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Show 1894.] ANATOMY OF ORNITHORHYNCHUS. 711 hand, and gives off a branch to the sup. longus and extensor carpi radialis longior. The lower trunk, formed by the seventh and eighth cervical, first dorsal, and a filament from second dorsal, is much the larger of the two. This trunk divides into three terminal branches about the middle of the arm :-(a) One passes downwards, enters forearm, and is distributed to the cleft between the index and middle fingers on palmar aspect; (/3) The next, also a cutaneous branch, is about the same size as the first; it is distributed to the third and fourth clefts. The above two nerves would seem to represent superficial portion of median and part of ulna, (y) The third branch of the lower trunk, the largest of the three, passing down arm, gives off two branches to the triceps, which curve round the lower border of latissimus dorsi. One of these is the posterior interosseous, which passes through triceps, winds round humerus something like the musculo-spiral of human anatomy. This nerve next passes between brachialis anticus and muscles arising from the radial condyle of humerus, and, passing into forearm, perforates the supinator brevis and supplies the usual muscles on the back of the forearm, with the exception of extensor carpi radialis brevior, extensor primi and secundi inter-nodii pollicis, all of which muscles are absent. The continuation of the third branch passes into forearm, beneath the anconeus internus, and, after supplying the muscle, gives off branches to the flexor carpi ulnaris and the ulnar portion of the flexor sublimis digitorum ; it then continues its course down forearm and ends by supplying the ulnar side of the fifth digit. It communicates in the forearm with the branch to the fourth cleft. Other branches from lower part of plexus :-The seventh nerve gives off a branch to latissimus dorsi before it joins the plexus. From the lower part of plexus a branch is given to the pectoralis major. The median nerve arises by two roots, and, after supplying the biceps, coraco-brachialis, and brachialis anticus, passes through supracondyloid foramen into the forearm. Lumbar Plexus. Obturator nerve.-Its chief origin is from the sixteenth dorsal nerve, allowing two lumbar vertebrae. The nerve passes out between the sixteenth and seventeenth dorsal vertebrae. It is joined by a slender branch from the seventeenth dorsal, a subcostal nerve. The nerve passes along pelvis in the usual position and supplies the usual muscles, together with a slender branch to the semi-membranosus. It also gives off a branch in the abdomen, which, after dividing into several branches, passes to psoas and iliacus. The anterior crural nerve arises by two fairly large branches from the seventeenth dorsal and the first lumbar nerves. The rest of the first lumbar together with the whole of the second pass into the pelvis, forming the lumbo-sacral cord, |
