In the erect position of the body, therefore, that vertebra tends to slide upon the sacrum ; and it does so slide until its articulating processes meet those of the latter bone. The sacrum then yields to the weight, and its promontory is depressed.

With regard to the ossa innominata, the sacrum is so placed that most of the surface by which it meets the last vertebra, lies in front of its lateral articulations with the ilia, and that two-fifths of the whole length of the bone project downward and backward, below and behind the sacro-iliac synchondroses.

Such being the position of the sacrum with regard to other bones, it must be liable to two kinds of displacement. On the one hand, the innominate bones separating from one another, it might slip bodily down between them; and on the other hand, supposing it to be secured from falling between those bones, and yet articulated to them, it might yield

Edinburgh Monthly Journal, August and September, 1855.
+ Cruveilhier's Descriptive Anatomy : Library of Medicine, vol. vii. p. 202.

so much to the weight of the vertebræ as to suffer an extreme degree of inclination, and thus become an insecure basis for sustaining the spinal column. Either, then, the ligaments of the sacrum, or the form of its lateral articular surfaces, must be adapted to resist those displacements.

The wavy auricular surfaces of most sacra, whilst unfitted for extensive motion, are yet adapted to permit a certain extent of the very displacements to which the position of the bone exposes it. Narrower from side to side at its pelvic than on its spinal surface, the sacrum is liable to drop into the pelvis. It could not, however, readily do so from the inequalities of its lateral articular surfaces. Again, in the direction of movement by which the sacrum would be too much inclined, there is a curved groove, fitted by a corresponding ridge on the ilium, and extending the whole length of the articular surface, first directly downward, and then obliquely backward and downward. By the anterior part of this groove, which is in the line of the centre of gravity, the promontory may descend vertically; by the posterior part, the coccygeal end of the bone may be projected backward and slightly upward. The whole movement is one by which the inclination of the sacrum is increased, as if the bone turned upon a pivot passing transversely through it and both ilia. The imaginary pivot would be situated above and a little behind the articulation. The principal convex portions of the sacral surfaces are a slight and short elevation of the posterior edge behind the lower part of the curved groove, and a more considerable projection of the anterior angle. The former is not a constant ridge, but when it exists, would permit the descent of the sacrum. The anterior first favours, and then resists, the inclination and depression of the bone.

All sacra are not formed precisely as has been described. In some, the pivot actually exists within the area of the joint; in others, the concavity is on the iliuin instead of the sacrum. But the movements of which they are all capable will be found to vary little from those of the most common and, as it inight be called, the typical shape, which is that above described.

The security of the articulations, then, depends not on the bones, but on the ligaments; and these are so situated as to be only brought into use by the displacements of the sacrum already described. It is the liability to displacement which gives the bone its greatest sta-bility.

From the tip of either lateral costiform process of the fifth lumbar vertebra, the ilio-lumbar ligament, exceedingly strong, and about an inch in length, passes outward, backward, and a little upward, to the crest of the ilium. Its attachment to the crest is in the vertical line above the sacro-iliac synchondrosis, and marks the anterior limit of that strong beam of bone of which the hinder part of the ilium is composed, and which, under the name of cotylo-sacral beam, reaches from the posterior tuberosities of the iliuin to the acetabulum, and, beyond that, to the tuber ischii.

Scarcely any ligament exists on the anterior and inferior aspect of the synchondrosis, but behind and above it the back of the sacrum is bound to the overhanging projection of the ilia by short, numerous, and exceedingly strong vertical fibres (superior sacro-iliac).

From the posterior superior spine of the ilium, a flat and strong ligament descends vertically behind the lowest part of the synchon. drosis, and is attached to the tubercles on the third and fourth verte bral segments of the sacrum (posterior sacro-iliac).

Lastly, the greater and less sacro-sciatic, originating in the ischium, and attached to the sacrum, cross from bone to bone, inward, backward, and upward, at a considerable distance below the level of the sacro-iliac joint. The shorter ligament, attached near the lower end of the sacrum, passes to the spine of the ischium almost horizontally. The use of these ligaments is as follows:

The sacrum is slung between the ilia by the short or superior sacroiliac ligaments; but those ligaments are so placed as not to interfere with that movement of the bone, as it were upon a pivot, which its auricular surfaces are adapted to permit. The weight received on the sacrum from the spine is by these ligaments transferred to the posterior overhanging tuberosities of the ilium, from which it descends by the strong cotylo-sacral beams to the acetabula in walking, and in sitting to the tubera ischii. These ligaments answer to the pivot upon which the sacrum might be imagined to turn.

