Licence: Public Domain Mark
Credit: The anatomy of the human body / By J. Cruveilhier. Source: Wellcome Collection.
Provider: This material has been provided by the University of Massachusetts Medical School, Lamar Soutter Library, through the Medical Heritage Library. The original may be consulted at the Lamar Soutter Library at the University of Massachusetts Medical School.
164/944 (page 140)
![side, and, on the other, the tendon of the infra-spinatus and the ^eat trochanter of the humerus. The study of the coraco-acromion vault cannot, therefore, be separated from the study of the scapulo-humeral articulation, either under an anatomical and physiolo- gical or surgical .point of view. The Coraco-acromial Ligament. This ligament (m, figs. 69 and 70) forms part of the vault we have described : it is a triangular bundle of radiating fibres, which extends from the apex of the acromion to the whole length of the posterior edge of the coracoid process. Its external edge becomes thinner, and is continued into an aponeurotic lamina below the deltoid muscle, and sep- arating that muscle from the joint. Its anterior and its posterior bundles are very thick, folded upon each other, and look like mother-of-pearl; its middle bundles are much less thick. It is lined below by a synovial membrane, and is separated from the clavicle by fatty tissue.* Mechanisiti of the Scapulo-humeral Articulation. The scapulo-humeral articulation admits of the most extensive movements of any joint in the body : it is capable of every kind of motion, viz., forward and backward, and also those of adduction, abduction, circumduction, and rotation. Forward and Backward Motions.—In these the head of the humerus rolls upon the gle- noid cavity, and moves round the axis of the neck of the humerus, while the lower extrem- ity of the bone describes the arc of a circle, of which the centre is at the joint, the radius being represented by the humerus, t The for^vard movement is very extensive, and may be carried so far that the humerus may take a vertical direction exactly opposite to the natural one. The motion backward is produced by the same mechanism ; the head of the humerus turns upon its axis. This movement is limited by the contact of the head of the humerus with the coracoid process, without which dislocation fon\^ard would he very easily produced. It should be remarked that, in any considerable movement of th€ humerus forward, the scapula is also moved, performing that sort of rotation which we spoke of when considering the mechanism of the shoulder. And this combination of the movement forward of the arm and the movement of rotation of the shoulder renders every kind of displacement extremely difficult in exercising the movements of the arm forward. The movement outward, or abduction, is the most remarkable. It belongs exclusively to animals possessed of a clavicle. In it the head of the humerus does not turn upon an axis; it glides do^^iiward upon the glenoid cavity, and presses upon the lower part of the capsule. The shape of the glenoid cavity, which has its long diameter vertical, and its broad part below, is advantageous as regards this motion. When abduction is car- ried so far that the humerus forms a right angle with the axis of the trunk, a great part of the head of the bone is below the glenoid canty. If, while in this condition, the arm be moved forward or backward, the great tuberosity of the humerus rubs against the coraco-acromial arch, and forms with it a sort of supplementary articulation, lubricated by the bursa situated between the coraco-acromion vault and this great trochanter.t The movement of abduction may be carried so far as to allow the arm to touch the head without dislocation; for the capsular hgament is sufficiently loose, especially below, to receive almost the Avhole head of the bone without being torn. It should be remarked, that during abduction the scapula is fixed, which explains the frequency of luxations of the humerus downward. Adduction is limited by the ai-m meeting with the thorax. VMien it is combined with ■ftie for\vard motion, the upper and back part of the capsule, and the muscles which cover it, are considerably stretched. The scapula does not participate in this movement, du- ring which luxation can be occasioned only by a very strong impulse on the arm upward and backward. Circumduction is nothing more than the transition of the humerus from one to another of these motions. The cone which it describes is much more extensive in front than behind, a circumstance tending greatly to facilitate the prehension of external objects, which is the chief purpose of the upper extremities. This predominance of the forward motions has been already noticed in the sterno-clavicular articulation, and wiM be fotiH^ also in many others. ♦ [This is the Ugamentum proprium imttriua of authors ; but the author has taken no notice of another ligw- ment proper to the scapula, viz., the hgamentum proprium posterius, a thin band of fibres stretched across the notch at the base of the coracoid process, which it thus converts into a foramen. The supra-scapular nerve generally passes below, and the arterj' abore it.] t It is through this ingenious and simple mechanism, of -which we shall soon see another example in the ftrticulation of the femur with the os innominatum, that the movement forward of the humerus may be carried fel- enoug-h to describe a demi-circle, without the bone being displaced. t If theory has led ns to believe that the coraco-acromion vault contributed to luxation, by offering- a poi-nt of support to the lever represented by the Ixumerus abducted from the body, a more careful observation, on the contrar}-, has demonstrated that this supporting of the humerus was impossible, as the anterior border of the coraco-acromion ligament is alone pressing against the humerus in the forcible abduction, and luxation is al- ways produced in a middle abduction of the arm.](https://iiif.wellcomecollection.org/image/b21196801_0164.jp2/full/800%2C/0/default.jpg)
