The anterior and posterior ligaments are connected, both externally and internally, with the lateral ligaments, so that no portion of the periphery of the articulation is devoid of ligamentous covering c. External Lateral Ligament of the Wrist Joint.—The external lateral ligament is funi cular in form, and connects the styloid process of the radius with the scaphoid and trapezium, the two most external of the carpal bones. The direction of its fibres is downwards and backwards. The radial artery crosses ob liquely over the external surface of this liga ment, and separates it from the tendons of the extensor ossis metacarpi and the extensor primi internodii pollicis. The synovial mem brane lines its inner surface. When the hand is fully adducted, this ligament is put upon the stretch.
d. The internal lateral ligament.—This li gament is larger and longer than the pre ceding ; for, notwithstanding the greater length of its styloid process, the ulna is more widely separated from the carpus than is the radius. This ligament connects the styloid process of the ulna with the cuneiform bone, _ giving a fasciculus to the Sprirorm bone and to the anterior annular ligament. The tendon of the extensor carpi ulnaris is posterior and parallel to this ligament. The internal lateral ligament limits abduction, so as to render it the least extensive of all the motions enjoyed by the wrist joint.
III. Synovial Membrane.— The synovial membrane of the wrist joint is very extensive, and contains a large quantity of synovia. Its anatomical disposition is so simple as not to require any special notice ; but it may be of practical importance to remember the prox imity of this structure to the synovial sacs between the lateral articulations of the carpal bones, and also its contiguity to the articula tion between the trapezium and the first metacarpal bone. The former circumstance explains the facility with which acute inflam mation, occurring in the smaller carpal articu lations as the result of imparonychia, may be propagated to the contiguous wrist joint, where, too frequently, the morbid action thus excited proves destructive to the articulation.
• The latter circumstance demands our atten tion, since it teaches that, in amputating at the first carpo-metacarpal articulation a care less use of the knife may inflict a wound upon the wrist joint itself, from which the most serious consequences may accrue.
The synovial membrane of the wrist joint is separated from that of the inferior radio ulnar articulation by the triangular intcrarti cular cartilage. In amputating at the wrist joint, if care be taken to leave the inter articular cartilage uninjured, the motions of pronation and of supination will be left to the forearm, and may therefore be communicated to the artificial substitute for the amputated hand. In this respect, the amputation through the joint offers an advantage which is sacrificed when amputation is performed through the continuity of the bones of the forearm.
IV. Mechanical Functions. — The wrist joint enjoys every variety of motion included under the head of gliding motions (Bichat) ; thus it is capable of flexion, extension, adduc tion, abduction, and circuniduction. In the motions of flexion and of extension, the carpus rolls either forwards or backwards on the lower articular surface of the radius. Flexion is limited, not only by the posterior ligament, but also by the lateral ligaments which are attached behind the centre of motion of the articulation. Extension is limited by the an terior ligament only. In adduction and abduc tion the carpal bones glide from side to side on the surface opposed to them. Of these motions abduction is the more limited, be cause, probably, the styloid process of the radius and the trapezium come sooner into mutual contact than the corresponding parts at the inner side of the joint do. In circumduction (a motion compounded of all the preceding ones), the hand moves through a circle re presenting the base of a hollow cone, the apex of which is at the joint. The free gliding of the carpal bones on each other causes this movement to appear more extensive than it is in reality, whilst it gives all that ease and grace to the movements of the wrist which are in so especial a manner its characteristics.
The wrist joint thus constituted would not, d priori, be supposed to enjoy that remarkable immunity from accidental luxation which must be conceded to it. In truth, this articu lation owes its remarkable strength and its freedom from accidental displacements to the mass of tendons which occupy its anterior and posterior surfaces principally, not to its ligamentous connections, nor to the form of its osseous surfaces. Those tendons, like so many vital ligaments kept tense as well by the tonic as by the active contraction of their muscles, and bound down by the annular liga ments, bear off, in a great degree, all violent shocks from the joint itself, whilst, by their antagonistic resistance to each other, they at the same time maintain the bones in accurate contact.
But whilst, owing to these circumstances, the impetus of shocks applied to the hand are borne off frOm the articulation of the wrist, they fall in many instances with resistless force upon the lower end of the radius, which, owing to the thinness of its compact tissue, is ill adapted for opposing an effectual resistance. It has already been shown that the ulna is but indirectly connected with the carpus ; and that a layer of highly-elastic tissue intervenes between the carpus and that bone. These circumstances taken in connection with the remarkable mobility of the lower radio ulnar articulation sufficiently explain why the ulna so frequently escapes injury from forces which act on the hand, and which suffice to fracture 'the radius. At the same time it is to be observed, that displacements of the lower extremity of the ulna are by no means unfre quent accompaniments of fractures of the radius.