OF DIDELPIIYS VIRGINIANA. 139 



motions of joints are but combinations of flexion and extension ; adduction and abduction 

 are only transverse instead of longitudinal flexion and extension ; circumduction comprehends 

 both, the distal extremity of a segment describing a curved instead of a straight line ; 

 and even simple rotation, as at shoulder and hip, will be found, when closely examined, to 

 be similarly reducible. 



The direct action of many flexors and extensors upon a succeeding segment is comple- 

 mented by an indirect action, whereby they become respectively extensors and flexors of a 

 preceding segment — that one upon which they lie. Such flexors and extensors are those 

 that take origin from a segment above that upon which they lie, and the second above that 

 upon which they primarily and directly act. Thus the biceps, arising from the scapula, is a 

 direct flexor of the forearm, and an indirect extensor of the arm, etc., etc. On the other 

 hand, those that extend over but two instead of three segments, have no such complemen- 

 tary action ; thus the brachialis anticus is purely a flexor of the forearm, and if it have any 

 " reverse action," as when it contracts from its insertion as a fixed point, such operation 

 still really tends to continued flexion of the forearm, by means of the flexion (retroduction) 

 of the humerus that is induced. These, and many correlated facts, have been ably eluci- 

 dated by Professor Wilder, in his paper " On Morphology and Teleology, especially in the 

 Limbs of Mammalia," and been made the basis of a division of limb-muscles into systems 

 of "long" and "short" flexors and extensors ; which classification is made to play an im- 

 portant part in his determination of the special homologies, and beyond a doubt affords 

 instructive indications in certain obscure cases. 



The foregoing appear to be the principal theorems and definitions that will come into use 

 in the present inquiry. 



The scapular arch is loosely appended to the trunk, and enjoys free movement in various 

 directions. "With the exception of the loose sterno-clavicular articulation, its attachment is 

 entirely by muscular tissue. The muscles are large, numerous, and varied in form, func- 

 tion and position ; they are eight in number, viz. : the cleido-mastoid, subclavius, 

 trapezius, rhomboideus, serratus with levator anguli scapulae, " atlanto-acromialis," 

 " atlanto-scapularis " (see page 112), and omo-hyoid. Of these eight, the first named maj^ 

 be morphologically a part of the sterno-mastoid, and the second appears referable, with ob- 

 vious propriety, to the intercostal series. The omo-hyoid has been conjectured to belong 

 to the same series, connecting as it does hoemal arches of contiguous cranial vertebrae — the 

 occipital and parietal. The levator anguli scapulse and serratus, believed by Professor 

 Wilder in the case of the human subject to be one and the same dismembered muscle, are in 

 the opossum actually one and the same ; there is no division of their common plane, which 

 extends unbroken from a long series of dorsal and cervical pleurapophysial elements, by 

 numerous converging fiiscicles, to the occipital pleurapophysis. It is not, perhaps, going 

 too far to suggest that these muscles may also be referable to the trunk series. Of the 

 four muscles remaining after these eliminations, two connect the scapula with the diapophy- 

 sis of the atlas alone, and two with the occiput and several spines of cervical and dorsal 

 vertebrae. Thus it is seen that all the muscles of the scapular arch appear to grow out of 

 the peculiar relations of this haemal arch to its own and other vertebrae. While it would 

 be premature and unwarrantable to assert that the scapular muscles are highly developed, 

 differentiated and specialized components of the vertebral or trunk series, it cannot be 

 denied that there are grounds upon which to base a belief that such may be the case. In 



