The caudal muscles are powerfully developed. The oblique fibres of the inferior or deflector muscles are shown at 53; they are removed on the other side to expose the anterior caudal nerves (i). The obliguus externus abdominis (3, 3) arises from all the vertebral ribs, except the first, and from the dilated extremity of the ilium; it is inserted by a strong tendon into the outer extremity of the marsupial bone (vr), then expands into an aponeurosis which is attached to the internal margin and base of the marsupial bone, and into the symphysis pubis, decussating with the tendinous fibres of the opposite muscle.
The obliguus internus (6) arises from the anterior part of the ilium, expands, and is inserted into the broad cartilages of the seven posterior ribs (v, v).
The transversus abdominis (7) is a thicker muscle, and arises from both the ilium and the transverse processes of the lumbar vertebrae ; its tendon passes behind the recti to blend with that of the opposite muscle, and with the aponeurosis of the obliqui externi, in the linea alba.
The pyramidalis, or superficial rectus (4), is here, as in the ordinary Marsupials, of very large size; it arises from the whole inner margin of the marsupial bone ; its fibres converge to wards and are confluent at the linea alba with those of its fellow, and it gradually terminates in apoint opposite the posterior part of the sternum. It depresses the ribs, shortens the abdomen, and protracts the marsupial bone.
The rectus abdominis, or posterior rectus (5), arises from the posterior margin of the marsu pial bone, and is inserted into the cartilage of the first rib, the manubrium stemi, and the coracoid bone.
The diaphragm presents the structure which is characteristic of the true mammiferous ani mal. The lesser muscle arises from the first lumbar and four last dorsal vertebrae, and ex pands to be inserted into the central tendon, which chiefly receives the fibres of the greater muscle arising from the cartilages of the eleven inferior pairs of ribs.
The pectoralis (2) is of very striking dimen sions; the origin of the superficial portion extends from the acromion, along the sternum and linea alba, almost to the pubis ; a deeper seated portion arises from the six osseous sternal ribs ; the fibres of both portions con verge to be inserted into the largely-developed pectoral or anterior crest of the proximal half of the humerus.
The pectoralis minor is attached to the cora coid, and the subclavius is likewise inserted, as in some other quadrupeds, into this bone, which is no longer a subordinate process of the scapula in the Monotremes.
The subscapularis is a narrow muscle, and narrower in reality than at first sight it appears to be, since the supra-spinatus, from the inflec tion of the spine and acromion, arises from the same aspect of the scapula, and appears to form the anterior fasciculus of subscapularis; its distinct insertion into the anterior tubercle of the head of the humerus points out its true nature.
The infra-spinatus (20) and the large teres major cover the whole external surface of the scapula.
The deltoid is divided into an anterior and a posterior portion. The anterior portion (19) arises from the anterior extremity of the cora coid, and is inserted into the summit of the deltoid crest of the humerus : the posterior part (21) arises from the anterior and superior apex of the scapula, and is inserted into the lower half of the deltoid crest. There are also two muscles to which the name coraco brachialis may be applied, a superior one (22) and an inferior one (25).
The biceps brachii arises by two heads ; one (23) arises from the sternal extremity of the coracoid, the other (24) also arises from the coracoid ; the common tendon is inserted into the middle of the radius.
The other muscles of the anterior extremity adhere closely to the mammalian type. The extensor carpi radialis (30) sends three ten dons, to be inserted respectively into the second, third, and fourth metacarpal bones. There is a single common flexor digitorum, as well as extensor digitorum (27).
The extensor digiti minimi (26), the indica tor (28), the extensor pollicis (29), the prona tor teres (32), and the flexor carpi radialis (33) are all remarkable for their strength in the Ornithorhynchus, and are still more powerfully developed in the Echidna.
The most remarkable muscle on the palmar aspect of the fore arm is the flexor carpi ulnaris, which arises by two separate heads, the longer one from the broad olecranon, the shorter one from the internal condyle of the humerus - the common tendon is attached to the os pisiforme and the metacarpals of the fourth and fifth psoas minor from its insertion into the pelvic arch should be regarded as a muscle of the pelvic extremity, and it is one of the largest of these muscles. It arises from the sides of five dorsal vertebrae, and its strong tendon is implanted in the remarkably-developed ilio pectineal process. It depresses the pelvis, and with it also the tail and the pelvic extre mities.