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Show 1894.] ANATOMY OF ORNITHORHYNCHUS. 697 Owen does not mention it in this animal. Mivart says it is absent in Echidna. Subscapularis.-There would appear to be a difference in the origin of the two muscles supra-spinatus and subscapularis in the two species of Monotremes. According to Mivart, in Echidna, supra-spinatus occupies the whole of the costal surface of scapula, while subscapularis is confined to outer surface. Owen states that subscapularis is a narrow muscle in Ornithorhynchus. It would appear, rather, to be a large muscular sheet arising from the so-called subscapular fossa, i. e., posterior part of outer surface, and from almost the Avhole of the costal surface, so-called supraspinous fossa. The two parts are intimately blended, as they pass beyond scapula to humerus, and are inserted together on the ulnar tuberosity. In the portion of the tendon which springs from the costal surface is a sesamoid bone; the other portion is inserted into the ulnar tuberosity just below the sesamoid bone. It is, however, an artificial dissection to separate the two portions. The larger portion, containing the sesamoid bone, would seem from its insertion to represent the subscapular muscle of human anatomy ; the portion arising from the so-called subscapular fossa, i. e., posterior part of outer surface, would seem to represent that portion of the subscapularis of human anatomy which arises close to axillary border of scapula. This portion is often separate, and is called in human anatomy subscapulo-capsularis by Macalister. It is in M a n inserted just below the chief portion of the subscapular muscle. In this animal it is also inserted below the rest of the muscle. If w e imagine the axillary border of the human scapula twisted backwards, so that it, together with the origin of triceps, comes to occupy the middle of the bone, then this subscapulo- capsularis w-ould be also twisted backwards and its origin would occupy the exact position which it has in Ornithorhynchus. The two portions of the muscular sheet are supplied by the separate branches of the same nerve which is a branch from the upper part of brachial plexus. Coues describes this muscle as upper part of teres major, i. e., the muscle which he speaks of as arising from both sides of scapula and embracing the bone. Since, however, the two portions of his teres major are quite distinct from their origin to their insertion, and separated, moreover, by the long head of the triceps, this would lead one to think that that portion Avhich passes to ulna tubercle is subscapularis. MUSCLES OF ARM. Deltoid.-Consists of two parts, anterior and posterior. They are both inserted into strongly marked deltoid ridge of humerus, and are quite distinct to their insertion. Biceps.-Arises by two heads ; both are coracoid in their origin and both are radial in their insertion. The larger arises from sternal extremity of coracoid, the smaller arises nearer the glenoid cavity from epicoracoid. They blend about the middle of the arm |
