OF DroELPHYS VIRGINIANA. 99 



going, but effectually a distinct muscle, by a difference in the direction of its fibres. On 

 account of the wider intercostal spaces below, the fleshy digitations are much broader than 

 those of the S. superior. They are four in number, arising from the tenth — thirteenth 

 rib, interdigitating with the obliquus abdominis. Although they decrease in length from 

 above downward, the lower one reaches nearest to the spine, in consequence of overbalanc- 

 ing decrease in the breadth of the aponeurotic attachment. The fibres of this muscle are 

 directly transverse ; and, while the S. superior lifts the ribs, this draws them directly away 

 from the median line, and so continues expansion of the chest below. 



'^ Sterno-costalis." — There is a small muscle upon the outside of the thorax, regarding 

 the name and homology of which I am in doubt. It arises by a thin aponeurosis of varying 

 width, from the second, and sometimes part of the third, sterneber, this aponeurosis lying 

 between the pectoralis major and rectus abdominis, and closely investing the latter. It 

 proceeds obliquely forward and outward ; at the external border of the rectus it becomes 

 fleshy, and proceeds to be inserted fleshy into the first rib, opposite the insertion of the 

 scalenus anticus. In general characters it recalls the pectoralis minor, and may possibly be 

 regarded as that muscle, arrested on its way to the coracoid, since the three pectorales 

 elsewhere described are really only dismemberments of one — the pectoralis major. 



Triangularis sterni. — This muscle is of considerable superficial extent, but very thin, 

 and usually divided into halves along the median line, by an interval corresponding to 

 the width of the sternebers. Each half arises as a continuous plane along the outer border 

 of the four lower sternebers (not from the xiphoid), and proceeds obliquely outward and 

 forward, lying upon the whole of the costal cartilages of the second — sixth ribs. 



Inter cosfales. — The externi terminate on the "true" ribs at some distance from the ster- 

 num (in general, along the outer border of the rectus abdominis) ; but on the floating ribs 

 they reach to the margin of this bone. The upper ones are most oblique and shortest ; the 

 lower ones are nearly vertical. The interni cover the whole of the inside of the thorax 

 from spine to sternum. The direction of their fibres decussates wdth that of the preceding 

 muscles in the usual way. 



Infracostales. — At the back of the thorax, near the spine, the internal intercostales run 

 over two or three contiguous ribs instead of being confined to a single costal interspace. 

 The plane of these elongated overlapping intercostals is directly continuous ; the inner sur- 

 face is blended with the successive true intercostals. The width of the series is least 

 above ; toward the bottom of the thorax it increases ; and similarly the individual infracostals 

 elongate, widen, and become more distinct. 



Levatores costariim. — ^A well marked series of little oblique muscles, running from the 

 apex of a transverse process to the succeeding rib. They arise fleshy, and are inserted 

 partly tendinous into the rib between the tuberosity and the angle. The lower ones are 

 longest, as well as largest and most distinct ; some of them have two heads, taking ori- 

 gin (by the outer head) from the rib above, as well as from the transverse process. Their 

 collective force in elevating the ribs would be considerable. 



Diaphrarjm. — With little modification the description of the human diaphragm would 

 apply to that of the opossum. The central tendon commonly resembles the trefoil in shape, 

 but varies with individuals from the trefoil through cordiform to broad reniform. It is equi- 

 distant from the circumference of the thorax, but the middle leaflet approaches the xiphoid, 

 and each lateral leaflet the two last ribs. The muscular fibres radiate in all directions from 



