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Show 1896.] ANATOMY OF PETROGALE XANTHOPUS. 709 the layers of the adductors and flexores breves to supply these well as the interosseous muscles. The internal anterior thoracic nerve is not nearly as large as it is in animals with a better developed panniculus : it rises by two roots from the 8th cervical and 1st thoracic nerves respectively; these roots unite and supply the posterior and deep parts of the pectoral mass as well as the ventro-lateral panniculus. The musculo-spiral nerve is formed by the union of two branches in front of the teres major muscle ; the upper of these comes from the 5th, 6th, and 7th cervical, the lower from the union of the 8th cervical and 1st thoracic nerves. The musculo-spiral winds round the back of the humerus, giving off branches to the triceps and dorso-epitrochlearisas well as a descending branch to the anconeus, but no filament to the brachialis anticus or cutaneous twigs. In front of the external supracondylar ridge a branch to the supinator longus is given off, after which the nerve passes deep to the supinator brevis and supplies all the muscles of the back of the forearm, as well as a cutaneous branch to the skin of that region. The circumflex nerve is given off from the upper branch of the musculo-spiral and so can only obtain fibres from the 5th, 6th, and 7th cervical nerves ; it accompanies the circumflex artery through the quadrilateral space, giving off branches to the teres minor and shoulder-joint; it then gives off a large branch to the skin of the outer side of the arm and finally supplies the three parts of the deltoid. The phrenic nerve comes from the junction of the 5th and 6th cervicals, and runs back ventral to the plexus and subclavian artery, to pursue its usual course through the thorax to the diaphragm. The posterior thoracic nerve cannot be seen in the axilla until just before its distribution; it rises from the 4th and 5th cervicals, and runs back in the substance of the scalenus longus to the serratus magnus. No intercosto-humeral nerve was seen. In studying the foregoing nerves the arrangement of the radial is worthy of special attention, because it is opposed to the law laid down by Faterson in his most interesting paper on the limb plexuses of Mammals'. In that paper the following passage occurs :-" In the case of the fore limb the nerves of distribution are derived from the inferior primary divisions of the hinder cervical and first, thoracic nerves. The nerves entering the plexus divide into ventral and dorsal parts, the ventral divisions of the nerves combining to form one set, the dorsal divisions combining to form another set of nerves of distribution. In no case do ventral divisions ever combine with dorsal divisions of adjacent nerves. In no case does a nerve of distribution derived in one animal from ventral divisions, in another spring from dorsal divisions aud vice versa." The musculo-spiral is rightly described 1 Journ. Anat. vol. xxi. p. 622. |
