The principal modifications of the muscles of the pectoral extremity are here described as they exist in the Perameles lagotis.
The trapezius is a broad and very thin mus cle, having its origin extended from the skull, along the cervical and dorsal spines, to the fascia covering the lumbar portion of the latis simus dorsi : its fibres converge to be inserted along the spine of the scapula, the anterior ones directly continued into the pectorales, whereby it becomes an extensor of the humerus and a protractor of the fore extremity.
The latissimus dorsi arises chiefly from the broad aponeurosis covering the muscles of the lumbar region of the spine, and from the spines of the six posterior dorsal vertebrae; the fibres gradually converge, the muscle increasing in thickness as it diminishes in breadth, and terminating in a strong flattened tendon one inch before its insertion at the upper third of the humerus.
The chief peculiarity of this muscle is its con nection with an accessory extensor (omo-anco neus) of the antibrachium. This extensor takes its principal origin by fleshy fibres from the terminal half inch of the fleshy part of the latissimus dorsi, and continues fleshy, slightly diminishing in size to its insertion at the apex of the olecranon; it may thus be considered as a slip detached from the latissimus dorsi, yet its fibres from their very origin run at right angles to those of that muscle, to which they are attached. To remedy the inconvenience of an origin from a yielding and flexible part, a thin aponeurotic slip, two lines in breadth and an inch in length, attaches a past of the base of the superadded muscle and the corresponding portion of the latissimus dorsi to the sheath of the teres major, and to the inferior costa of the scapula near its posterior angle.
The serratus magnus offers no peculiarity worthy of notice.
The supra-spinatus, a strong penniform mus cle, exceeds the infra-spinatus in breadth by as much as the supra-spinal fossa is broader than the infra-spinal one: it has a broad and strong insertion into the great outer tuberosity of the humerus. The infra-spinatus is inserted into the upper and posterior part of that tubero sity.
The deltoidesis a comparatively small muscle; it arises from the anterior half of the spine of the scapula and from a fine aponeurosis covering the infra-spinatus ; its fibres converge to be in serted in the upper part of the deltoid ridge.
A thin small strip of muscle arises from about the middle of the inferior costa of the scapula, beneath the infra-spinatus ; its fibres pass forwards and join the lower margin of the small deltoid, thus bracing and enclosing the tendon of the infra-spinatus.
The subscapularis offers no peculiarity.
The teres major is a strong sub-compressed muscle arising from near the posterior half of the inferior costa of the scapula, and joining, as before stated, the tendon of the latissimus.
The triceps extensor has its long portion arising from the anterior third of the inferior costa of the scapula; its second head comes from the posterior part of the proximal third of the humerus; the third portion takes its origin from the whole of the posterior part of the humerus ; in addition to these, the olecranon receives the above described fourth superadded slip from the latissimus dorsi.
The pectoralis major is, as usual in the Mar supial and many of the Placental quadrupeds, a very complicated muscle; it consists of an anterior or superficial, and a posterior or deeper portion ; the anterior portion receives the strip of fibres before mentioned from the trapezius, there being no clavicle or clavicular ossiele interposed in the Perameles : its fibres con verge, increasing in thickness as they dimi nish in breadth, and are inserted into the anterior and outer part of the strongly deve loped pectoral ridge. The second and main portion of, the pectoralis arises from the whole extent of the sternum; its fibres are twisted obliquely across each other as they converge to be inserted into the inner part of the pectoral ridge ; some of the internal and posterior fibres of this portion of the twisted pectoral pass ob liquely upwards and behind the anterior fasci culi, and are inserted into the coracoid process, thus representing the pectoralis minor.* Be neath this latter portion of the twisted pectoral, a long and slender muscle passes to be inserted into the anterior part of the tuberosity of the humerus; this may likewise be regarded as a dismemberment of the pectoralis major, but it arises from the fascia, of the rectus abdominis, below the cartilages of the lower ribs. Thus the strong pectoral ridge of the humerus is acted upon by muscles having a range of origin from the occiput and cervical vertebra: along the whole extent of the chest to the beginning of the abdomen.