MARSUPIALIA. 



289 



by the large elliptic abdominal ring (/), bends 

 round the marsupial bone near its free ex- 

 tremity, and expands upon the tunica vaginalis 

 testis. In the female it has the same origin, 

 course, and size, but spreads over the mam- 

 mary glands at the back of the pouch. If the 

 anterior fascicles of the diverging and em- 

 bracing fibres be dissected from the posterior 

 ones, the appearance of the cremaster dividing 

 into two layers is produced. 



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 vertebra ; 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 part 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-spinatM) 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-spmatus is inserted into 

 the upper and posterior part of that tubero- 

 sity. 



The deltoides is 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 



VOL. III. 



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 ossicle 

 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 pecloralis 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 vertebrae along 

 the whole extent of the chest to the beginning 

 of the abdomen. 



The biceps is a powerful muscle, although 

 its short head from the coracoid process is sup- 

 pressed. The long head has the usual origin 

 and relation to the shoulder-joint ; its tendon 

 is very thick and short. The fleshy belly joins 

 that of the strong brachialis interims, situated 

 at the external side of the humerus, whence it 

 takes its principal origin from the short deltoid 

 ridge, closely connected there with the second 

 portion of the triceps, and deriving some fleshy 

 fibres from the lower and outer third of the 

 humerus. The portion of the biceps arising 

 by the long head soon resolves itself into two 

 distinct penniform muscles ; the tendon of the 

 outer one joins that of the brachialis, and this 

 conjoined tendon simply bends the fore-arm, 



* Professor Vrolik found that the pectoralis 

 minor in the Kangaroo was inserted not into the 

 coracoid process, but into the humerus between the 

 biceps and the tendon of the pectoralis major. 



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