When the body of the fifth lumbar vertebra, by which the weight of the trunk reaches the sacrum, slides forward over or with the upper surface of that bone, the ilio-lumbar ligaments are tightened, and the ilia are pulled toward this vertebra and one another. But the ilia, in closing together, jam the sacrum between them, and fix it by lateral pressure with a security proportioned to the weight of the last vertebra and all it bears.

Whilst this movement is taking place at the upper part of the articulation, others may occur below. The coccygeal end of the suorum may be tilted upward and backward in the opposite direction to the depression of the promontory, or the whole sacrum may press downward and backward in the direction of its length. The latter displacement is stopped by the vertical or posterior sacro-iliac ligaments; but so far as it does occur, it necessarily tends to separate the ilia, and consequently to tighten the ilio-lumbar ligaments: these, as We have se'n, draw the crests of the ilia together,

Suill, as the lio-lumbar ligaments secure the approximation of only the upper parts of the innominata, might not the depression of the Weign like serem separate the innominata below! Provision is made

ainsi such a separation, and at the same time against too great an elevation of the cameral end of the sacrum, by the sacro-sciatic liggements For thom being useless in the mechanism of the pelvis, as ( wilher Ver they are most important parts of that mechanism, Bucurring with the alt-lunular ligaments to jam the serum between the Am intramuta, anel at the same time, in the reverse direction, In n ing the language of the steram. The same weight which dePas the fifth lumber vertelia and the promontory, and tightens

the ilio-lumbar ligaments, also elevates the coccyx and tightens the sacro-sciatic ligaments. The greater the weight, therefore, the tighter will be the ilio-lumbar and sacro-sciatic ligaments, and consequently the more securely will the sacrum be fixed. Moreover, it will be seen that the sacro-sciatic ligaments, by the same mechanism, become important ligaments of the spine, since they support the column in its last curvature, and render the sacrum a firm basis upon which the vertebral bodies may be safely piled.

The foregoing explanations relate to the mechanism of the sacrum only as it is a part of the spinal column, and in its connexion with both the innominate bones at once. It remains to consider the mode in which the weight of the trunk is conveyed from the top of the sacrum to either femur singly. It might be the case that the pelvic circle of bones would act as a whole, and bear in all its parts the weight which arrives only on the sacrum. In that case the weight would be divided between the two innominata, and the whole mechanism already described would come into action equally when we stand on one leg or on both. But it is probable that only the hinder and stronger portion of the pelvis is really concerned in progression. A boy who had been crushed between a dray and a wall, walked into the surgery of the Middlesex Hospital scarcely lame; yet after death he was found to have had the pubes and ischial rami of both sides broken off from the back of the pelvis. A specimen is preserved in the museum of the same hospital, in which also the anterior part of the pelvis is broken off from the posterior, yet the man had walked with only a moderate limp. The weight in such cases must have descended from the vertebræ to either femur through the ilium of its own side alone. Dr. Matthews Duncan has satisfactorily shown that the weight in descending thus to one side does not pass through the sacro-iliac synchondrosis. It is conveyed by the superior sacro-iliac ligaments to the tuberosities of the ilium, which over hang the back of the sacrum, and then descends to the acetabulum through the thick beam of bone which composes the posterior part of the ilium.

A misapprehension as to the direction of this beam-the cotylo-sacral beam-of bone, has led Dr. Duncan into what I conceive to be an error with regard to the development of the pelvis, and the cause of a part of the deformity in Nägele's oblique pelvis. He describes this beam as curved outwards between its upper extremity, at which the superior sacro-iliac ligaments pull upon it, and its lower end, which rests on the head of the femur; and he attributes its curvature to the outward pressure of the sacrum between these two points-namely, at the sacro-iliac synchondrosis. But it is surely erroneous to say that bones depend upon pressure for their regular development. Undue pressure may deform them, but not that which they are made to resist : had they no power of growing into a definite shape, and maintaining it, they would be incapable of bearing the very forces to which they are ordinarily subject. Moreover, Dr. Duncan's explanation is misapplied. The cotylo-sacral beams are not curved at all. From their extremities at the tuberosities of the ilium to the acetabulum, although not

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