OF DIDELPHYS VIRGINIANA. 91 



particularly described presently, and then spreads forward with a nearly longitudinal direc- 

 tion of its fibres, over the temporal fascia, covering the latter from the parietal crest to the 

 zygoma, on a line between the ear and eye. Along the median line of the skull it is 

 loosely connected with its fellow by a fascial expansion, and it seems to constitute an 

 '"occipito-frontalis." I cannot trace it beyond the frontal crests, while the plane first 

 described is easily demonstrated to extend quite to the snout. 



Auricular muscles. — Three muscles of the ear may be described, besides the superficial 

 panniculus that suri-ounds the conch, and is subservient to its general movements ; only 

 one of them, however, is very distinct, or has bony origin. This is a detrahens aurem ; it 

 is a stout, terete, somewhat flattened band, lying beneath, and entirely distinct from, all 

 the foregoing layers. It arises from the lower border of the zygoma, just at the anterior 

 margin of the glenoid fossa ; proceeds directly upward and spreads a little, after reaching 

 the conch, upon the back part of the latter. It draws the ear directly downward — what 

 would be forward were the animal erect — and seems to represent our attrahens. 



The plane of fibres that arises from the ligamentum nuchoe, sends, from its anterior mar- 

 gin, a large, fan-shaped, flattened fasciculus that mounts and spreads over the back part of 

 the conch. The portion of this fasciculus that lies upon the ear itself is tolerably distinct ; 

 but in the rest of its extent the muscle is scarcely separable without violence from the 

 general plane. It draws the conch upward and backward, uniting the offices of attoUens 

 and retrahens. 



The forward motion of the ear is subserved by the transverse plane of fibres that mount 

 the side of the head from the middle line of the throat ; these having an insertion along the 

 border of the conch. 



Orbicular muscles. — The orbicularis oculi is small and pale, and might readily elude 

 observation by its intimate connection with the panniculus, beneath which it lies, were it 

 not for its definite bony attachment in front. The fibres, after encircling the eyelids, con- 

 verge to a point and are inserted into the lachrymal bone, just anterior to the foramen. 

 The orbicularis oris lies between the mucous membrane of the lips and the panniculus, 

 which, as already stated, I'eaches to the edge of the true skin all around the mouth. Upon 

 carefully lifting it away the orbicularis is exposed, forming a cylindrical band of fibres 

 around the margins of the lips. These fibres cannot be fiiirly traced around the snout from 

 one side of the mouth to the other, being lost, both above and below, in the thickened sub- 

 cutaneous tissue. 



Buccinator. — This is, in effect, a backward prolongation of the orbicularis into the cheeks, 

 between the panniculus and the mucous membrane. There is no crossing of the fibres of 

 the orbicularis at the corner of the mouth, nor other interlacing with the cheek muscle. 

 The fibres of the latter form a series of semicircular loops, thrown successively further and 

 further back from the orbicularis, till the angle of the mouth is reached. This muscle is 

 much thickened along its periphery, by which it has definite bony attachment to nearly the 

 whole length of the horizontal ramus and anterior border of the coronary process of the 

 lower jaw, as well as to the greater part of the alveolar process of the upper jaw. 



The two foregoing muscles close and appress the lips and cheeks. They are directly 

 antagonized by the panniculus, the line of traction of which, and insertion into the lips, are 

 such that they draw the lips apart, and the angle of the mouth backward, producing the 

 characteristic expression of the opossum's face — the "risus sardonicus." This "showing of 



