OF DIDELPHYS VIRGINIANA. 89 



almost as free as a metatarso-phalangeal joint, in the line of flexion and extension, and 

 enjoys, moreover, considerable motion in other directions. 



Besides these seven bones, there is an accessory ossicle (fig. 27, h) for the support of 

 the rudimentary spur. It is a small conoidal bone, with an oval base, by which it is articu- 

 lated with the inner aspect of the ento-cuneiform. It enjoys little, if any, motion. The 

 spur is a small, oval, concavo-convex cartilage, lying closely appressed upon the muscle, 

 over the base of the ball of the great toe, and covered by the common integument. Its 

 position is indicated in the sole by the most internal of the tegumentary callosities. It is 

 chiefly interesting as an indication of the well developed spur of one sex of the only lower 

 mammalian order — the Mouotremata. In the opossum the spur is not characteristic of sex. 



Metatarsus. — The metatarsals are nearly straight, but a slight downward projection of 

 the articular surfaces at either extremity gives some lengthwise concavity underneath, 

 which, with the lateral convexity, produced by the shape of the extremity of the tarsus, 

 produces a hollowing of the sole that is filled by the flexor tendons, lumbricales, and inter- 

 osseous muscles. The second is the longest, the fourth is nearly as long, the third, fifth and 

 first successively decrease in length ; the last, besides being shortest, is much stouter than 

 the rest, and depressed, being wider than deep. Its divergence from the others, and pecu- 

 liar articulation, have already been noticed. The proximal extremity of the fifth is en- 

 larged, and projects outside the cuboid ; the articular fiicet only occupies a part of it. The 

 heads of the second, third and fourth are small ; that of the third, in particular, being 

 scarcely wider than the shaft, though projecting considerably downward. Other minor 

 differences in the heads of these three metatarsals need not l)e particularized. 



Phalanges. — The four lesser toes have each three phalanges, the first of which is long 

 and slender, surpassing the other two together ; the third is simply a small, compressed, 

 somewhat falciform ossicle supporting the claw. The hallux has but two phalanges, of 

 which the last is a small nodule, bearing no claw. This toe terminates in a rounded 

 clubbed extremity, upon the superior surface of which the place of a claw is indicated by a 

 curved groove in the integument. The articulations of the phalanges with the metatarsus, 

 and with each other, offer nothing peculiar. 



Sesamoids. — The metatarso-phalangeal articulation of each of the five toes bears two 

 sesamoids upon its plantar aspect. They are rather large, oval, placed side by side, their 

 long axes parallel with the axes of the toes. Those of the great toe are embraced by the 

 tendons of the _^exor hrevis ; the others, by the inter ossei. Their presence is not invaria- 

 ble. The sesamoids of the knee joint have been already noticed. 



PART II.— THE MUSCLES. 



The following account of the muscles is designed to embrace descriptions of all those 

 belonging to the general system. Most of the proper muscles of particular organs, as, e.g., 

 the eye, ear, larynx, heart, etc., and all those of unstriped fibre, are omitted ; such being 

 regarded as more properly falling in the province of a special treatise on these parts. 

 Some, however, are noticed in considerable detail ; particularly the muscles of the peri- 

 nseum and genital organs. 



An arbitrary division of the body into muscular " regions " being not entirely commend- 



HEUOIRS HOST. SOC. NAT. HIST, VOL. II. 28 



