Licence: Public Domain Mark
Credit: The mechanism of joints / by Harrison Allen. Source: Wellcome Collection.
Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.
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![THE MECHANISM OF JOINTS. The1 limb being subservient to both support and motion,it is reasonable to expect that in certain joints the former function should be observed, and in others the latter. When the apparatus for support is conspicuous, the joint may be said to be of static value. But when this purpose is subordinate to flexion (i. e. deviation from the axial line of the limb), the joint may be said to be of dynamic value. The most striking distinctions between the static and dynamic articu- lations lie in the relations of the opposed surfaces. To explain this por- tion of my remarks a few words of a general character are necessary. I premise that the typical “ ball and socket” joints consist of well-defined balls embraced by perfect sockets. No such joints are found outside of the vertebral column. The articulations between the bodies of the vertebrae, although spoken of as amphiarthroses, are in truth “ balls and sockets.” The central intervertebral mass is the “ball;” the op- posed vertebral surfaces and the peripheral interlacing fibrous bands of the disk make up the “ socket.” I also premise that the simplest forms, or at least the first forms, of joints appear in the vertebral column, since this structure answers to the axial line of the body. The movable union between any pair of segments or bodies of this axis may be taken as typical of what is possible under more complex conditions elsewhere. If it is remembered that a rod or axis cannot be projected far without segmenting, the best basis is secured upon which to rest any consideration of arthrosis. Obviously motion between a pair of bony segments is the main fact to consider in every problem. The questions which I believe can be asked with reference to further development of this theme are, in what way is support secured through a series of such mobile joints, and how can the apparatuses of motion be varied ? The limb is a special structure appended to the body. It moves chiefly at the joint between its first and second bones. This joint, in both the anterior and posterior extremities, is a ball and socket, as usually de- scribed. But it evidently in each instance is a portion of a ball opposed to a partial socket; since only small portions relatively of the head of the humerus or femur can be brought in relation to the socket at one time. . So generally throughout the limbs (although in less marked de- gree), it is found that segments of spheroids play within concave sur- faces. I propose to call all such surfaces modified balls and sockets (the type of which is exhibited only in the vertebral column), and proceed to define the two main varieties, named above, to wit: (1) Where the rela- 1 [This paper is an abstract of a more elaborate study which the author reserves the right of publishing hereafter in a separate form.—Editor.]](https://iiif.wellcomecollection.org/image/b22459029_0007.jp2/full/800%2C/0/default.jpg)
