LAMENESS IN THE FORE LEG.
Anatomo-Physiological Review of parts of the Fore Leg.
For supporting weight, whether the subject is at rest or in motion, the bony column of the leg, together with attached ligaments, tendons and muscles, is wonderfully well adapted by nature for the function which they perform. The several bones which go to make up the supportive portion of the leg, are so joined at their points of articulation, that a minimum degree of strain is put upon each attachment.
The upper third of the scapula, with its cartilage of prolongation, is sufficiently broad and flattened that it fits snugly against the thorax without necessity for a complicated method of attachment—the clavicle being absent, attachment is muscular.
Smith has very aptly stated that:
“It seems quite legitimate to regard the muscular union between the thorax and forelimb as a joint. There are no bones resting on each other, no synovia; but where the scapula has its largest range of movement there is a remarkable amount of areolar tissue, which renders movement easy. The whole central area beneath the scapula and humerus not occupied by muscular attachment, is filled with this easy-moving, apparently gaseously distended, crepitant, areolar tissue over which the fore legs glide on the chest wall as freely as if the parts were a large, well lubricated joint.”
The scapulohumeral articulation (shoulder joint) is an enarthrodial (ball and socket) joint but because of its being held more or less firmly against the thoracic wall by muscular and tendinous attachment, and because a part of this attachment affords a means of support for the body itself, there is no need for binding ligaments and movement is possible in all directions even though restricted as to extent.
[Illustration: Fig. 2—Muscles of Left Thoracic Limb from Elbow Downward; Lateral (External) View.
a, Extensor carpi radialis; g, brachialis; g’, anterior superficial pectoral; c, common digital extensor; e, ulnaris lateralis. (After Ellenberger-Baum, Anat. fuer Kuenstler.) (From Sisson’s “Anatomy of the Domestic Animals").]
[Illustration: Fig. 3—Muscles of Left Thoracic Limb from Elbow Downward; Medial (Internal) View.
The fascia and the ulnar head of the flexor carpi ulnaris have been removed. 1, Distal end of humerus; 2, median vessels and nerve. (From Sisson’s “Anatomy of the Domestic Animals").]
Undue extension, (by extension is meant such movement as will cause the long axis of two articulating bones to assume a position which approaches or forms a straight line—opposite to flexion), of the scapulohumeral joint is impossible while weight is borne, because of the normally flexed position of the humerus on the scapula; whereas flexion, beyond desirable limits, is inhibited by the biceps brachii (flexor brachii or coracoradialis) muscle.