MONOTREMATA. 



381 



The transversalis cervlcis and trnchelo-mas- 

 toideus are blended into a single oblong muscle 

 arising from the anterior dorsal and the trans- 

 verse processes of the six lower cervical ver- 

 tebrae, and inserted into the mastoid process. 



The sterno-mastoid is a double muscle on 

 both sides, one portion being superficial (8), 

 the other deep-seated ; each arises separately 

 from the episternal bone, and is separately 

 inserted into the mastoid tubercle, behind the 

 tympanic cavity. The omo-hyoideus and mylo- 

 hyoideus (10, 10) have a common insertion into 

 the os hyoides. A muscle (1"), arising from 

 the hyoid bone and expanding to be inserted 

 into the lower lip, serves to retract this part. 

 The sterno-hyoideus (11) joins the hyo-glossus. 

 The genio-hyoideus (12) and the stylo-tiyoideus 

 (13) have the normal relations: the biventer 

 maxilla (14) is a short thick muscle, inserted 

 near the bend, representing the angle, of the 

 jaw. 



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 obliquus externus uhdominis 

 (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 (vi), 

 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 obliquus interims (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 a point 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 sterni, 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 the 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 radiulis (30) sends three ten- 

 dons, to be inserted respectively into the second, 

 third, and fourth metacarpal bones. There is 

 a single common Jlexor digitorum, as well as 

 extensor digitorum (27). 



The extensor digiti minimi (26), the indica- 

 tor (28), the extensor pollicis (29), the prona- 

 tor feres (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 forearm is \hejlexor 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 

 digits. 



The 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 , 



